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

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


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:

  1. Prokaryotes
  2. Eukaryotes

Prokaryotes: 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.

Eukaryotes: 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
  2. Bacilli
  3. Vibrios
  4. Spirochaetes

Intermediate shape like cocco-bacilli also exist.MORPHOLOGICAL CLASSIFICATION


Bacteria are divided into Gram- negative and Gram-positive on the basis of their cell wall structure.

Gram-Positive: Bacteria staining purple in Gram Stained smear. They have thick layer of peptidoglycan.

Gram-Negative: Bacteria staining pink in Gram stained smear. Gram positive bacteria, when dead may stain red. They have thick outer membrane.

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Gram-Variable: The organisms is Gram positive but appear Gram Negative or is Gram Negative but appear Gram Positive.


  1. Strict Aerobes: These do not grow in the absence of oxygen.
  2. Anaerobes: These can be two types:
  3. Strict (Obligatory) anaerobes: Bacteria that can grow only in the absence of oxygen.
  4. Facultative anaerobes: These can grow both in presence or absence of oxygen. Most of the commonly isolated bacteria belong to this group.
  5. Carboxyohillic: These require presence of high percentage (10%) of carbon dioxide.
  6. Microaerophilic: These require only small amount of oxygen for their growth and higher concentration of the oxygen will kill the organism.


Based on the temperature requirement for their growth bacteria are classified into following three groups:

  1. Mesophilic
  2. Psychrotrops (facultative psychrophiles)
  3. Psychrophilic
  4. Thermophilic
  5. Hypethermophiles


1. Gram Positive Cocci

a. Aerobes (Facultative anaerobes)

      • Staphylococcus Species
      • Streptococcus Species
      • Enterococcus Species

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

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
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      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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