Classification Of Bacteria

Classification Of Bacteria Overview

  • Classification Of Bacteria are classified and recognised to separate one organism from another and to categorise similar species according to standards relevant to microbiologists or other scientists.
  • Numerous purposes are served by bacterial categorization. Bacteria may be categorised using a wide range of different typing methods as a result of their diversity.
  • Commonly used categorization criteria might be based on:

Morphologic Characteristics

  • Wet-mounted and appropriately stained bacterial cell suspensions can give a plethora of information.
  • The Gram reaction, acid fastness, motility, flagellar arrangement, existence of spores, capsules, and inclusion bodies, as well as the organism’s shape, can be revealed by these simple tests.
  • This information frequently enables the identification of an organism to the genus level or minimises the chance that it belongs to a particular group.

Growth Characteristics

  • The method of growth an organism uses—aerobic, anaerobic, facultative (i.e., in the presence or absence of oxygen) or microaerobic—is a distinguishing characteristic (i.e., in the existence of a partial pressure of oxygen below that of the atmosphere). Ideal atmospheric conditions are critical for the isolation and identification of bacteria, ideal atmospheric conditions are critical.
  • The incubation temperature, pH, necessary nutrients, and antibiotic resistance are all crucial growth evaluations. For instance, the causative agent of diarrhoea, Campylobacter jejuni, thrives at 42° C in the presence of several drugs, but Y. enterocolitica thrives at 4° C better than the majority of other bacteria. Unlike E. coli and the majority of other Enterobacteriaceae, Legionella, Haemophilus, and certain other pathogens may thrive on low-cost media.

Antigens and Phage Susceptibility

  • Cell wall (O), flagellar (H), and capsular (K) antigens are utilised to aid in the classification of particular organisms at the species level, to serotype strains of clinically relevant species for epidemiologic reasons, and to determine serotypes of public health importance.
  • In some cases, serotyping is used to identify strains that are particularly virulent or important for the public’s health, such as with V. cholerae (O1 is the pandemic strain) and E. coli (enterotoxigenic, enteroinvasive, enterohemorrhagic, and enteropathogenic serotypes).
  • Staphylococcus aureus, mycobacteria, P. aeruginosa, V. cholerae, and S. Typhi-related disorders have mostly been monitored epidemiologically using phage typing, which analyses an isolate’s sensitivity pattern to a group of certain bacteriophages. It has also been employed as an epidemiologic strain identifier to determine bacteriocin susceptibility. Phage and bacteriocin typing have mostly been replaced by molecular techniques in recent years.

Biochemical Characteristics

  • The bulk of bacteria are identified and classified based on their responses to a battery of biochemical tests.
  • Certain tests (oxidase, nitrate reduction, amino acid degrading enzymes, fermentation, or carbohydrate utilisation) are often used for a variety of bacterial families, whilst others are exclusive to a particular family, genus, or species (coagulase test for staphylococci, pyrrolidonyl arylamidase test for Gram-positive cocci).

Classification on the Basis of Gram Stain and Bacterial Cell Wall

The Gram stain is the only one of the various classification schemes that has stood the test of time. H.C. Gram made the discovery in 1884, and it’s still a significant and practical method today.

Based on their morphology and unique staining characteristics, it enables a significant fraction of clinically significant bacteria to be categorised as either Gram positive or negative.

In that order, crystal violet, iodine, alcohol destaining, and safranin counterstaining are applied to slides. Gram negative bacteria stain it red, while Gram positive bacteria stain it a blue-purple colour.

It is believed that the variation between the two groups is due to Gram positives possessing a significantly larger peptidoglycan (cell wall). In contrast to Gram-negative bacteria, where crystal violet is readily eluted, iodine and crystal violet precipitate in the thicker cell walls of Gram-positive bacteria and are not eluted by alcohol.

Bacteria can therefore be identified based on their shape and staining characteristics.

Because of the peptidoglycan’s high lipid content, it is lipid-rich and several bacteria, including mycobacteria, cannot be successfully stained. As a result, alternate staining methods (such as Kinyoun or acid fast stain) are employed to benefit from the resistance to destaining following longer initial staining.

Classification of Bacteria on the Basis of Shape

According to their forms, the scientist Cohn divided bacteria into four main categories in 1872, as follows:

A) Cocci:These unicellular, spherical or elliptical-shaped bacteria are one of two species. They can either stay as a single cell or can be grouped together for different layouts. These are what they are:

Monococcus: Also known as micrococcus, they are identified by a single, distinct round Micrococcus flavus, for instance.

