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.

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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.

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