Category Archives for "Microbiology"

Economic Importance of Viruses

Virus is an infectious agent with microscopic size that needs living cells of plants, animals or bacteria to multiply. The name virus is derived from Latin word meaning “poison” or “slimy liquid”.  

They are not animals, plants or bacteria and they can`t do metabolic process without a host cells. The body of virus contains nucleic acid (DNA or RNA) and protein. Hence, there are two types of viruses such as DNA virus and RNA virus.

Image of DNA Viruses

Classification of DNA Virus

image of RNA viruses

Classification of RNA Virus

Virus influence the global biogeochemical cycles and they can transfer their genetic material into a host cell when they attach with host cell.

They produce diseases in an organism using their different mechanisms. Some viruses produce chronic infections to the hosts such as hepatitis B virus and hepatitis C virus infections.

If people chronically infected then they serve as reservoirs of infectious virus and they are known as carriers. In this case, the disease is called endemic if the population with a high proportion of carriers.

Importance of Viruses

Viruses have both harmful and beneficial roles in human life. Actually no virus directly involves in human welfare, in most cases scientists use them as beneficiary agents in different fields.

Harmful Effects of Viruses

A vast number of viruses cause human and animal diseases. Some viruses are epidemic which spreads rapidly to many people and some viruses are pandemic which spread diseases worldwide. COVID-19 (corona virus disease) is very life threatening disease that already has taken the lives of nearly few lakhs people worldwide. In 2003, the severe acute respiratory syndrome (SARS) also took the lives of nearly 800 people worldwide.

Besides, a vast number of viruses cause plant diseases as rice tungro baciliform virus (RTBV), tobacco mosaic virus (TMV), tomato/papaya ring spot virus, tomato leaf curl, potato leaf roll virus, etc.

The following table shows the various virus diseases in man, animals and plants:


Name of diseases


Influenza virus



Herpes virus



Hepatitis B, C virus






Variola virus

Small pox


Rubeola virus



Polio virus



Rabies virus



Yellow fever virus

Yellow fever


Flavi virus



Vaccinia virus

Cow pox


Foot and mouth virus

Foot and mouth disease


Papaya ring spot virus

Papaya ring spot disease


Tobacco mosaic virus

Tobacco mosaic disease


Bean mosaic virus

Bean mosaic disease


Tungro virus

Tungro disease of rice


Bushystant virus

Bushystant disease of tomato


Bancy top virus

Bancy top disease of banana


Some morphological and pathogenic characteristics of the major hepatitis viruses:


Virus Name

Hepatitis A virus (HAV)

Hepatitis B virus (HBV)

Hepatitis C virus (HCV)


27 nm

42 nm


Nucleic acid





Endemic and epidemic



Incubation period

2-7 weeks

1-6 months

2-8 weeks


Fever, Gastro-intestinal tract disorders

Fever, rash, arthritis

Fever, rash, arthritis


1 in 10




Acute, short

Gradual, chronic

Acute to chronic



Chronic active hepatitis, hepatic cancer

Chronic inflammation, cirrhosis

Poultry birds are also severely infected by viruses. The avian influenza H5N1 (hemagglutinin type 5 and Neuraminidase type1) produces birds flu and the poultry sector.

The grasseric disease of silkworm (Bombyx mori) is caused by a nuclear polyhedrosis virus (NPV), sometime which is the only reason of declination of silk industry.

Besides these, bacteriophases attack the nitrogen fixing bacteria of the soil and are responsible for reducing fertility of the soil.

Viruses are also used as biological warfare and weapons in many countries. Viruses are tiny but they have the ability to produce diseases which can cause death and damage to huge populations in epidemics and pandemics.

Beneficial Effect of Viruses

There are many industrial uses of viruses. Viruses are used in preparation of sera and vaccines to be used against diseases like rabeis, polio, hepatitis B, papillomavirus, etc. The multiplication of viruses in bactereial cells is also utilized in the production of antibodies.  It is used in various pharmaceutical products such as proteins, vaccine, antigens and antibodies.

Viruses are also used in biological studies. They have gained a prominent position in world because of their value as biological research tools.

They are broadly used in research in the field of genetic engineering, molecular biology and medicine due to their capacity to fast reproduction and plainness structure.

Viruses are used in bacteriophage therapy. Bacteriophages have been researched for their use in therapy.

There are many uses of viruses in medicine. In this case, it is being used as vectors or carriers that take the required material for treatment of a disease to various target cells.

In the life science, it plays an important role to understand the basic mechanisms of molecular genetics. It is also used in genetic engineering for the production of cloning.

It is used for gene therapy. In this case, viral genes are replaced by the human gene.

Viruses are used in biological control by human in eradicating pests like insects (by NPV) and in controlling the population of organisms such as rabbits by inducing viral infection.

The viruses also played a central role in the early evolution, before the diversification of bacterial, archaea and eukaryotes, at the time of the last universal common ancestor of life on earth. Viruses are still one of the largest reservoirs of unexplored genetic diversity on earth.

Viruses are also used in aquatic environment to recycling carbon. About one million of viruses are found in one teaspoon of seawater. Among them, majority are bacteriophages. They are not harmful to animals and plants and they play an important role to regulate the freshwater and saltwater ecosystem by destroying bacteria and recycling carbon in the aquatic ecosystem.

Economic Importance of Bacteria

Bacteria are single-celled microscopic organisms which can live in different types of environment and survive in extreme conditions. They contain high protective coating in their body, which enhances to live any severe conditions. Many bacteria possess flagella which help them to move around, but some have hair-like structures which help them live in hard surfaces and the cells of the human body. Bacteria possess significant economic importance concerning human being. Bacteria cause many harmful effects, including diseases in man, animals and plants. At the same time, they show many beneficial effects.

Beneficial Effects of Bacteria

Role of Bacteria to Increase in Soil Fertility

Plants do not utilize atmospheric nitrogen directly. In this case, there are many symbiotic and free-living bacteria which can fix free nitrogen from the atmosphere and converts it into nitrogenous compounds. Azotobacter is soil-inhabiting bacteria which fixes the atmospheric free nitrogen and turns it into organic forms; some amount of which mixes with the soil and increase the fertility of the soil. Another free-living soil bacterium is Clostridium, which also increases soil fertility. Rhizobium is a symbiotic bacterium which lives in the root nodules of leguminous plants.

Nitrifying Bacteria: Nitrifying bacteria are chemolithotrophic aerobic microorganisms (family Nitrobacteraceae) which play an essential role in the nitrogen cycle, convert the soil ammonia to nitrates and make the usable compounds by plants. Some notable nitrifying bacteria are Nitrosococcus, Nitrosomonas,  Nitrobacter, Nitrococcus, etc. These bacteria receive their energy from the inorganic nitrogen compounds. They can convert the soil ammonia into nitrate by the oxidation.  In this case, Nitrosomonas bacterium can convert ammonia to nitrous acid by the oxidation while the Nitrobacter can convert nitrous acid to nitric acid by the oxidation.

Proteins of dead plants and animals are converted into organic nitrogenous substances and amino acids by the action of some saprophytic bacteria. Besides these, many ammonifying bacteria such as Clostridium sp., Proteus vulgaris, Bacillus ramosus, etc. can convert amino acids R-CH(NH2)-COOH] into ammonia(NH3). This ammonia is mixed with carbon dioxide (CO2) and water (H2O) to form (NH4)2CO3 (ammonium carbonate) which are used as a nitrogen source by many plants.

There are many nitrogen-fixing bacteria which can fix nitrogen from the direct atmosphere.  Among them,  Rhizobium bacteria live in the root nodules of leguminous plants(Fabaceae)  such as alfalfa, peas, beans, lentils, soy, and peanuts, etc. They have the ability to fix atmospheric nitrogen about 100-400 kg/hectare/year. Besides these, other nitrogen-fixing filamentous bacteria, Frankia sp. that live in root nodules of legumes (family Fabaceae) or actinorhizal plants which can fix nitrogen about 150 kg/ hectare/year from the atmosphere with the help of nitrogenase enzyme. These bacteria provide sufficient nitrogen for the host plants. For this reason, actinorhizal plants can quickly grow and colonize in any low nutrient soil conditions. Clostridium and Azotobacter are also free-living nitrogen fixers which can fix nitrogen about 25-50 kg/hectare/year.

The following table shows the list of some Nitrogen Fixing Bacteria:

Name of Bacteria

Where Living

Mode of Life

Rhizobium leguminosum (Rhizobiaceae)

Root nodules of Legumes

Mutualistic symbiotic

Rhizobium gallicum (Rhizobiaceae)

Root nodules of legumes

Mutualistic Symbiotic

Rhizobium bangladeshense (Rhizobiaceae)

Root nodules of lentils

Mutualistic Symbiotic

Bradyrhizobium japonicum

Root Nodules of LegumesSoybean

Mutualistic Symbiotic

Frankia spp.

Nodules of Casuarina, Alnus, etc

Mutualistic Symbiotic

Azobacter agilis

Aerobic and soil inhabiting

Free living

Clostridium pneumoniae

Anaerobic and soil inhabiting

Free living

Role of Bacteria in Dairy Industry

Lactic acid bacteria (LAB) play an important role in the fermentation process in the dairy industry. Some lactic acid bacteria (LAB)  such as Streptococcus lactis, S. thermophilic, Lactococcus lactis, Lactobacillus plantarum, Lactobacillus casei, Lactobacillus acidophilus, Lactobacillus helveticus,  Lactobacillus bulgaricus, etc. are used to produce butter, cheese, curd, etc.  These bacteria make fermentation the lactose in the milk to produce lactic acid, which helps in curd coagulation and texture formation during the cheese production. During cheese production, the low pH helps to keep in check the growth of pathogen and spoilage organisms. In this case, the LAB converts lactose sugar to glucose and then glucose to lactic acid. Lactic acid sours the milk and coagulates the casein (milk protein) and forms the curd.

