This contains brief classification of Bacteria based upon different factors including morphology, staining property, nutrition, etc. with special focus on morphology. Other basis of classification are introduced with some examples.

1. Classification of Bacteria
Ritendra Badu
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2. Classification
Bacteria can be classified into varies types depending on various basis such as:
A) Morphology
B) Staining Property
C) Cultural Characteristics
D) Antigenic Structure
E) Others: Oxygen dependence, H+ Ion concentration(pH), Temperature, Falgella
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3. A. Morphology
Depending upon the shapes, bacteria are divided into various types as given below:
1. Cocci
- These are bacteria having spherical, oval or generally round shape. These are of
multiple genera and can grow in pairs, chains or clusters depending on their
orientation and attachment during cell division. These can also be Gram-positive or
Gram-negative. Cocci are of following types:
a) Monococci – Remains single. Eg. Monococcus sp.
b) Diplococci – Remains in pairs. Eg. S. pneumoniae, N. gonorrhoeae, etc.
c) Streptococci – Occurs in chains. Eg. S. pyogenes
d) Staphylococci- Occurs in grapes like clusters. Eg. Staphylococcus aureus
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4. Morphology Contd…
e) Tetrads – Cluster of four cocci. Eg. Micrococcus sp.
f) Sarcina – Found in cuboidal arrangement of 8 cocci. Eg. Sarcina ventriculi
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5. Morphology Contd…
2. Bacilli
- A genus of gram-positive, rod shaped bacteria which includes bacillus and
lactobacillus. On the basis of arrangement they are of following types:
a) Diplobacilli
b) Streptobacilli
c) Coccobacilli – Eg. Coccust bacillus
d) Palisades
The examples of bacilli are B. anthracis (Anthrax causing bacteria), B. luteus, B.
agri, B. pumilus, B. graminis, etc.
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7. Morphology Contd…
3. Vibrio
- This is comma shaped, nearly rod like bacteria. The most common is Vibrio
cholera ( Cholera causing bacteria). These are mostly gram-negative and contain
single flagella at one end.
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8. Morphology Contd…
4. Spirilla
- These are small, regularly coiled, rigid bacteria normally motile and having flagella
at both ends. These are usually gram-negative. Eg. Spirillum minus
5. Spirochetes
- These are also flexible, regularly coiled (more than spirillum) motile bacteria. Eg.
Treponema pallidum, borrelia sp. etc.
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9. Morphology Contd…
6. Actinomycetes
- Branching filamentous bacteria, called so because of their resemblance to the
radiating rays of the sun when seen in tissue lesions. They look like fungi (Actin-ray;
Mykes-fungi).
7. Mycoplasma
- These lack rigid cell wall hence do not possess stable morphology and are highly
pleomorphic. They occur usually in round or oval bodies and in interlacing filaments.
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10. B. Staining Property
- On the basis of stain, bacteria are of following types:
1. Gram stain
2. Acid Fast Stain
a) Acid-fast bacilli- M. tuberculosis
b) Non acid-fast bacilli- Staphylococcus aureus
Cocci Bacilli
Gram-positive Streptococcus, Micrococcus,
Sarcina
B. anthracis, Streptobacillus,
Nocardia
Gram-negative Meningococcus, Gonococcus Bordetella pertussis, V. cholera,
Helicobacter
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11. C. Cultural Characteristic
- Hemolysis on Sheep blood agar.
a) α Hemolysis
b) β Hemolysis
c) γ Hemolysis
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12. D. Others
1. Nutrition
a) Autotrophs
b) Heterotrophs
c) Phototrophs
d) Chemotrophs
2. Temperature
a) Psychrophiles – Cold temperature (15-20C). Eg. Saprophytes
b) Mesophiles – Modetare Temperature (25-40C). Eg. Human pathogenic Bacteria
c) Thermophiles – High Temperature (50-60C). Eg. Thermococcus
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13. D. Others
3. H+ Ion Concentration (pH)
a) Acidophiles – Eg. Lactobacilli
b) Neutrophiles – E. coli
c) Basophiles – V. cholera
4. Oxygen Dependence
a) Obligate aerobes – Grow only in presence of O2. Eg. Pseudomonas
b) Facultative anaerobes – Grow in presence of O2 but can also grow in its absence. Eg. E. coli
c) Obligate anaerobes - Grow only in absence of O2 . Eg. Clostridium
d) Aerotolerant anaerobes - Do not utilize O2 but can grow in its presence. Eg. Lactobacillus
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D. Others
5. Flagella
a) Atrichous – No flagella. Eg. Lactobacillus
b) Monotrichous – Single flagella at one end. Eg. V. cholerae
c) Amphitrichous – One flagella at both ends. Eg. A. faecalis
d) Lophotrichous – Tuft of flagella at one end. Eg. Spirillum
e) Peritrichous – Flagella all over the surface. Eg. E. coli, S. typhi

15. References
1. https://www.slideshare.net/shyrosh/classification-of-bacteria-49373508
2. https://en.wikipedia.org/wiki/cocci
3. https://en.wikipedia.org/wiki/Bacilli
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