Bacteria can be classified in several ways based on their morphology, staining properties, motility structures, metabolic properties, temperature and pH optima, salt and oxygen requirements. Morphological classification divides bacteria into cocci, bacilli, spirochetes, mycoplasma and actinomycetes. Metabolic classification distinguishes phototrophs, chemotrophs, autotrophs and heterotrophs. Physiological classification separates bacteria based on their optimal growth temperatures, pH levels, salt concentrations and oxygen requirements.

1. Classification of Bacteria

2. Classification of bacteria on the basis of Morphology
1. Coccus:
These bacteria are spherical or oval in shape
On the basis of arrangement, cocci are further classified as-
i) Diplococcus: coccus in pair. E.g, Neissseria gonorrhoae,
ii) Streptococcus: coccus in chain. E.g. Streptococcus salivarius
iii) Staphylococcus: coccus in bunch. E.g. Staphylococcus aureus
iv) Tetrad: clusters of 4 cocci arranged with in the same plane.E.g. Aerococcus
v) Sarcina: cocus in cubical arrangement of cell. E.g. Sarcina ventriculi

3. 2. Bacilli:
These are rod shaped bacteria
On the basis of arrangement, bacilli are further classified as-
i) Coccobacilli: Eg. Haemophilus Influenzae
ii) Streptobacilli: chain of rod shape bacteria: Eg. Bacillus subtilis,
iii) Comma shaped: E.g. Vibrio cholarae
iv) Chinese letter shaped: Corynebacterium dephtherae

4. 3. Mycoplasma
They are cell wall lacking bacteria.
Do not stain with the Gram stain.
E.g. Mycoplasma pneumoniae
4. Spirochaetes:
They are spiral shaped bacteria.
E.g. Spirochaetes, leptospira.

5. 5. Rickettsiae and Chlamydiae;(Chemically Unique Cell Wall)
They are obligate intracellular parasites resemble more closely to viruses
than bacteria. Stains Gram -ve.
Rickettsiae : Cell wall contains diaminopimelic acid . Rickettsia rickettisii
Chlamydiae : lacks peptidoglycan . Chlamydia trachomatis
6. Actinomycetes
They have filamentous or branching structure.
They resemble more closely to Fungi than bacteria
Example: Actinomyces bovis, A. dentalis.

6. Classification of bacteria on the basis of Gram staining
1. Gram positive bacteria:
cell wall of these bacteria is composed of peptidoglycan layer only.
E.g. Actinomyces, Bacillus, Clostridium, Enterococcus, Staphylococcus,
Streptococcus.
2. Gram negative bacteria:
cell wall of these bacteria is composed of Peptidoglycan and outer membrane.
E.g. Acetobacter, Bortadella, Chlamydia, Enterobacter, Escherichia,
Helicobacter, Pseudomonas, Rickettsia, Salmonella, Vibrio.

7. Classification of bacteria on the basis of Flagella
1. Monotrichous bacteria:
bacteria having single flagella in one end of cell.
e.g. Vibrio cholera, Pseudomonas aerogenosa
2. Lophotrichous bacteria:
bacteria having bundle of flagella in one end of cell.
e.g. Pseudomanas fluorescens

8. 3. Amphitrichous bacteria:
bacteria having single or cluster of flagella at both end of cell.
e.g. Aquaspirillium
4. Peritrichous bacteria:
bacteria having flagella all over the cell surface.
E.g. E.coli, Salmonella, Klebsiella
5. Atrichous bacteria:
bacteria without flagella.
E.g. Shigella

9. Classification of bacteria on the basis of mode of
nutrition
1. Phototrops:
Those bacteria which gain energy from light.
Phototrops are further divided into two groups on the basis of source of
electron.
Photolithotrops: these bacteria gain energy from light and uses reduced
inorganic compounds. Eg. Chromatium okenii
Photoorganotrops: these bacteria gain energy from light and uses organic
compounds.E.g. Rhodospirillum.