Role of Bacteria in the Vinegar industry

Acetic acid bacteria (AAB) are used in the vinegar industry for the production of certain foods and vinegar. These are Gram-negative bacteria which belong to the family Acetobacteraceae. They can produce acetic acid during oxidative fermentation by performing oxidation reaction producing vinegar as a byproduct. For the production of ascorbic acid, gluconic acid, dihydroxyacetone, and cellulose, acetic acid bacterias` oxidation mechanism is utilized. Besides these,   acetic acid bacterias are also used as biocatalysts for the improvement of eco-friendly fermentation processes.

The following list shows the most frequently used acetic acid bacteria (AAB) species for the production of vinegar:

  • Acetobacter aceti,
  • A. cerevisiae,
  • A. malorum,
  • A. oeni,
  • A. pasteurianus,
  • A. pomorum,
  • Gluconacetobacter entanii,
  • G. liquefaciens,
  • G.  oxydans,
  • Komagataeibacter europaeus,
  • K. hansenii,
  • K. intermedius,
  • K.  medellinensis,
  • K. oboediens,
  • K. xylinus

Role of Bacteria in Medicine

Antibiotics are the chemical materials which are produced from microorganisms for preventing the growth and development of other microorganisms. Many bacteria are used in the pharmaceutical industry for the production of antibiotics, probiotics, drugs, vaccines, starter cultures, insecticides, medically-useful enzymes, etc. Bacteria can also be programmed to make various medicines in genetic engineering technology. Antibiotics like Tetracyclines, erythromycin, streptomycin, rifamycin and ivermectin are produced by using bacteria Streptomyces spp. while bacitracin and polymyxin are produced from Bacillus and Paenibacillus species. Bacteria are also used in the manufacture of vaccines. These vaccines are used against infectious diseases such as whooping cough, diphtheria, typhoid fever, tetanus, and cholera. In the twentieth century, the widespread uses of antibiotics and vaccination (immunization) against infectious disease have radically increased the lifespan of individuals in developed countries.

The following table shows the list of notable bacteria which are useful sources of antibiotics:

Name of bacteria



Bacillus subtilis


Syphilis, Lymphonema or Reticulosis.

Bacillus polymyxa



Streptomyces rumosus


Intestinal and Urinary Infections

Streptomyces griseus


Pneumonia, Meningitis, Tuberculosis and Local Infection.

Streptomyces fradiae


Hepatic encephalopathy, skin infections, ear infections

Streptomyces venezualae, S.lavendulae



Typhus, Typhoid, Whooping cough, Bacterial Urinary Infections, A typical Pneumonia

Micromonosopora purpurea


Effective against Gram (+) bacteria

Streptomyces erythreus


Typhoid, Common Pneumonia and Diphtheria, Whooping Cough, etc.

Streptomyces aureofaciens


Osteomyelitis,  Whooping Cough, Viral pneumonia, and Eye infections.

Aspergillus fumigatus


It is used  against Salmonella and Shigella.

Penicillium chrysogenum, P.notatum


Gnonorrhea, Rheumatic Fever Tonsilitis, Sore Throat,  some Pneumonia types.

Role of Bacteria in the Production of Vitamins

Different probiotic bacteria have several health benefits, including vitamin production. These bacteria have commercial importance for the production of Vitamin including the species Lactococcus lactis, Lactobacillus gasseri, Lactobacillus reuteri, Pseudomonas denitrificans, Clostridium butylicum and Bifidobacterium adolescentis. They are able to synthesize vitamin K and B vitamins, such as biotin, nicotinic acid, cobalamin (Vitamin B12), panthotenic acid, folates, pyridoxine, riboflavin (Vitamin B2), and thiamine, etc. Among these bacteria, Pseudomonas denitrificans is used to produce Cobalamin (Vitamin B12) while Clostridium butylicum is used to synthesize Riboflavin (Vitamin B2).

The following table shows the list of bacteria with corresponding yields of Vitamin):

Name of Bacteria


Yield (mg/l)

Clostridium acetobutylicum

B2 (Riboflavin)


C. butylicum

B2 (Riboflavin)


Mycobacterium smegmatis

B2 (Riboflavin)


Bacillus megaterium



Streptomyces olivaceus



Butyribacterium rettgeri



Micronospora sp.



Propionibacterium freudenreichii



P. shermanii



Pseudomonas denitrificans



Role of Bacteria in Wastewater Treatment

Wastewater is very detrimental to the environment and acts as a source of several types of waterborne diseases.  There are many well-known bacteria which play an essential role in keeping sewage clean. In this case, putrifying bacteria treat and purify the wastewater and make it less harmful to our surrounding environment. These bacteria work under the anaerobic condition to remove the solid and semi-solid constituent of sewage. After finishing the treatment process, the constituents get decayed and liquefied which are filtered, and the liquid is drained out in the river.

Role of Bacteria in  Butanol and Acetone Production

Butanol or butyl alcohol (C4H9OH) and acetone or propanone [(CH3)2CO] are produced by using bacteria in a different industry.  In this case, Clostridium acetobutylicum is the most well-known and widely used species for the production of Butanol and acetone as a commercial basis.  Clostridium beijerinckii is also used to produce Butanol and acetone with excellent results.

Role of Bacteria in Fiber Retting

Microbiological processes are used for discharge of the fiber. In this case, there are many bacteria help in the retting of jute, hemp and flax fibers. These bacteria grow under low oxygen condition, which can cause hydrolysis of the pectic substances that help to blind the fibers with the stem and make easy for the discharge of the fibers.  The following list shows some notable bacteria species which play an important role in the process of fiber retting. 

  • Achromobacter parvulus
  • Aerobacter cloacae
  • Aerobacter aerogenes
  • Bacillus brevis
  • Bacillus cereus
  • Bacillus megaterium
  • Bacillus sphaericus
  • Bacillus subtilis
  • Clostridium butylicum
  • Clostridium beijerinckii
  • Clostridium saprogenes
  • Clostridium perenne
  • Pseudomonas aeruginosa
  • Pseudomonas pseudomallei

Role of Bacteria for Curing of Tea and Tobacco

Tea and tobacco are cured to give particular taste, flavor or smell by using some bacteria. In this case, the helpful bacteria are Bacillus megatherium and Micrococcus candisans which are used in the curing and fermentation of tea and tobacco leaves for commercial purposes.

Role of Bacteria for Biological Control of Insect

Biological control is also known as biocontrol. It is the process for controlling different types of pests like insects, weeds, mites, and plant diseases by using other organisms. Many microorganisms such as a bacterium, virus, fungus, or protozoan, etc. are used as the active ingredient for the preparation of microbial insecticides to control many different kinds of pests. These microbial insecticides are essentially nonpathogenic and non-toxic to humans, wildlife, and other organisms. Bacillus thuringiensis is more effective to control Aedes aegypti while B. sphaericus strain is effective to control Culex quinquefasciatus. In this case, Bacillus thuringiensis secrete proteinaceous substances which are highly toxic to caterpillars and insects under the order Lepidoptera.

The following table shows the list of Bacteria, product name and their uses.

Name of Bacteria

Product Name


Bacillus thuringiensis var. kurstaki

Bactur®, Caterpillar Killer®, Bactospeine®, Bioworm®, Javelin®, Dipel®, Futura®, Thuricide®,SOK-Bt®, Tribactur®, Worthy Attack®,Topside®,

Effective for foliage-feeding caterpillars and Indian meal moth of stored grain.

Bacillus thuringiensis var. israelensis

Aquabee®, Bactimos®, Gnatrol®, LarvX®, Skeetal®, Mosquito Attack®, Vectobac®, Teknar®,

Effective against larvae of Aedes and Psorophora mosquitoes(Psorophora ciliata), black flies, and fungus gnats only.

Bacillus thuringiensis var. tenebrinos

Foil®, M-Track®,

It is highly effective against larvae of Colorado potato beetle and the elm leaf beetle(Xanthogaleruca luteola).

Bacillus thuringiensis var. aizawai


It is used only for control of wax moth infestations in honeybee hives.

Bacillus popilliae and Bacillus lentimorbus

Grub Attack®

Effective against larvae (grubs) of Japanese beetle

Bacillus sphaericus

Vectolex CG®,
Vectolex WDG®

Effective against larvae of Culex, Psorophora, and Culiseta mosquitos, larvae of some Aedes spp.

Role of Bacteria in the Degradation of Petroleum

Hydrocarbon contamination is one of the major environmental problems today. It occurs due to the accidental releases of petroleum products from the petrochemical industry, oil tankers, ships, etc. The ultimate natural mechanism is the microbial degradation which can clean up the pollutants of petroleum hydrocarbon from the environment. There are many indigenous microorganisms which live in water and soil, and they can eliminate hydrocarbon contaminants. The following bacteria species can degrade hydrocarbon pollutants from crude oil:

  • Pseudomonas fluorescens, 
  • P. aeruginosa,
  • Bacillus subtilis
  • Alcaligenes sp.
  • Acinetobacter lwoffi
  • Flavobacterium sp.
  • Micrococcus roseus, and
  • Corynebacterium sp

Role of Bacteria for Decomposition of Dead Animals

Decomposition is the natural process which breaks down the dead animal or plant tissue by the action of different types of the organism such as invertebrates, fungi and bacteria. These organisms are known as decomposers. In this case, some bacteria play an essential role to decompose the dead organic matters into an inorganic form which increase the fertility of the soil by mixing with it, and finally the plants absorb it as nutrients.

Harmful Effect of Bacteria

Role of Bacteria in Food Spoilage

Some bacteria cause food Spoilage. Micrococcus can cause vegetable spoilage, Pseudomonas, Clostridium can cause deterioration of meat while Enterobacter causes decay of syrup, Acetobacter can cause decay of orange. Streptococcus, Micrococcus and Lactobacillus also can cause decay of milk and different milk products.  Sometimes foods are poisoned by the bacteria like Streptococcus aureus and Clostridium botulinum. Clostridium botulinum causes botulism disease by producing exotoxin showing the symptoms like double vision, respiratory disturbances, and swelling of the tongue.  By releasing exotoxins in foods which makes the food unsuitable for the consumption of human being. Under favourable temperature and conditions, bacteria grow in food materials and change the appearance, flavour and smell of food.  By consuming those foods, different types of diseases such as gastroenteritis, dysentery, etc. are exposed, even death may occur.

