Classification of Bacteria
1. On the Basis of Gram Stain
- Gram Positive Bacteria
- Gram Negative Bacteria
2. On the basis of Morphology
a. Cocci
- Diplococci:Those Cocci that remain in pairs after dividing
- Streptococci:Those cocci that divide and remain attached in chain like patterns
- Tetrad:Those cocci that divide into two planes and remain in groups of four
- Sarcinae: Those cocci that divide in three planes in cube like groups of eight
- Staphylococci: Those cocci that divide in multiple planes and form grape like clusters
b. Bacilli
- Single: E.coli, Klebsiella etc
- Diplobacilli: Coxiella burnetii
- Streptobacilli :, Streptobacillus moniliformis, Bacillus subtilis
- Cocco-bacilli: Acinetobacter, Haemophilus influenzae, Gardnerella vaginalis, and Chlamydia trachomatis
- Pleomorphic:- Proteus species, Pseudomonas aeruginosa
c. Vibrio (Comma shaped): Vibrio cholerae, Vibrio parahemolyticus
d. Spirilla (wave shaped): Spirillu minus
e. Spirochaete (Spiral): Treponema , Borellia, Leptospira
f. Filamentous: Actinomycetes, Nocardia and Streptomyces
3. On the basis of Oxygen Requirement
a. Obligate aerobes: Pseudomonas aeruginosa
b. Obligate anaerobes: Clostridium species
c. Facultative anaerobes: E.coli, Staphylococci
d. Capnophilic:- Requires high concentration of CO2 for growth: Hemophilus infuenzae, Neisseria gonorrhoea
e. Microaerophilic:- Campylobacter jejuni, and Helicobacter pylori
f. Aerotolerant: Anaerobic bacteria but can grow in the presence of air. Lactobacilli species
4. On the basis of flagellar arrangement
a) Aflagellate: Cornybacterium diphtheriae
b) Flagellate:
i. Monotrichous (having one flagellum) : Vibrio Cholerae
ii. Amphitrichous (single flagellum at both ends: Rhodospirillum rubrum.
iii. Lophotrichous (numerous flagella as a tuft): Spirillium, Pseudomonas fluorescens
iv. Peritrichous (flagella distributed all over): E.coli, Proteus
5. On the basis of nutitional requirement
a) Phototrophs: Those bacteria which gain energy from light.
b) Chemotrophs:Those bacteria gain energy from chemical compounds. Nitrosomonas, Pseudomonas pseudoflava.
c) Autotrophs: Those bacteria which uses carbondioxide as sole source of carbon to prepare its own food.
d) Heterotrophs: bacteria which uses organic compound as carbon source. Most of the human pathogenic bacteria are heterotropic in nature.
6. On the basis of temperature requirement
a) Psychrophiles: (4-25)°C Examples: Vibrio psychroerythrus, vibrio marinus, Polaromonas vaculata, Psychroflexus.
b) Mesophiles: Those bacteria that can grow best between (25-45)o C but optimum temperature for growth is 37°C. Most of the human pathogens are mesophilic in nature.
c) Thermophiles: Those bacteria that can best grow above 50°C.
Examples: Streptococcus thermophiles, Bacillus stearothermophilus, Thermus aquaticus.
7. On the basis of pH requirement:
a) Acidophiles: Acidic pH E.g. Lactobacillus
b) Alkaliphiles: alkaline pH e.g. Vibrio cholerae optimum ph of growth is 8.2.
c) Neutrophiles: Neutral pH (6.5-7.5). Most of the pathogenic bacteria
8. Halophiles: Need high amount of salt for growth e.g. Archeobacteria, Halobacterium, Halococcus
9. On the basis of spore formation
a) Non sporing
b) Spore forming
Exospore forming bacteria: Spore is produced outside the cell. Actinomycetes
Endospore forming bacteria: Spore is produced within the bacterial cell.Examples. Bacillus, Clostridium, Sporosarcina etc
- Terminal endospores: At the poles of cells e.g. Clostridium tetani
- Subterminal : Between the center and terminal end e.g. Clostridium perfringens, Bacillus subtilis
- Central endospores: In the middle. Eg. Bacillus cereus
Shape of spore | Bacteria |
Tennis racket shape | Clostridium tertium |
Club shaped | C. perfringens |
Spindle | C. bifermentans |
Drum-stick | C. tetani |
10. On the basis of presence or absence of Capsule
Chemical composition of some bacterial capsules
Bacterium | Capsule composition | Structural subunits |
Gram-positive Bacteria | ||
Bacillus anthracis | polypeptide (polyglutamic acid) | D-glutamic acid |
Bacillus megaterium | polypeptide and polysaccharide | D-glutamic acid, amino sugars, sugars |
Streptococcus mutans | polysaccharide | (dextran) glucose |
Streptococcus pneumoniae | polysaccharides | sugars, amino sugars, uronic acids |
Streptococcus pyogenes | polysaccharide (hyaluronic acid) | N-acetyl-glucosamine and glucuronic acid |
Gram-negative Bacteria | ||
Acetobacter xylinum | polysaccharide | (cellulose) glucose |
Escherichia coli | polysaccharide (colonic acid) | glucose, galactose, fucose glucuronic acid |
Pseudomonas aeruginosa | polysaccharide | mannuronic acid |
Azotobacter vinelandii | polysaccharide | glucuronic acid |
Agrobacterium tumefaciens | polysaccharide | (glucan) glucose |