Classification of Bacteria: on the Basis of Morphology, Gram Staining, Oxygen Requirement & Important Groups

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Classification of bacteria is a complex and evolving field that involves categorizing bacteria into different groups based on various criteria, including morphology, Gram staining, oxygen requirement, and other factors.

Microorganisms are very small microscopic structures that are capable of free living. Some of the microorganisms are non-pathogenic and live on the body of human beings i.e. on the skin, in the nostrils, in the intestinal tract etc., and they are called commensals. The organisms that are capable of causing disease are called pathogenic organisms. These are two groups depending upon the structure of cells:


This group includes those organisms that have a very simple cell structure and nuclear material is in the form of single chromosomes but is not surrounded by a nuclear membrane. They divide by simple binary fission. Example are bacteria, Mycoplasma, Chlamydia and Rickettsiae.


These organisms have complete cell structure similar to higher organisms. The nuclear material is bounded by a nuclear membrane to form a nucleus. They have more than on chromosomes, complete enzyme system of their own and divide by mitosis. Examples are fungi and protozoa.


Bacteria can be classified depending upon:

  • Morphology
  • Gram Staining
  • Requirement of Oxygen
  • DNA Homology


On the basis of morphology bacteria are divided into the following groups:

  1. Cocci: Spherical-shaped bacteria.
  2. Bacilli: Rod-shaped bacteria.
  3. Spirilla: Spiral or helical-shaped bacteria.

Intermediate shape like cocco-bacilli also exist.MORPHOLOGICAL CLASSIFICATION


Bacteria are classified based on their response to the Gram staining technique, which helps differentiate them into two main categories: Gram-positive and Gram-negative bacteria. This classification is fundamental in microbiology and has implications for bacterial cell wall structure, as well as their response to antibiotics and various aspects of their biology.

  1. Gram-Positive Bacteria:

    • Gram Stain Result: When subjected to the Gram staining procedure, Gram-positive bacteria retain the crystal violet stain, appearing purple or blue under a microscope.
    • Cell Wall Structure: Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet stain. They lack the outer membrane seen in Gram-negative bacteria.
    • Examples: Staphylococcus, Streptococcus, Clostridium, Bacillus, and Listeria are common Gram-positive bacteria.
  2. Gram-Negative Bacteria:

    • Gram Stain Result: Gram-negative bacteria do not retain the crystal violet stain and appear pink or red under the microscope.
    • Cell Wall Structure: Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that contains lipopolysaccharides. This outer membrane makes them more resistant to some antibiotics and contributes to their distinctive staining pattern.
    • Examples: Escherichia coli, Salmonella, Pseudomonas, and Helicobacter pylori are examples of Gram-negative bacteria.


  1. Aerobic Bacteria:

    • Oxygen Requirement: Aerobic bacteria require oxygen for growth and metabolism. They use oxygen as the final electron acceptor in their respiratory chain, allowing them to generate energy efficiently through aerobic respiration.
    • Examples: Common examples of aerobic bacteria include Mycobacterium tuberculosis, Mycobacterium leprae, and Nocardia.
  2. Anaerobic Bacteria:

    • Oxygen Requirement: Anaerobic bacteria do not require oxygen for growth and may even be harmed by exposure to it. They utilize alternative electron acceptors, such as nitrate or sulfate, for their respiratory processes.
    • Examples: Clostridium species (e.g., Clostridium botulinum, Clostridium tetani) are examples of anaerobic bacteria.
  3. Facultative Anaerobes:

    • Oxygen Requirement: Facultative anaerobes can grow in the presence or absence of oxygen. They possess the metabolic flexibility to switch between aerobic and anaerobic respiration, depending on oxygen availability.
    • Examples: Escherichia coli and many Enterobacteriaceae are facultative anaerobes.
  4. Microaerophiles:

    • Oxygen Requirement: Microaerophiles require low levels of oxygen for growth. They can grow in an environment with reduced oxygen tension but are inhibited by high oxygen concentrations.
    • Examples: Helicobacter pylori is a microaerophile commonly found in the stomach.
  5. Aerotolerant Anaerobes:

    • Oxygen Requirement: Aerotolerant anaerobes can tolerate the presence of oxygen but do not use it for respiration. They typically generate energy through fermentation.
    • Examples: Lactic acid bacteria, such as Lactobacillus, are aerotolerant anaerobes.


Bacteria can be classified based on their temperature requirements for optimal growth. This classification is related to their adaptation to temperature ranges and is often divided into the following categories:

  1. Psychrophiles: Psychrophilic bacteria are adapted to cold temperatures and thrive at or below 20°C (68°F). They are commonly found in polar regions and deep-sea environments, where low temperatures prevail.