Diplococcus: The Diplococcus cell splits into individual cells in a certain plane, and following division, the cells continue to be joined together. An illustration is diplococcus pneumonia.

Streptococcus: In this organism, cells continually divide along the same axis to produce chains of cells. Streptococcus pyogenes, for instance.

Tetracoccus: This microorganism has four spherical cells that are arranged at right angles in two planes. For instance, Gaffkya Tetragena. Staphylococcus: In this organism, the cells are separated into three planes, creating an irregular shape that resembles bunches of grapes. Staphylococcus aureus, for instance.

Sarcina: In this instance, the cells split into three planes but still create an arrangement resembling a cube with eight or sixteen cells and a regular shape. For instance, Sarcina lutea.

B) Bacilli:These rod- or cylindrical-shaped bacteria can exist alone or in pairs. For instance, Bacillus cereus

C) Vibro:The Vibro are a single genus of bacteria that have a curved, comma-shaped form. For instance, Vibro cholerae.

D) Spirilla:These bacteria resemble spirals or springs and have many curved surfaces as well as terminal flagella. Spirillum volutans, for instance.


Actinomycetes When seen in tissue lesions, branching filamentous bacteria known as actinomycetes are said to resemble the sun’s radiating beams (from actis, meaning ray, and mykes, meaning fungus).

Mycoplasmas are bacteria that lack a stable shape due to their lack of a cell wall. They appear as interlaced threads with circular or oval bodies.

Classification of Bacteria on the Basis of Mode of Nutrition

1. Phototrophs

  • Those microorganisms that use light for energy
  • Based on the source of the electron, phototropes are further split into two classes.
  • Photolithotrophs are bacteria that use light to produce energy and use reduced inorganic substances like H2S as an electron source. For example, Chromatium okenii
  • These bacteria utilise organic compounds such as succinate as an electron source and derive their energy from the sun.

2. Chemotrophs

  • These bacteria use chemical substances as fuel.
  • They are unable to perform photosynthesis.
  • Based on the source of the electron, chemotrops are further split into two types.
  • Chemolithotrophs:they use the reduction of inorganic substances like NH3 as an electron source and obtain energy from the oxidation of chemical molecules. Consider Nitrosomonas.
  • Chemoorganotrophs:they employ organic substances like glucose and amino acids as sources of electrons and obtain their energy from chemical components. For example, Pseudomonas pseudoflava

3. Autotrophs

Those microorganisms that produce their own nourishment only from carbon dioxide.

Autotrophs are divided into two types based on the quantity of energy needed to digest carbon dioxide. Examples include photoautotrophs as well as chemoautotrophs.

Photoautotrophs: they absorbed CO2 by using light. On the basis of the electron sources, they are divided into two more classes. In particular, photoorganotropic and photolithotropic autotrophs.

Chemoautotrophs: They absorb CO2 by using chemical energy.

4. Heterotrophs

  • microorganisms that utilise organic compounds as a source of carbon.
  • They are incapable of repairing CO2.
  • The majority of bacteria that are harmful to humans are heterotropic in nature.
  • Certain heterotrops are simple because they have basic dietary needs. However, certain bacteria—known as fastidious heterotrophs—need particular nutrients for their development.

Classification of Bacteria on the Basis of Temperature Requirement

According to how they react to temperature, bacteria may be divided into the primary categories listed below.


Psychrophiles are bacteria. It can survive temperatures as low as 0°C and as high as 20°C, with 15 °C or less being the ideal temperature for growth.

Psychrophiles’ cell membranes contain polyunsaturated fatty acids, which give them a fluid character even at low temperatures.

Examples include Polaromonas vaculata, Psychroflexus, Vibrio psychroerythrus, and Vibrio marinus.

2. Psychrotrops (facultative psychrophiles):

Even at 0°C, certain bacteria may thrive, although 20–30°C is the ideal temperature for growth.

3. Mesophiles

While certain bacteria can grow best at temperatures ranging from 25–40, 37C is the ideal temperature for growth.

Mesophilic organisms make up the majority of human diseases.

  • coli, Salmonella, Klebsiella, and Staphylococci are a few examples.