Role of Bacteria in Water Pollution

Water is polluted by different bacteria which make the water unsuitable for drinking. Those polluted water are transmitted by drinking and can cause diseases like cholera (Vibrio cholera), typhoid (Salmonella typhi) and bacillus dysentery (Shigella dysenteriae).

Role of Bacteria in the Reduction of Soil Fertility

Moist soil-inhabiting bacteria are capable of transforming soil nitrates into gaseous nitrogen. This process is called denitrification, and those bacteria are called denitrifying bacteria. By the process of denitrification, the following bacteria such as Pseudomonas, Micrococcus, Thiobacillus, Achrobacter Thiobacilus, Micrococcus, Achrobacter, Bacillus denitrificus etc. convert nitrates of the land into gaseous nitrogen; as a result, a good loss of nitrogen occurs from the soil causing reduction of soil fertility.

Role of Bacteria as Disease-Causing Agents

Many parasitic bacteria induce diseases in plants and animals, including human. Diseases cause by different types of bacteria in plants decrease the yield of crops.  A list of pathogenic bacteria, the respective diseases and their site of infection are shown in the following table:                                            

Name of Bacteria


Site of Infection

Plant Pathogens:


Streptococcus scabies

Scab disease of potato

Potato tuber

Corynebacterium rependonicium

Ringrot disease of potato

Potato tuber

Xanthomonas campestris

Black rot disease of cabbage

Young branches, leaves and fruits of citrus

Erwinia atroseptica

Black rot disease of potato

Stem and tuber of potato

Erwinia amylovora

Pear diseases(Pyrus)

Vascular tissue of pear

Pseudomonas maculicola

Cauliflower spot disease


Pseudomonas solanacaerum

Wilt disease of potato

Potato, tomato

Bacterium stewartii

Wilt disease of corn(Stewards disease)

Vascular tissue of corn

Agrobacterium rhizogens

Hairy root disease of apple

Meristematic tissue of apple

Pathogens for man


Salmonella typhi


Alimentary canal of man

Vibrio cholerae

Asiatic cholera

Intestinal tract of man

Mycobacterium tuberculosis


Lungs of man

Mycobacterium leprae


Skin of man

Diplococcus spp.


Lungs of man

Corynobacterium diptheriae


Throat of man

Streptococcus sp.


Joints tendons, ligaments, bones, and muscles

Clostridium septicum

Gas gangrine

Muscle tissue

Clostridium tetani


Blood vascular system of man

Bacillus dysenteriae


Intestinal tract of man

Nesseria gonorrhoea


Urethra, rectum or throat

Leptospina hemorrahagiae



Animal Pathogens


Bacillus antracis

Anthrax disease


Yersinia pestis



Coxiella burnetti

Q fever

Birds and rats

Leptospira interrogans



image of Black rot disease of cabbage

Black rot disease of cabbage

image of Black rot disease of potato

Black rot disease of potato

Concluding Remarks

Bacteria are single-celled microscopic organisms which can live in different types of environment and survive in extreme conditions. They contain high protective coating in their body, which enhances to live any severe conditions. Many bacteria live in the stomach and mouth of a human. They are also found in soil, water, food and surface areas of our environment. There are many bacteria which live in the digestive system or gut of the human body. They help digestion of food and make the body healthy. Some bacteria provide oxygen which is used to make antibiotics while some other bacteria can decompose dead leaves, release CO2 and nutrients in the environment, which is essential for the plant's growth. Besides these, many bacteria show harmful effects on plants, animals and human being.

Fungus: Definition, Characteristics, Types and Its Economic Roles

Fungi (singular fungus) are the eukaryotic organisms that belong to the Kingdom Fungi. They have no chlorophyll pigments and vascular tissues. The body consists of a single cell to branched filamentous hyphae that often produce specialized fruiting bodies. The special characteristic of fungi is that they grow rapidly and die soon. In nature, there are over a million species of fungi, but only 10% of known fungi species are found in the scientific literature. There are over 300 species of fungi that are infectious to humans. Each year 85 billion tons of CO2 released in air through decomposing plants done by fungi. They are widely distributed organisms with excellent medical and environmental value.

Most of the fungi freely live in soil or water, but many form symbiotic or parasitic relationships with animals and plants. When they make a symbiotic relationship with the roots of plants, then it is known as mycorrhizal fungi. The study of fungi in a discipline of botany is called Mycology. Edible mushrooms, yeasts, rusts, smuts, mildews, black mold, toadstools, and the Penicillium notatum, which produce the antibiotic drug, penicillin, all belong to the domain Eukaryota under the kingdom Fungi.

Characteristic Features of Fungi

  • They are unicellular or multicellular eukaryotic organisms.
  • The multicellular body is filamentous like, and it is composed of individual microscopic filament, which is known as hyphae. In this case, hyphae show apical growth and branches to produce lots of hyphae network, known as mycelium.
  • They are saprophytic, symbiotic, or parasitic organisms.
  • They get their nutrition through the process of absorption.
  • They cannot make food through photosynthesis due to a lack of chlorophyll pigments.
  • Primarily, the cell wall composed of chitin and glucagon, but in some species such as Oomycetes, the cell wall contains cellulose.
  • Typically, they have a haploid nucleus, but hyphal components are multinucleated. In some species, such as Oomycota and some yeast, have diploid nuclei.
  • The body of fungi can store trehalose, glycogen, sugar alcohols, and lipids.
  • They can produce spores by sexual and asexual reproduction. In this case, sexual spores are Zygospores, Oospores, Ascospores, Basidiospores, etc. and asexual spores are Zoospores, Aplanospores, Sporangiospores, Conidia, etc.
  • They are typically non-motile and have no embryonic stage.
  • In the life cycle of the fungus, the phenomenon of alteration of generation occurs. In this case, they show both haploid and diploid stages.
  • They are heterotrophic organisms, and they get their food from organic substances, animal, and plant matters.
  • Fungi can tolerate acidic pH and prefer to grow in an acidic environment.
  • Optimum growth temperature for most saprophytic fungi is 20-30°C, and a parasitic fungus is 30-37°C.
    Fungus shows a slower growth rate than that of bacteria.
  • Fungi can live a dry environment and tolerate high sugar concentration.

Types of Fungi

Based on their structure, commonly, the following three types of fungi are seen within the fungi kingdom.

  • 1. Yeasts
  • 2. Molds and
  • 3. Mushrooms

1. Yeasts

  • They are eukaryotic, unicellular microorganisms.
  • About 1500 species of yeasts are currently described, which constitute 1% of all described fungal species.
  • . In nature, the sizes of the species vary, which can grow up to 40 µm in diameter.
  • Most of the yeasts species reproduce asexually by mitosis, and some by budding.
  • The best growth temperature for yeast species varies greatly. The best growth temperature for Saccharomyces telluris is ranged from 41 to 95 °F (5 to 35 °C), for Leucosporidium frigidum is 28 to 68 °F (−2 to 20 °C), and for Candida slooffi, it is ranged from 82 to 113 °F ( 28 to 45 °C).
  • Some yeast species have a beneficial role, and some have harmful effects on human health. For example, Saccharomyces cerevisiae, is also known as Brewer’s or baker’s yeast, which is commercially used to produce bread in the food factory. In many cases, Saccharomyces boulardii is found in the intestine as normal flora. It is also found in the mouth and esophagus. It can become infectious due to weak immunity system.
image of Saccharomyces cerevisiae

Saccharomyces cerevisiae

2. Molds

  • Molds are multicellular organisms with different colors, including green, gray, or black.
  • Over 100,000 mold species have been identified.
  • They have negative health effects on human health.
  • They are found throughout the environment. Naturally, they inhabit the soil and break down decaying vegetable matters.
  • They can rot leaves, wood, and other organic debris to form humus in the soil. They also spoil food and animal feed materials.
  • They reproduce by creating spores which vary in shape that range from 2 to 100 µm in size. Spores of mold spread through the air, water, or on animals and can cause health issues by triggering allergies to humans.

Mold Types

Generally, mold species are grouped into the following three types:

1.Allergenic: Aspergillus, Cladosporium, and some Penicillium species produce airborne spores that can act as allergens. Other common molds that can also act as allergens include Helminthosporium, Mucor, Epicoccum, Rhizopus, Fusarium, and Pullularia, etc.

2.Pathogenic: Aspergillus fumigatus and Aspergillus flavus are the leading cause of invasive aspergillosis. It can also cause chronic pulmonary infections. Other pathogenic species, Histoplasma capsulatum causes infectious disease to human, which is known as histoplasmosis.

3.Toxigenic: Some mold species are toxic to humans and other animals that come in contact with them. Some toxigenic molds are Fusarium solani, Fusarium oxysporum, Fusarium moniliforme, Penicillium brevicompactum, Penicillium chrysogenum, Penicillium citrinum, Penicillium corylophilum, Penicillium cyclopium, Penicillium expansum. Penicillium fellutanum, Penicillium spinulosum, Penicillium viridicatum, Aspergillus versicolor, Aspergillus niger, Aspergillus flavusand and Stachybotrys chartarum.

image of Aspergillus niger

Aspergillus niger

3. Mushrooms

The term "mushroom" is derived from the French word ‘mousseron’ meaning moss. It belongs to the order Agaricales of Phylum Basidiomycota. There are different types of mushroom, among them, some are edible, some may be poisonous, or unpalatable. Mushroom is also known as toadstool, which grows on soil and trees. They have fleshy, spore-bearing, and umbrella-shaped fruiting bodies. On the planet, mushrooms are one of the most health-promoting superfoods which have more than 100 different beneficial effects on health. It is used for improving overall health by preventing and treating serious health conditions.

image of Edible mushrooms

Edible Mushromms

Mushrooms offer commercial, aesthetic, and ecological values. Edible mushroom varieties are tasty, and they supply vitamin B and minerals such as phosphorus, potassium, iron, and selenium with low calories. Some mushrooms show diverse forms and colors and exhibit wondrous nature for humans.