  2. Mesophiles: Mesophilic bacteria are adapted to moderate temperatures and grow optimally between 20°C (68°F) and 45°C (113°F). Most bacteria, including many human pathogens, fall into this category. Mesophiles are often associated with environments like the human body.

  3. Thermophiles: Thermophilic bacteria are adapted to high temperatures and have an optimal growth range between 45°C (113°F) and 80°C (176°F). They are commonly found in geothermal environments, such as hot springs and deep-sea hydrothermal vents.

  4. Hyperthermophiles: Hyperthermophilic bacteria can withstand extremely high temperatures and grow optimally above 80°C (176°F). These bacteria are often found in extreme environments like volcanic vents and hydrothermal systems.


  1. Pathogenic Bacteria: Bacteria that can cause diseases in humans and animals.
  2. Non-Pathogenic Bacteria: Bacteria that are not associated with causing diseases.


  1. Extremophiles: Bacteria that thrive in extreme conditions (e.g., thermophiles, acidophiles).
  2. Nitrogen-Fixing Bacteria: Convert atmospheric nitrogen into a form usable by plants.
  3. Photosynthetic Bacteria: Bacteria that can perform photosynthesis, such as cyanobacteria.
  4. Methanogens: Bacteria that produce methane as a metabolic byproduct.
  5. Halophiles: Bacteria that thrive in high-salt environments.


Bacteria can be classified based on the presence or absence of endospores, which are highly resistant, dormant structures that some bacterial species can form under adverse environmental conditions. Endospore formation is a survival strategy, allowing certain bacteria to withstand extreme conditions and remain viable for extended periods. Based on endospore formation, bacteria are categorized into two primary groups:

  1. Spore-Forming Bacteria:

    • Endospore Formation: Spore-forming bacteria have the capability to form endospores, which are highly resistant structures containing the bacterial genome and some cellular components.
    • Examples: The most well-known spore-forming bacterium is Clostridium botulinum, which can produce the deadly botulinum toxin. Other examples include Clostridium perfringens, Clostridium difficile, and Bacillus anthracis (causing anthrax).
    • Exospores Formation: Exospores are formed by certain filamentous bacteria, particularly in the genera Streptomyces and related actinomycetes. These exospores are involved in the reproductive cycle of these bacteria.
  2. Non-Spore-Forming Bacteria:

    • Endospore Absence: Non-spore-forming bacteria lack the ability to produce endospores. They do not form these highly resistant structures and are more susceptible to adverse environmental conditions.
    • Examples: The majority of bacterial species fall into the non-spore-forming category, including Escherichia coli, Salmonella, Streptococcus, and many others.


1. Gram Positive Cocci

These bacteria retain the crystal violet stain during the Gram staining process and appear purple or blue. They have a thick peptidoglycan layer in their cell walls.

a. Aerobes (Facultative anaerobes)

b. Anaerobes (obligatory)

      • Peptococcus Species
      • Peptostreptococcus Species
      • Ruminococcus Species

2. Gram Positive Rods (Bacilli)

 a. Aerobes (facultative anaerobes)

      • Corynebacterium Species
      • Bacillus Species
      • Listeria Species
      • Lactobacillus Species
      • Nocardia Species

 b. Anaerobes (Obligatory)

      • Clostridium Species

3. Gram Negative Cocci

These bacteria do not retain the crystal violet stain and appear pink or red. They have a thinner peptidoglycan layer and an outer membrane in their cell walls.

a. Aerobes (facultative anaerobes)

      • Neisseria Species
      • Moraxella Species

b. Anaerobes (obligatory)

      • Veillonella Species

4. Gram Negative Rods (Bacilli)

a. Aerobes (facultative anaerobes)

      • Escherichia coli
      • Klebsiella Species
      • Proteus Species
      • Shigella Species
      • Salmonella Species
      • Vibrio Species

5. Gram Negative Cocco-Bacilli

a. Aerobes (facultative anaerobes)

      • Haemophilus Species
      • Bordetella Species
      • iii.Brucella Species
      • Legionella Species
      • Francisella Species

 b. Strict Aerobes

      • Aeromonas Species
      • Plesiomonas Species
      • Mycobacterium Tuberculosis
      • Pseudomonas Species

c. Anaerobe (obligatory)

      • Bacteroides Species
      • Fusobacterium Species
      • Prevotella Species

d. Microaerophilic

      • Campylobacter Species
      • Helicobacter pylori

6. Spirochaetes

  a. Aerobic

      • Leptospira Species
      • Microaerophilic
      • Treponema Species
      • Borrelia Species

7. Intracellular Organisms

      • Bartonella Bacilliform
      • Chlamydia Species
      • Rickettsia Species

8. Cell Wall Deficient Organisms

      • Mycoplasma Species
      • ‘L’ forms of bacteria


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