4. Thermophiles

  • microorganisms that thrive at temperatures higher than 45°C.
  • Those thermophiles known as facultative thermophiles are capable of growing in the mesophilic range.
  • Since they are obligatory thermophiles, true thermophiles are known as stenothermophiles.
  • The cell membrane of thermophiles includes saturated fatty acids, preventing it from becoming overly fluid even at higher temperatures.
  • Examples include Bacillus stearothermophilus, Thermus aquaticus, and Streptococcus thermophiles.

5. Hypethermophiles:

  • those microorganisms whose optimum growth temperature is more than 80°C.
  • Archeobacteria are often hyperthermophiles.
  • Archeobacteria have monolayer cell membranes that are more heat-resistant and have adapted to growing in higher temperatures.
  • Examples include Pyrolobus fumari, Thermotoga, Thermodesulfobacterium, and Aquifex.

Classification of Bacteria on the Basis of Oxygen Requirement

Obligate Aerobes:

  • need oxygen to survive.
  • For instance, Pseudomonas, a typical nosocomial infection.

Facultative Anaerobes:

  • can grow without oxygen yet can use it.
  • They have an intricate array of enzymes.
  • Examples include yeasts, E. coli, Staphylococcus, and several gut bacteria.

Obligate Anaerobes

  • are injured by the presence of harmful forms of oxygen and are unable to use oxygen.
  • For instance, the Clostridium bacteria that cause botulism and tetanus,

Aerotolerant Anaerobes

  • can tolerate the presence of oxygen but cannot utilise it.
  • may disintegrate oxygen in hazardous forms.
  • For instance, Lactobacillus does fermentation in the absence of oxygen.


  • requires oxygen, but only in small amounts.
  • sensitive to harmful oxygen forms.
  • For instance, Campylobacter

Classification of Bacteria on the Basis of pH of Growth

  1. Acidophiles:
  • The pH must be acidic for these bacteria to flourish.
  • These bacteria have an acidic cytoplasm by nature.
  • Thermoacidophiles are a class of acidopiles that are naturally thermophilic.
  • Examples include Thermoplasma, Sulfolobus, Thiobacillus thioxidans, Thiobacillus ferroxidans, and others.
  1. Alkaliphiles:
  • The pH should be alkaline for these bacteria to flourish.
  • The ideal pH for Vibrio cholerae growth is 8.2, for instance.
  1. Neutrophiles
  • At a pH of 7, these bacteria thrive the best (6.5-7.5).
  • At a pH of 7, the majority of bacteria can thrive.
  • Example: E. coli

Classification of Bacteria on the Basis of Osmotic Pressure Requirements


  • require salt concentrations that are moderate to high.
  • The glycoprotein that makes up the cell membrane of halophilic bacteria has a high concentration of negatively charged aspartic and glutamic acids. In order to protect the -ve charge, a sufficient concentration of Na+ ions is needed.
  • 5 percent of the ocean’s water is salt. The majority of these microorganisms are found in the sea.
  • Archeobacteria, Halobacterium, Halococcus

Extreme or Obligate Halophiles:

  • call for extremely high salt concentrations (20 to 30 percent ).
  • Dead sea brine vats are home to bacteria.

Facultative Halophiles:

  • They can withstand up to 2 percent salt or more but do not require high salt concentrations for development.

Classification of Bacteria on the Basis of the Number of Flagella

On the basis of flagella, the bacteria can be classified as:

  1. Atrichos:These bacteria don’t have flagella, says Atrichos. As an example, Corynebacterium diptherae
  2. Monotrichous:One flagellum is linked to one end of the bacterium’s cell, making it monotrichous. For instance, Vibro cholerae.
  3. Lophotrichous:The bacterial cell has a cluster of flagella connected to one end. For instance, Pseudomonas
  4. Amphitrichous:a group of flagella that emerge from the bacterium’s two ends. Rhodospirillum rubrum, as an example.
  5. Peritrichous:Flagella are equally spaced all over the bacterial cell in a peritrichous state. Bacillus, for instance.

Classification of Bacteria on the basis of Spore Formation 

1. Spore-forming bacteria:

microorganisms that, when conditions are not favorable, they generate spores.

These are further divided into two groups:

i) Bacteria that create endospores:The spore is formed inside the bacterial cell.

Examples. Sporosarcina, Bacillus, Clostridium, etc.

ii) Bacteria that create exospores:Spores are produced outside of the cell.

For instance, methylosinus

2. Non-sporing bacteria

those microorganisms that don’t make spores

Salmonella, E. coli, etc.


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