Mushrooms make an important source of nutrition and energy in terrestrial food chains because many animals like rodents and birds also eat mushrooms. Besides these, mushrooms also contain toxic substances, which can cause human sickness or even death. The most deadly mushrooms are the angel and the death cap, which belong to the genus Amanita.

Economic Roles of Fungi

Beneficial Roles of Fungi

Fungi show the following both ecological and economic roles:

  • Fungi play an important role in our environment. They are used in the food factory for the production of bakeries and breweries. They are also used in food processing and in pharmaceutical industries.
  • Fungi also exhibit a symbiotic relationship with a large range of organisms. The symbiotic relationship helps in the absorption and retention of moisture. Some fungi live in the roots of some plants, which help the plants to take up nutrients from the soil. This relationship is known as a mycorrhizal association.
  • Fungi play an important role in to recycle nutrients and decompose of plant debris.
  • Some fungi inhabit the soil that has a beneficial effect on commercial agriculture. In this case, fungi help to increase soil fertility.
  • Saprophytic fungi inhabit in acidic soils which decay and decompose dead plants and their waste.
  • They break down organic materials and continue the recycles of nutrients through the ecosystem.
  • Symbiotic fungus inhabits in the roots of the many vascular plants and supplies essential nutrients.
  • Many fungi species, such as Agaricus volvarella, Morchella, are used as human food.
  • Many fungi species such as Penicilium notatum, Penicilium chrysogenum are used to produce the antibiotic drug, penicillin. The anti-fungal drug is also produced from Penicillium griseofulvum. Some fungi are used in the production of vitamins such as riboflavin, and various important drugs such as ergotamine and cortisone.
  • Many fungi species take part in the elimination of various diseases like malaria. Some fungi species attack harmful insects that can cause crop disease, and in such a way, they play an important role in the enrichment of the economy.
  • Many fungi species (such as Ascherronia deyroides, Beauveria bassiana, Isara ferinosa, Empusa sepulchralis) act as bio-controlling agents. In this case, they are used for controlling insect pests of crops.
  • The yeasts are used as important model organisms for studying problems in molecular biology and genetics. Many varieties of Saccharomyces cerevisiae, such as AH109, PJ69-4 alpha, Y187, etc. are used for higher research in molecular biology.
  • Many fungal species such as Aspergillus, Penicillium, Saccharomyces are used extensively for producing important industrial products such as gluconic, citric, formic acid, lactic, and malic acids.
  • Some fungi are used to produce certain types of cheese and soft drinks.
  • Saccharomyces cerevisiae is known as baker`s yeast. It is used to make commercially baked bread, cakes, etc.
  • Alcoholic beverage products are also produced from Saccharomyces sp. through fermentation.
  • Blue cheeses like Roquefort, gorgonzola, Danish blue, etc are produced from Penicilium camemberti, and Penicilium rosqueferti.
  • Fungi also have a great source of food. They are a rich source of protein and vitamins with excellent flavor. Besides these, some fungi (mushrooms) are deadly poisonous and cause even death.
  • Many fungi species act as a good source of enzymes like lipase, amylase, cellulases, invertages, proteases, etc. Besides these, they provide an important source of plant hormones like Gibberella fujikuroi

Harmful Roles of Fungi

  • Many fungi species cause diseases in humans. Some fungal species such as Aspergillus niger, Aspergillus fumigates act as common human pathogens, and they cause diseases like aspergillosis of the throat, bronchi of lungs, and well-known skin disease "ringworm". Some species produce mycotoxins and affect human health in various ways.
  • Some species of fungus such as Aspergillus, Mucor, Rhizopus, etc. can cause domestic animal diseases.
  • Some fungi affect commercial agriculture and cause different diseases both in crops and animals, which results in economic losses. The most common and significant crop diseases are: potato blight, Downy mildews of grapes, Ergot disease of rye, Apple scab, Rust diseases, Blackarm, wilt and root rot of cotton, rusts, root and stem rots, brown leaf spot disease of rice, etc.
  • Fungi also release toxic substances that destroy the plasma membrane of the liver cells and also cause kidney and intestine diseases.
  • Many species of fungi spoil food materials if the food does not properly store. In this case, Aspergillus, Rhizopus, Mucor, yeasts, Penicillium, etc. damage many food items, such as fruit, fruit-derived food, cooked food, bread, fish and meats, sweetmeats, etc.
  • Many personal commodities such as paper, books, wearing clothes, leather goods, camera, microscope, television, radio, electric wearing, wood, and furniture, etc. are destroyed by many fungal species.

Concluding Remarks

Fungi are the eukaryotic organisms that have a great role in our economy. They have both beneficial and harmful effects in our daily life. They take part in the decomposing of organic material to form the humus. They also involve in the recycling process and release nutritional elements in the soils, which are essential for the growth of plants. Besides these, many fungi are used as a research tool in the study of fundamental biological processes.

Denitrifying Bacteria

Nitrogen is the necessary component of life, which is essential to all life-forms. The naturally available environmental nitrogen is inorganic, which convert into organic compounds like ammonia by the presence of nitrogen-fixing organisms. These natural nitrogenous products are taken up by plants and animals. The excess amounts of these products are removed in the form of nitrogenous waste. When nitrogenous waste is not broken down, it leads to depletion of the nitrogen source. In this case, denitrifying bacteria prevent this situation and convert the organic nitrate compounds into its inorganic form. In this way, nitrogen levels in the atmosphere are refilled, and the nitrogen cycle is completed.

There is a diverse group of bacteria that are capable of performing the denitrification process. Over 50 genera with over 125 different species of denitrifying bacteria have been identified, which represent 10-15% of the bacteria population in water, soil, and sediment. Denitrifying bacteria are also known as NRB or nitrate-reducing bacteria, that can convert nitrates in the soil to free atmospheric nitrogen and deplete soil fertility and finally reduce the productivity in the agricultural sector. By using various enzymes, these bacteria can metabolize nitrogenous compounds and turn nitrogen oxides back to nitrogen gas or nitrous oxide (N2O).

You might also read: Endotoxins Vs Exotoxins

Some notable denitrifying bacteria such as Micrococcus denitrificans, Thiobacillus denitrificans, some species of Serratia, Alkaligenes, Bacillus, Achromobacter, Hyphomicrobium, Paracoccus, and several species of Pseudomonas, and others are involved in denitrification process. In this case, Pseudomonas aeruginosa can reduce the amount of fixed nitrogen in the soil by up to 50% under anaerobic conditions.

Broad ranges of inorganic and organic compounds are used by denitrifying bacteria as a source of carbon and energy. These bacteria can be detrimental to agricultural soil and can also produce greenhouse gas, such as a nitrous oxide (N2O). In many cases, denitrifying bacteria are used for removing fixed nitrogen pollutants from ecosystems such as sewage sludge. Denitrifying bacteria can carry out the process of denitrification, which can occur in soil, in freshwater and marine environments.

Some Notable Denitrifying Bacteria

1.Thiobacillus denitrificans

Systematic Position
  • Domain: Bacteria
  • Phylum: Proteobacteria
  • Class: Betaproteobacteria
  • Order: Hydrogenophilales
  • Family: Hydrogenophilaceae
  • Genus: Thiobacillus
  • Species: Thiobacillus denitrificans

Thiobacillus denitrificans is a gram-negative, short rod-shaped bacterium that belongs to the family Hydrogenophilaceae under the phylum Proteobacteria. It is an obligately chemolithoautotrophic bacterium that grows as a facultatively anaerobic chemolithotroph.

This species is widely distributed in both soil and water habitats. Generally, it is found in mud, marine and freshwater sediments, sewage, digestion tanks, sewage lagoons, industrial waste treatment ponds, and even abandoned mines. It is considered as an easily cultivable bacterium, and in artificial habitats, it was first cultured by Beijerinck in 1904. The optimal temperature and pH for growth of this bacterium is 28 - 32 0C and 6.8 - 7.4, respectively.

This bacterium can help the denitrification process under the optimum conditions with a pH of 6.85 and a temperature of 32.8°C. It is one of the best-studied obligately chemolithoautotrophic bacterium that can combine inorganic sulfur-compound oxidation with the process of denitrification. Thiobacillus denitrificans plays a great role in the biogeochemical cycles on a global scale. It also takes part in the nitrate dependent oxidation of FeSx of subsurface ecosystems. Generally, it is often found in the communities of nitrate-rich habitats as a dominant organism.

2. Paracoccus denitrificans

Systematic Position
  • Domain: Bacteria
  • Phylum: Proteobacteria
  • Class: Alphaproteobacteria
  • Order: Rhodobacterales
  • Family: Rhodobacteraceae
  • Genus: Paracoccus
  • Species: Paracoccus denitrificans

Paracoccus denitrificans is a gram-negative, non-motile, and well-known nitrate-reducing coccoid bacterium. It belongs to the family Rhodobacteraceae under order Rhodobacterales of class Alphaproteobacteria. It has a typically rod-shaped body and becomes spherical shape during the stationary phase. The cell has double membranes with a cell wall. It was first isolated by a Dutch microbiologist, Martinus Beijerinckin, in 1910. Formerly, it was known as Micrococcus denitrificans, but D.H. Davis reclassified it to Paracoccus denitrificans in 1969.

This bacterium is found in the soil under both aerobic or anaerobic environments. It is known to be an extremophile because it has the ability to live in many different kinds of media and environments. It is very flexible and obtains energy both from organic (such as methanol and methylamine) and inorganic compounds (such as hydrogen and sulfur).

This bacterium takes part in the denitrification process and reduces the nitrogen fertilizers in agricultural soil. In this case, nitrogen is converted to dinitrogen and produce nitric oxide and nitrous oxide that cause damage to the atmosphere.

3. Pseudomonas denitrificans

Systematic Position
  • Domain: Bacteria
  • Phylum: Protobacteria
  • Class: Gammaproteobacteria
  • Order: Pseudomonadales
  • Family: Pseudomonadaceae
  • Genus: Pseudomonas
  • Species: Pseudomonas denitrificans

Pseudomonas denitrificans is a gram-negative, aerobic, heterotrophic bacterium that takes part in the denitrification process of the nitrogen cycle by which nitrate is reduced into nitrogen gas (N2). It has a rod-shaped body with a polar flagellated cell which can produce vitamin B12. The size of the cell ranges around 1.05 x 0.8 µm, and chemically, it is composed of up to 48% lipids. The suitable growth temperature for Pseudomonas denitrificans is 25°C. It was first separated from garden soil in Vienna, Austria. It has medical and environmental significance, and it has industrial use to synthesize vitamin B12. It also has potential nitrate toxicity or wastewater treatment applications.

This bacterium is also used to produce commercial compounds such as 3-hydroxypropionic acid. It also takes part in the process of denitrification. In many cases, its denitrification capabilities help in wastewater management. It may cause diseases like meningitis in humans, and it may also inhabit the intestines of fish. It is found in a variety of habitats, including soils, surface waters, wastewaters, and also bottom lake sediment. It can also be able to live at low and high levels of oxygen. Generally, it is considered a decomposer, which also acts as an antagonist against the plant fungus (Vertilcillum lateritum ) through the production of anti-fungal metabolites.

Concluding Remarks

Environmental nitrogen is a fundamental natural particle that is found in proteins and nucleic acid. They are a significant piece of every living thing. Denitrification is a natural procedure where nitrate (NO3-) is changed over to nitrogen (N) gases by the activity of denitrifying microscopic organisms. Thus, the decrease of nitrogen happens in the climatic condition. For the most part, denitrification happens when denitrifying microorganisms use nitrate (NO3-) for their breath in the place of oxygen in the air. This procedure happens most quickly in warm, wet soils with plenty of nitrates (NO3-).

Endotoxins Vs Exotoxins

Micro-organisms such as bacteria, fungi produce toxic substances that boost up the infection and diseases through damaging the host tissues and by troubling the immune system. The most potent and known natural bacterial toxin is Botulinum neurotoxin which has essential uses in research and medical science. At present, toxins are used as tools in cellular biology and neurobiology to develop anticancer drugs and other medicines. Toxins from bacteria act as virulence factors that influence the functions of the host cell to help microbial infections. Toxins produced from bacteria can be either endotoxins or exotoxins. These toxins play a pivotal role in leading various diseases and infections.


Endotoxins are lipopolysaccharides toxic substances which are part of the outer membrane of the gram-negative bacteria. Generally, it is released when the bacterium is killed, or cell lyses by the immune system due to action of phagocytic digestion or specific antibiotic actions. It is found in the body of the following bacteria species:

  • Escherichia coli
  • Shigella
  • Salmonella typhi
  • Pseudomonas
  • Haemophilus influenza
  • Neisseria
  • Vibrio cholerae and
  • Bordetella pertussis
image of E coli

Escherichia coli,

image of Clostridium tetani

Clostridium tetani

image of Vibrio chlorae

Vibrio chlorae

image of Salmonella

Salmonella typhi

Image Showing Some Bacteria Species Which Secrete Toxic Substance

Endotoxins have different physical and chemical characteristics which consist of 2 components, namely hetero-polysaccharide and lipid A. Endotoxins are stable heat proteins that make the body wall of gram-negative bacteria. They are one kind of pyrogens that act as fever-causing agents. Fatty acids and disaccharide phosphates are the main components of the endotoxins. It also bears core polysaccharides and O antigen.

During the development period of bacteria, they release small quantities of endotoxins which help to enhance natural immunity. The host body shows a response to endotoxins and makes severe inflammation. Generally, the inflammation is beneficial to the body of the host, but it can lead to sepsis if the inflammation is severe enough.

image of Endotoxin

Image Showing the Effects of Endotoxins and Exotoxins

Some Clinical Features and Significance of Endotoxins

  • It is characterized as lipopolysaccharides (LPS).
  • They are present in the cell wall of bacteria.
  • Endotoxins are released by the gram-negative bacteria during cell lysis.
  • They are heat-stable proteins.
  • They can remain active up to 100 °temperatures.
  • Endotoxins are lethal to the host body when it is released large quantities.
  • They show a non-specific effect on the body of the host.
  • Generally, antibodies cannot neutralize endotoxins.
  • There is no specific receptor for the entrance of endotoxins to the host body.
  • They show a poor antigenic nature.
  • The host body is susceptible to fever when the body gets in touch with the endotoxins.
  • Releasing of endotoxins mainly depend on the genes of the bacterial chromosome.


Exotoxins are one of the most contagious toxic substances, which are secreted by both gram-positive (+) and gram-negative (–) bacteria into the surrounding at the exponential phase of the bacterial cell. They have proteins or polypeptides in nature and show enzymatic or direct action of the host cell. They can cause extreme injury to the host body by damaging the cells or by obstructing the metabolism of cells.

Various bacteria release exotoxins which cause diseases to the host. Many types of exotoxins are identified. Among them, the most common ones are:

  • Botulinum Toxin
  • Enterotoxin
  • Cholera Toxin
  • Diphtheria Toxin
  • Tetanospasmin

Some important health disorders by exotoxins include Tetanus, Cholera, and Diphtheria. Among them, the notable bacteria Clostridium tetani produce tetanus toxins, Vibrio cholerae produces cholera toxins while Cornybacterium diphtheria produces diphtheria toxin. Exotoxins are of three categories, such as Enterotoxins, neurotoxins, and cytotoxins. In this case, Enterotoxemic action is found in the gastrointestinal tract; neurotoxins show their functions on neurons while cytotoxins prohibit the function of the host cell.

image of Exotoxin

Some Clinical Features and Significance of Exotoxins

  • They are characterized as protein in nature.
  • During releasing exotoxins, the bacterial cell does not need to undergo lysis.
  • Exotoxins are heat sensitive, and if you expose them to heat with a temperature beyond 600 C, it becomes destroyed.
  • They have specific effects on the host body, and they enter into the host cell by using specific receptors.
  • They are extremely lethal to host with a small amount.
  • It has a high antigenic nature, and you can neutralize it by using antibiotics.
  • Sometimes, they cause fever to host, and there are available vaccines for neutralizing the action of exotoxins in the market.
  • They are secreted into the surrounding by both gram-positive (+) and gram-negative ( –) bacteria. 

Differences between Endotoxins and Exotoxins

The following table shows the differences between Endotoxins and Exotoxins


Key Features


It is characterized as Lipopolysaccharides (LPS).

Chemical Nature

It is protein in nature.

It is produced by gram-negative bacteria.


It is produced by both gram-positive and gram-negative bacteria.

It releases enzymes like Catalase, IgA / IgG proteases, Fibrolysin, etc.

It releases enzymes like Hyaluronidase, certain protease, Collagenase, Nuclease, Neuraminidase, Phospholipase A, etc.

It is found inside the cell membrane of gram-negative bacteria. It is released after lysis of the cell wall.


t is released into the surrounding by both gram-negative and gram-positive bacteria.

They are heat tolerant and can stable up to 2500 C temperatures for one hour. 

Heat sensitivity

They are heat liable and can get destroyed at a temperature between 60 and 800 C.

It does not produce antitoxins and shows weak immunogenicity.


They show high immunogenicity and secrete antitoxins to neutralize the toxin by stimulating the immune system.

The molecular weight of endotoxin is 50-1000 KDa.

Molecular Weight

The molecular weight of exotoxin is 10 KDa.

It shows poor antigenicity.


It shows high antigenicity.

Its effect is non-specific.


It is specific to particular bacterial strain.

It does not show any enzymatic activity.

Enzyme activity

It always shows enzymatic nature during activities.

Often it can cause fever in the host.

Pyrogenic / Fever

It does not cause fever or occasionally cause fever in the host.

It cannot be denatured on boiling.


It gets denatured on boiling.

The genes are located in the bacterial chromosome which is responsible for producing endotoxins.


The genes are located in the plasmid of the bacteriophage genome, which stimulates to produce exotoxins.

It is detected by using limulus lysate assay test.

Detection of toxins

It is detected by using various methods like ELISA-based method, Precipitation, Neutralization, etc.

For entering into host body, generally, it has no specific receptors.


For entering into the host body, generally, exotoxins use specific receptors.

It becomes lethal only with large amount.

Deadly behavior

It becomes lethal only with very minute quantities.

It does not shows any affinity towards specific tissues of the host.


It shows an affinity towards specific tissues of the host.

It can cause various health disorders such as urinary tract infections, Coronary artery disease, Neonatal Necrotising Enterocolitis, typhoid fever, meningococcal meningitis, Cystic Fibrosis, Crohn’s Disease and Ulcerative Colitis, Meningococcaemia, sepsis, haemorrhagic shock, etc.


It can cause the following health disorders: Diphtheria, Gas gangrene, Antibiotic-associated diarrhea, Scalded skin syndrome, Scarlet fever, Botulism, tetanus, etc.

Generally, antibodies cannot neutralize the endotoxins.


It can be neutralized using antibodies.

There are no available vaccines against endotoxins.


There are available vaccines against exotoxins.

It does not convert to toxoids.


It converts to toxoids.

Escherichia coli, Salmonella typhi, Shigella, Psedomonas, etc.


Bacillus anthracis, Bacillus cereus, Staphylococcus aureus, Streptococcus pyogenes, Vibrio cholerae, etc.

Concluding remarks

There are numerous bacterial toxins which hamper the host cells in different manners. They inhibit protein synthesis, destroy the host cells, and cause various health disorders. Toxins delivered from microscopic organisms can be either endotoxins or exotoxins. Endotoxins upgrade or restrain the pathogenicity of disease. Exotoxins are protein in nature which can also damage the host cell membranes and interfere with the host cell functions. Exotoxins become deadly with limited quantity while endotoxins are less deadly than exotoxins, yet they can make fever the host.

Virus Structure: Tobacco Mosaic Virus (TMV) and Bacteriophase (T2 Virus)

A virus is an infectious biological agent of small size.    The name ‘Virus’ comes from the Latin word meaning ‘poison’ or ‘slimy liquid’. The size of the viruses ranges from 20-300 nm. The chemical composition of all true viruses is nucleic acid either DNA or RNA and protein. They cannot multiply and continue the metabolic process without a host cell. About 5000 species of viruses have been identified. The shape of viruses varies from simple helical and icosahedral to more complex. They can cause diseases in man, plants and other animals.

External structure of Virus

Externally, the virus may be spherical, rod-shaped, cuboidal or tadpole-shaped.

Internal structure of Virus

Entire virus is called viron and consists of nucleic acid which is surrounded by a protective coat of protein called a capsid. An electron microscope shows that a virus particle is made up of two parts, an inner core of nucleic acid and the outer coat of protein. The nucleic acid component is either RNA or DNA, which lies centrally and surrounded by a membrane made up of protein.

The two main parts of the virus body are:

  • Capsid or outer membrane
  • Nucleoid- the parts lying within the capsid
image of Virus Structure

Structure of Virus

Structure of Capsid: The protein coat of a virus particle is called the capsid. The membrane is composed of a number of identical subunits known as the capsomeres. The nucleic acid core and capsid together form the nucleocapsid. The number of capsomeres, their shape and component differ considerably in various kinds of viruses. The capsid of a virus is physiologically inactive. Capsid protects the nucleic acid from unfavorable conditions and helps the virus to penetrate within the body of the host cell. This part is left outside the host cell at the time of reproduction.

Each capsomere is composed of a few monomers and both of which are attached together by bonds. The virus particles in some cases on the outer side of the capsid are surrounded by means of an envelope. The envelope is composed of viral protein and lipid of the host cell. 

In some virus such as Vaccinia virus, the capsid is composed of carbohydrate, fat, biotin, riboflavin and copper with 80% protein. The capsid of influenza virus possesses starch and fatty substances. Besides this, they possess numerous projected structure or spikes. The spikes are 10 nm long and situated 7.8 nm apart. In the Tobacco mosaic virus, the coat contains carbohydrate and other substances in addition to protein. Each virus capsid possesses a specific number of enzymes so that a virus can enter the host cell.

In some animal viruses, like Herpes virus, pox virus, the envelope situated outside the capsid and they are composed of viral protein and lipid which are derived from the host cell. The existence of lipids keeps the envelope loose and flexible.

The structural units of the envelope of the Lipoviruses are called pelpomeres. Pelpomeres are found in the viruses like Influenza virus, Herpes virus, Mumps virus, etc.

Structure of Nucleoid: The central core of the virus is composed of nucleic acid which is also called nucleoid. This part is covered by means of an outer coat of protein which is also called a capsid.

The nucleic acid is composed is Ribonucleic acid (RNA) and Deoxyribonucleic acid (DNA). Most of the plant viruses possess RNA but cauliflower Mosaic virus contains DNA while bacterial virus or Bacteriophase possesses DNA.   On the other hand, Animal virus contains DNA (pox virus, herpes virus) or RNA (influenza virus, poliovirus).

Structure of Tobacco Mosaic Virus (TMV)

image of Tobacco Mosaic Virus (TMV)

Tobacco Mosaic Virus (TMV)

The virus responsible in causing disease of tobacco plants result in chlorosis of leaves and also alternate patches of yellow and green color in leaves giving mosaic appearance is called Tobacco Mosaic Virus (TMV).

Tobacco Mosaic Virus (TMV) was first isolated in the crystalline state from the sap of infected tobacco plants in 1935. In 1936, N.W. Pirie and F.C. Bawden found out the nucleoprotein nature of Tobacco Mosaic Virus. Studies by A.King and D.L.D. Casper showed the present of sub-units around the nucleic acid core. The structure of TMV was first observed by Takahasi and Rawlins.

Structure of TMV: Structurally TMV is very simple, with a helical hollow rod with approximately 300 nm length and 17 nm in diameter. TMV is composed of an outer protective protein coat and an inner core of single-stranded nucleic acid, RNA. The capsid is made up of capsomeres, arranged in a closely helical or spiral fashion along the entire length of the rod. The capsomeres are in turn composed of 158 amino acids arranged in definite sequence. Each nucleic acid, RNA core contains 400 units (nucleotides). In TMV, about 95% of the molecule is protein and 5% is a nucleic acid,

Contamination: Tobacco Mosaic Virus (TMV) is extremely infectious and stable. Inside the host cell separation of outer protein coat from nucleic acid take place. New particles formed from the host cell have the capability of causing infection again.

Structure of Bacteriophase (T2 virus)  

image of Bacteriophage (T2)

Structure of Bacteriophase (T2 virus) 

Generally, the bacterial virus is called bacteriophage or T2 virus. The term bacteriophage was given by Felix d`Herelle in 1917. The bacteriophage was first successfully isolated by M. Schlesinger in 1933.

The bacteriophage is composed of DNA and protein. DNA consists of one continuous molecule which is about 50 µm long, tightly condensed in the shape head and enveloped with a layer of protein. They have a tadpole shape with hexagonal heads and a long tail.

Body Parts

The body of bacteriophage consists of :

  • Head
  • Neck
  • Tail and
  • Base plate

Head: The head is 6 faced with two protein layers. Inside the hollow cavity of the head lies the double-stranded DNA. The capsid of the head is formed of several capsomeres. Capsid generally possesses a cuboidal symmetry.

Neck: The neck is a tube-like structure which connects with the head and the tail portion. It is surrounded by a circular disc, the collar at the mid part. The posterior end of the neck is projected into the front hole of the tail core.

Tail: It is a tube-like portion which consists of two parts, the inner hollow core and the surrounding outer protein sheath called the tail sheath.

Base plate: The tail end in a base plate bears long slender thread-like tail fibers which help in anchoring the virus to the host cell. The base plate has a pin or spike attached to its lower surface at each corner. The appropriate length and width of a bacteriophage are 200-265 µm and 50-70µm respectively.

Bacteria : Characteristics and Classification

Bacteria (singular-bacterium) are the microscopic unicellular and prokaryotic organisms. It is also known as a microbe. The term bacteria were first coined by F.J. Cohn in 1854.  Bacteria come from the Greek word manning rod. It was discovered by Antonie Von Leeuwenhoek in 1976.  It has a plant like cell-wall and autotrophic mode of nutrition. They are aerobic or anaerobic and found in air, water, soil and living organisms. They possess both plant and animal characters. They do not possess a true nucleus. The branch of science which deals with bacteria is called Bacteriology.

Salient characteristics of bacteria

  • They are unicellular and microscopic organisms whose measuring diameter is less than 3 µm.
  • Body of the bacteria is covered by a cell wall.
  • Most of them are heterotrophic but a few are autotrophic in nature.
  • They have no true nucleus.
  • They do not possess cell organelles like mitochondria, Golgi bodies, endoplasmic reticulum, chloroplasts etc.
  • They can take only liquid food matters.
  • They mainly reproduce by means of binary fission.
  • They are attacked by the bacteriophage virus.
  • Most of the bacteria lead to parasitic life and can cause diseases of other living organisms.

You might also read: Denitrifying Bacteria

Where they occur

They are widely distributed in nature under diverse conditions. They are aerobic or anaerobic and found in air, water, soil, living organisms, dead bodies, and organic matters. Some are thermophilic, some others are psychrophilic while some others are mesophilic. 

Size of bacteria

The size of bacteria varies from species to species. The disease-causing bacteria grow up to 10µ in length and 1 µ in diameter. On the other hand, the diameter of ro- shaped bacteria ranges from 0.2-2µ. The length of the largest bacteria (Bacillus butschilli) is 80µ while the smallest bacteria (Mycoplasma) is 0.1µ in diameter.  

Classification of Bacteria

A. Cohn (1872) classified the bacteria into four groups on the basis of the shape such as:

  • Coccus
  • Bacillust
  • Spirillum
  • Vibrio or Comma

1. Coccus 

This type of bacteria is unicellular and spherical or oval shaped. They are of the following types:

(1) Micrococcus: It is also known as monococcus which belongs to the family Micrococcaceae. It is widely distributed in nature. They are gram-positive bacteria which range from 0.5-3.5 in diameter. They lie individually or separately such as Micrococcus flavus.

(2) Diplococcus: Diplococcus is a round shaped bacterium which typically lies in the form of two cocci together. The word "diplococcus" comes from 'Diplo' meaning double and 'coccus' meaning spherical, ovoid or round shape. They are a gram negative or gram positive bacteria and can cause many diseases to the human body such as Diplococcus pneumonia.

(3) Streptococcus: It is a gram-positive bacterium which belongs to the family Streptococcaceae under order Lactobacillales.  They occur with many bacteria united together like a chain. They are spheroidal bacteria and contain large numbers of species. Among them, some can cause disease in humans and other animals. Some important diseases caused by streptococcus are rheumatic fever, impetigo, scarlet fever, tonsillitis, puerperal fever, strep throat, and other upper respiratory infections.

(4)Staphylococcus: It is a gram-positive bacterium which belongs to the family Staphylococcaceae. They are round shaped bacteria which are frequently found in the upper respiratory tract and on the skin of humans. They occur many united together like a bunch of grapes. Streptococcus aureus can cause lots of disease to humans such as pneumonia,   endocarditis, toxic shock syndrome, bacteremia, osteomyelitis, meningitis, sepsis etc.

(5) Sarcina: It is a gram-positive bacterium which belongs to family Clostridiaceae. When many bacteria occur together and form a cuboidal shaped structure, then it is called Sarcina. It is found in the skin, stomach, large intestine of human, rabbits and guinea pigs. Some are found in soil. 

2. Bacillus

These types of bacteria (Genus Bacillus) looks rod-like and their body contains one or many flagella. They are gram-positive and can survive aerobic and anaerobic condition. They are frequently occurred in chains and found in water and soil. The largest known Bacillus species is Bacillus megaterium which is about 1.5 µm by 4 µm long. Among Bacillus species, some are harmful to humans, plants or other organisms such as Bacillus cereus which can cause spoilage in canned foods, food poisoning and Bacillus anthracis cause anthrax; Bacillus typhi causes typhoid disease.   Some Bacillus bacteria are used to produce antibiotics such as Bacillus subtilis.

3. Spirillum

The word "spirillum" comes from the Greek word 'speria' meaning spiral. They are gram-negative and have a rigid spiral shape or corkscrew-like body which belongs to the family Spirillaceae.  This type of bacteria bears tufts of whiplike flagella at each end. The larger spirillum species is Spirillum volutans which is 5-8 µm across by 60 µm long. Some species of Spirillum such as Spirillum minus is found in the blood of mice and rats and some species are free-living which are frequently found in water such as Aquaspirillum and Oceanospirillum spp.

4. Vibrio or Comma

Vibrio (Genus Vibrio) is a comma-shaped gram-negative bacteria which belongs to the family Vibrionaceae. They are facultative aerobic bacteria which contains one to three whip-like flagella at each end of the body. This type of bacteria grows 0.5 µm across 1.5 to 3.0µm long. O. F. Müller (1773) described eight species of Vibrio, among them, three species are spiraliform. Vibrios are generally aquatic and cause serious diseases to humans and other animals such as Vibrio cholerae causing cholera disease in humans.

image of Monococcus


image of Diplococcus


image of Streptococcus


image of Staphylococcus


image of Carcina


image of Baccilus


image of Coma or vibriao

Comma or Vibrio

image of Spirillum


B. Bacteria are also classified into five types on the basis of presence and absence of flagella:

(1) Monotrichous: When one flagellum is present at one end of the bacterial cell, then the bacterium is called monotrichous type such as Vibrio cholera.

(2) Lophotrichous: When a bundle of flagella is present in one end of a bacterial cell, then the bacterium is said to be a lophotrichous type such as Pseudomonas fluorescence.

(3) Amphitrichous: When single flagellum is present at both ends of the bacterial cell, then the bacterium is said to be an amphitrichous type such as Spirillum spp.

(4) Peritrichous: When many flagella are present all over the bacterial cell surface, then the bacterium is said to be peritrichous type such as  Escherichia coli, Salmonella typhi 

(5) Atrichous: When bacterium cell does not possess any flagellum, then the bacterium is said to be atrocious type such as Diptheria bacillus.

image of Amphitricous


image of Atrichous


image of Lophotrichous


image of Monotrichous


image of Peritrichous


C. On the basis of gram staining process, bacteria are classified into two types:

(1) Gram-positive Bacteria

Hans Christian Gram (Danish Bacteriologist) discovered a method of staining to distinguish the bacteria using a dye called crystal violet and iodine in 1884.  According to Gram, Gram-positive bacteria are those bacteria which take Gram stain made up of crystal violet and iodine.  These types of bacteria have a cell wall composed of a thick layer of a particular substance called peptidoglycan. The Gram-positive bacteria include Staphylococcus, Streptococcus, Pneumococcus etc. They are more easily effective for antibiotics. Staphylococcus aureus can cause inflammatory diseases such as skin infections, pneumonia, septic arthritis, osteomyelitis, abscesses. and food poisoning etc

(2) Gram-negative Bacteria

Some bacteria do not retain the violet stain, are known as gram-negative bacteria. These types of bacteria are less effective for antibiotics. They cause infections to humans. Escherichia coli can cause of food-borne disease while Vibrio cholera causes cholera. Some gram-negative bacteria can also cause respiratory infections, such as pneumonia, and sexually transmitted diseases such as gonorrhea.

image of E. coli

Escherichia coli (Gram-negative)

image of Vibrio cholerae

Vibrio cholerae (Gram-negative)

image of Staphylococcus

Staphylococcus (Gram-positive)

image of Streptococcus

Streptococcus (Gram-positive)

D. Bacteria are also classified into four groups on the basis of mode of nutrition:

(1) Heterotrophic Bacteria: These types of bacteria use organic compound from a carbon source. They require special nutrients for their growth. Hence, these types of bacteria are also known as fastidious heterotrophs.  They don’t have the ability to fix CO2. In nature, most pathogenic bacteria are the heterotropic type.

(2) Chemo-autotrophic Bacteria: These types of bacteria get energy from the inorganic compounds using the process of oxidation. In this case, they utilize sulfur, ferrous iron and ammonium ion (NH4+) as inorganic compounds.  Generally, bacteria use this energy to make carbohydrate and sugars. These types of bacteria can also live in tremendous environments. Hence, it is also known as extremophiles bacteria.

(3) Paratrophic Bacteria: These types of bacteria cannot grow on synthetic nutritional media. They need media containing nutrition for their growth such as blood, ascitic fluid, protein hydrolysates, or other such substances. Generally, these bacteria are pathogenic and cause diseases to man and animals.

(4) Photo-autotrophic Bacteria: These types of bacteria (such as cyanobacteria) can make their own food by the process of photosynthesis using sunlight. Hence, they are also known as photo-autotrophic bacteria. These types of bacteria can survive in all types of environments due to their varying mode of nutrition.  

E. Bacteria are also classified into the following types on the basis of the optimum temperature for growth.

(1) Psychrophilic Bacteria: Psychrophilic bacteria are those bacteria which can grow at 0°C or below temperature. Their suitable temperature for growth is 59° F (15° C).  They can also live at the temperature of 20°C. Cell membranes of these types of bacteria have poly-unsaturated fatty acids which help to live them at a lower temperature. Examples of these bacteria are Polaromonas vaculata, Vibrio psychroerythrus, Vibrio marinus etc.

(2) Psychrotrophic Bacteria:  These types of bacteria are known as extremophilic bacteria which can capable of growth and reproduction in low temperatures. Their growth temperature varies from −20 °C to +10 °C but the optimum temperature ranges between 20 and 30°C. They are widespread in natural environments and foods. They prefer to live in permanently cold places such as polar region.  They can spoil the refrigerated foods.

(3) Mesophilic Bacteria: Mesophilic bacteria are those bacteria which can live and thrive in medium or moderate temperature that should be varied from 20-45 °C. Their optimum temperature for growth is 37°C.  These type of bacteria very harmful which can cause chronic diseases to humans such as Escherichia coli, Salmonella typhi etc.  

(4) Thermophilic Bacteria: Thermophilic bacteria are those bacteria which can live and thrive at comparatively high temperature. Generally, they are heat-loving bacteria whose optimum growth temperature is 50 degree Celsius or more, but they can live at a minimum of about 20 degree Celsius temperature and a maximum of up to 122 degree Celsius or more. Cell membranes of these bacteria contain saturated fatty acids which help them to thrive at a higher temperature. Examples of these bacteria are Streptococcus thermophiles, Bacillus stearothermophilus, Thermus aquaticus etc.

5. Hyperthermophilic Bacteria: Hyperthermophilic bacteria are those bacteria which can live and thrive at a higher temperature. Their optimum temperature for growth is above 80 0C. They are able to thrive in such extreme temperature because they have monolayer cell membranes with some enzymes which help to grow them at a higher temperature. Examples of these bacteria are Aquifex, Pyrolobus fumari, Thermotoga etc.

F. Bacteria are of three types on the basis of optimum pH for growth

1. Acidophilic Bacteria: These types of bacteria can thrive under highly acidic conditions.  The optimum pH for growth is usually 2.0 or below because cytoplasm of these bacteria is acidic in nature. Some acidophilic bacteria are thermophilic in nature; hence they are called thermoacidothilic bacteria. Example of this bacteria is Thiobacillus acidophilus. 

2. Alkaliphilic Bacteria: Alkaliphilic bacteria typically grow up to pH values as high as 12-13. Their optimum growth pH is 9.0.  They are of three types such as obligate alkaliphilic ( they require high pH), facultative alkaliphilic (they require high pH but they can survive at normal pH) and haloalkaliphilic (they require high salt content pH). Examples of alkaliphilic bacteria are Bacillus, Micrococcus, Pseudomonas, Streptomyces etc.

3. Neutrophilic Bacteria: Neutrophilic bacteria can thrive in a neutral pH environment that should be between 6.5 and 7.5. Examples of neutrophilic bacteria are Escherichia coli, staphylococci, Salmonella spp  etc.

G. Bacteria are of two types on the basis of salt requirement

1. Halophilic Bacteria: They are salt-loving bacteria which need high concentrated salt (NaCl) for growth. The cell membranes of these bacteria contain glycoprotein with glutamic acid and aspartic acids which help them to thrive in high concentrated  NaCl. Example of halophilic bacteria is Halobacterium, Halococcus etc.

2. Halotolerant bacteria: These types of bacteria can grow under saline conditions but they do not require salt (NaCl) for growth. They are both Gram-positive bacteria and Gram-negative bacteria such as  Staphylococcus, Micrococcus, Bacillus,  Pseudomonas etc.

H. Bacteria are also classified on the basis of gaseous requirement

1. Obligate aerobic bacteria: Obligate aerobic bacteria are those bacteria which require oxygen to grow. Their energy production and respiration depend on the oxygen. Examples of obligate aerobic bacteria are Mycobacterium tuberculosis, Nocardia asteroides, Bacillus subtilis, Pseudomonas aeruginosa, Mycobacterium tuberculosis, Acidithiobacillus ferrooxidans etc.

2. Facultative anaerobic Bacteria: Facultative anaerobic bacteria can live and thrive in the presence of oxygen. Generally, they use oxygen for aerobic respiration but they also survive in the absence of oxygen during fermentation process. Examples of facultative anaerobic bacteria are Escherichia coli and Salmonella aureus.

3. Obligate anaerobic bacteria: Obligate anaerobic bacteria can only survive in the surroundings which lack oxygen. They survive in the absence of oxygen with the help of the fermentation process. These types of bacteria are very harmful because they cause many diseases to humans.  Examples of this bacteria are Peptococcus, Clostridium etc

4. Aerotolerant anaerobic Bacteria:  These types of bacteria do not necessitate O2 for growth. They can also survive in the presence of O2. They use fermentation to produce energy.  Example of an aerotolerant anaerobic bacterium is Streptococcus mutans.

5. Microaerophilic Bacteria: Microaerophilic bacteria require oxygen to survive. They can also survive lower levels of oxygen. Example of this bacteria is Campylobacter sp

6. Capnophilic Bacteria:  The term Canophilic comes from Gr. kapnos, smoke, and  philein, to love. They are CO2 loving bacteria and grow best in an atmosphere containing CO2. Examples of canophilic bacteria are   Helicobacter pylori, Brucella abortus etc.

You might also read: Economic Importance of Bacteria

Virus : Definition, Characteristics and Classification

For proper survival of the plants and animals, they have to struggle against the environment, their different factors and other living organisms. Viruses are microscopic organisms and can be seen only under electron microscope.In nature, any living organism is affected by virus, then it will take a serious turn. Viruses are mysterious biological agents which do not show any sign of life in free-state and are seen to remain as non-living things. There are million species of virus in nature, among them, about 5000 species have been described in details. The term virus is derived from a Latin word, meaning slimy liquid or poison. This term was first given by  Dutch microbiologist Martinus Beijerinck. The branch of science which deals with the virus is called Virology and the specialists of this branch are designated as Virologist.

image of TMV

Structure of Tobacco Mosaic Virus (TMV)

Definition of Virus

Viruses are biological agents as they exhibit characters of both non-living and living organisms. Some definitions of viruses are given below:

  • Viruses are ultra-microscopic disease causing nucleo-protein agents, capable of being introduced into the living cells of specific organisms and capable of multiplying or being multiplied within the living host cells.
  • Virus is a kind of micro-organism of at least less than 200 mµ in size, lives parasitically on a definite host (Bawden, 1949).
  • Viruses are some ultra-microscopic entities, which multiply only after entering the cell of a specific living organism (Luria, 1953).
  • Viruses are ultra-microscopic , filterable ( through bacterial filter), acellular, antigenic, obligatory parasitic, infectious, nucleo-protein particles which have the power to multiply only within the specific living host cell, lies in between inanimate and animate objects.

Salient Features of Virus

  • Viruses are only visible under electron microscope, a kind of organism lies in between living and non-living objects.
  • Viruses show their existence in water, land and air.
  • As they are ultra microscopic particles, hence they can be filtered through the bacteria proof micro-filter easily.
  • Cytoplasm is absent within the body of viruses, hence it is non-cellular.
  • Its body is composed of nucleo-protein.
  • They are very infectious obligatory parasites.
  • The disease producing virus particles can only multiply within living organisms.
  • They do not show any sign of life in free state and are seen to remain as non-living entities.
  • Any metabolic activities within the virus body are not visible.
  • As sunlight has no direct influence over the viruses, hence they can resist a very high temperature.
  • Viruses can be transferred from an infected body to healthy plants, animals or human beings.
  • Virus contains either one type nucleic acid-DNA or RNA. Plant viruses are generally RNA viruses and animal viruses are generally DNA viruses.
  • Virus reproduces only by the replication of gene or chromosome.
  • Living characters of viruses are reproduction and mutability.

Nature of Virus

  • Virus which lies between living and non-living objects, possess a high molecular mass of protein particles.
  • They do not show any sign of life in free state and are seen to remain as non-living things.
  • Nucleo-protein is the important element of the virus body, which is composed of nucleic acid and protein.
  • Virus lives on the host cell totally as an obligate parasite.
  • Virus can multiply and reproduce as soon as they enter within the host cells, when they are considered to be living.
  • They reproduce by replication of gene or chromosome.
  • Viruses are indeed smallest, non-cellular, non-cytoplasmic organisms which can pass easily through bacteria proof filter.
  • Viruses liberate their own protein membrane during infection outside the host cell and pass only the nucleic acid (DNA and RNA) within the host cell for reproduction.
  • Their living characters are reproduction and mutability.
  • Viruses have two phases in their life cycle- intracellular and extracellular, which are noted within the host cell and outside the body of the host.

Where They Occur

  • The existence of viruses is noted in aquatic, terrestrial and aerial conditions.
  • Virus lives generally within the alimentary tract of human being and other animals.
  • The existence of virus is also noted in different vegetables, fruits, milk and different types of foods and drinks.
  • The presence of viruses is also noted within the residual product, urine, saliva, etc of a viral attacked patient.

Characteristics of Virus

Virus shows the both living and non-living characteristics.

Living characteristics are:

  • Virus body contains DNA or RNA and protein.
  • They can reproduce within the host body.
  • They exhibit mutability and can infect other living organisms.

The non-living characteristics are:

  • Virus molecules can be converted into crystals.
  • The body is not covered by cell membrane.
  • Virus body does not contain cytoplasm.
  • They have no power of locomotion.
  • They can`t respond to external stimuli.
  • Virus body does not show any metabolic reactions.
  • They remain inactive outside of the host body.

Size of virus

Virus particles are ultra-microscopic entities that can be seen only under electron microscope. They are very small and vary in size appreciably. The average diameter of a virus lies between 8-280 mµ. The foot and mouth disease virus of cattle is the smallest known virus which is nearly 8-12 mµ in size and the larger sized virus particles are noted in the vaccinia and Variola viruses (280-300 mµ). 

The size of the tobacco leaf mosaic virus is 17 mµ. On the other hand, parrot fever virus (Psittacosis) Chlamydia psittaci is the largest among the viruses, which is a large as 450 mµ in diameter. According to Salle (1974), these are not virus. They can easily seen under a normal compound microscope. In this case, the larges virus will be Lymphogranuloma venereum (size: 300-400 mµ).

Shape of Virus

Virus body shows four different shapes such as spherical, rod-shaped, cuboidal and tadpole-like structure.

Spherical shaped virus: These type of virus are more or less round, like small golf ball such as influenza virus, Polio virus, Encephalitis virus, Tumor virus, etc. They show 18-150 mµ in diameter.

Rod shaped virus: This type of virus looks like small rod such as Tobacco mosaic virus (TMV), Mumps virus etc. They can grow 800 mµ in length and 15 mµ in diameter.

Cuboidal shaped virus: This type of virus is cubic shaped such as small pox viruses like Vaccinia and Variola, Canary pox, Herpes etc. Their size ranges between 210-305 mµ.

Tadpole shaped virus: This type of virus looks like spermatozoa or tadpole such as bacteriophase virus. They have head and tail in their body. The head measures 47-104 mµ and the tail measures 10-225 mµ.

image of measles virus

Measles Virus

image of polio virus

Polio Virus

image of influenza virus

Influenza Virus

image of bacteriophase


Classification of Virus

On the basis of nucleic acid, virus is divided into two types such as:

  • Deoxyvirus: They compose of Deoxyribonucleic acid or DNA.
  • Ribovirus: This type of virus composes of Ribonucleic acid or RNA.

On the basis of the virus host, they can be divided into:

  • Plant virus: This type of virus can reproduce within the plant body. They can cause plant diseases. Plant viruses include Tobacco Mosaic Virus(TMV), Bean Mosaic Virus(BMV), Pea Mosaic Virus (PMV), Cauliflower Mosaic Virus (CMV).
  • Animal Virus: This type of virus reproduces within the animal body and can cause diseases of animal. Some animal viruses are influenza virus, Polio virus (Polio myelities), Measles virus, small pox virus (variola, Vaccinia).
  • Bacterial virus: This type of virus attacks and multiply within the body of bacteria, hence it is called bacteriophase.

On the basis on Genome structure, virus is classified into the following types:

Virus name



Retroviruses, Rabies virus


Smallpox virus, Herpesviruses


Retroviruses, Rabies virus


Herpesviruses, smallpox virus


Herpesviruses, Rabies virus, retroviruses, smallpox virus


Many bacteriophages, Papillomaviruses

Genome with a single segment

Parainfluenza viruses

Genome with multiple segments

Influenza viruses

On the basis on Capsid Structure, virus is classified into the following types:

Virus Name


Naked icosahedral

Hepatitis A virus, polioviruses

Naked helical

Tobacco mosaic virus (TMV)

Enveloped icosahedral

Yellow fever virus, Epstein-Barr virus, rubella virus,  herpes simplex virus,  HIV-1

Enveloped helical

Rabies virus, Influenza viruses, measles virus, mumps virus

Complex with many proteins; some have combinations of icosahedral and helical capsid structures

Hepatitis B virus, T4 bacteriophage, Herpesviruses, Smallpox virus

According to David Baltimore Virus is  classified into following categories: 

Virus Categories


Virus Examples

Group I

Double-stranded DNA

Herpesvirus  (Herpes simplex)

Group II

Single-stranded DNA

Parvovirus (Canine parvovirus)

Group III

Double-stranded RNA

Rotavirus (Childhood gastroenteritis)

Group IV

Positive Signed Single stranded RNA

Picornavirus (Common cold)

Group V

Negative Signed Single stranded RNA

Rhabdovirus (Rabies)

Group VI

Single stranded RNA viruses with Reverse Transcriptase Enzyme

HIV (Human immunodeficiency virus)

Group VII

Double stranded DNA viruses with Reverse Transcriptase Enzyme

Hepadnavirus (Hepatitis B virus)

Advantages of Virus

  • Bacteriophage or T2 Virus is used in carbon cycling;
  • They are used in scientific research in the field of cell biology and molecular biology;
  • They are used as vectors for treatment of various diseases;
  • Virus is used to deliver the gene to target cells.
  • Virus plays important role in scientific researches of gene therapy.

Disadvantages of Virus

  • In nature, there are many harmful viruses which can cause diseases in plants and animals. Some notable human disease causing viruses are HIV, influenza, herpes, hepatitis, Polio virus (Polio myelities), Measles virus, small pox virus (variola, Vaccinia) while in plants, notable viruses are tobacco mosaic virus, Bean Mosaic Virus(BMV), Pea Mosaic Virus (PMV), Cauliflower Mosaic Virus(CMV).
  • Virus also can cause cancer in human and other animals;
  • Virus transmits diseases from person to person;

You may also read: Virus Structure