10. 2. Chemotrops:
Those bacteria gain energy from chemical compounds
They cannot carry out photosynthesis
Chemotrops are further divided into two groups on the basis of source of
electron.
Chemolithotrops: they gain energy from oxidation of chemical compound
and reduces inorganic compounds. E.g. Nitrosomonas
Chemoorganotrops: they gain energy from chemical compounds and uses
organic compound such as glucose and amino acids.
E.g. Pseudomonas pseudoflava

11. 3. Autotrops:
Those bacteria which uses carbon dioxide as sole source of carbon to
prepare its own food. (and water: C-H-O as source of CHO and sugar).
Autotrops are divide into two types on the basis of energy utilized to
assimilate carbondioxide. i.e. Photoautotrops and chemoautotrops.
Photoautotrops: they utilize light energy to assimilate CO2. Cyanobacteria.
Chemoautotrops: they utilize chemical energy for assimilation of CO2.
Nitrosomonas

12. 4. Heterotrops:
Those bacteria which uses organic compound as carbon source.
They lack the ability to fix CO2.
Most of the human pathogenic bacteria are heterotropic in nature.
E.g. Streptococcus, Clostridium, Mycobacterium tuberclosis.
However there are some bacteria that require special nutrients for their
growth; known as fastidious heterotrops. E.g. Haemophilus sp. (need
haemolyzed blood).

13. Classification of bacteria on the basis of optimum temperature
of growth
1. Psychrophiles:
Bacteria that can grow at 0°C or below but the optimum temperature of
growth is 15 °C or below and maximum temperature is 20°C are called
psychrophiles.
Psychrophiles have polyunsaturated fattyacids in their cell membrane which
gives fluid nature to the cell membrane even at lower temperature.
Examples: Vibrio psychroerythrus, vibrio marinus, Polaromonas vaculata,

14. 2. Psychrotrops (facultative psychrophiles):
Those bacteria that can grow even at 0°C but optimum temperature for
growth is (20-30)°C e.g. Vibrio rumoiensis
3. Mesophiles:
Those bacteria that can grow best between (25-40) °C but optimum
temperature for growth is 37°C
Most of the human pathogens are mesophilic in nature
Examples: coli, Salmonella, Klebsiella, Staphylococci

15. 4. Thermophiles:
Those bacteria that can best grow above 45°C.
Thermophiles capable of growing in mesophilic range are called facultative
thermophiles.
True thermophiles are called as Stenothermophiles.
Thermophiles contains saturated fatty acids in their cell membrane so their cell
membrane does not become too fluid even at higher temperature.
Examples: Streptococcus thermophiles, Bacillus stearothermophilus, Thermus
aquaticus,

16. 5. Hyperthermophiles:
Those bacteria that have optimum temperature of growth above 80°C.
Mostly Archeobacteria are hyperthermophiles.
Monolayer cell membrane of Archeobacteria is more resistant to heat
and they adopt to grow in higher temperature.
Examples: Thermodesulfobacterium, Aquifex, Pyrolobus fumari,
Thermotoga

17. Classification of bacteria on the basis of optimum
pH of growth
1. Acidophiles:
Those bacteria that grow best at acidic pH
The cytoplasm of these bacteria are acidic in nature.
Some acidopiles are thermophilic in nature, such bacteria are called
Thermoacidophiles.
Examples: Thiobacillus thioxidans, Thiobacillus ferroxidans,
Thermoplasma, Sulfolobus.

18. 2. Alkaliphiles:
Those bacteria that grow best at alkaline pH.
Example: vibrio cholerae: optimum pH of growth is 8.2
3. Neutriphiles:
Those bacteria that grow best at neutral pH (6.5-7.5).
Most of the bacteria grow at neutral pH
Example: E. coli

19. Classification of bacteria on the basis of salt requirement
1. Halophiles:
Those bacteria that require high concentration of NaCl for growth.
Cell membrane of halophilic bacteria is made up of glycoprotein with high
content of negatively charged glutamic acid and aspartic acids. So high
concentration of Na+ ion concentration is required to shield the –ve charge.
Example: Archeobacteria, Halobacterium, Halococcus
2. Halotolerant:
Most of the bacteria do not require NaCl but can tolerate low concentration of
NaCl in growth media are called halotolerant. E.g. Alteromonas, Lactobacillus.

20. Classification of bacteria on the basis of gaseous
requirement
1. Obligate aerobes:
Those bacteria that require oxygen and cannot grow in the absence of O2.
These bacteria carryout only oxidative type of metabolism.
Examples; Mycobacterium, Bacillus

21. 2. Facultative anaerobes:
These bacteria that do not require O2 but can use it if available.
Growth of these bacteria become better in presence of O2.
These bacteria carryout both oxidative and fermentative type of
metabolism.
Examples: Klebsiella, Salmonella
3. Aerotolerant anaerobes:
These bacteria do not require O2 for growth but can tolerate the
presence of O2.
Growth of these bacteria are not affected by the presence of O2.
These bacteria have only fermentative type of metabolism.
Example: lactobacillus

22. 4. Microaerophiles:
Those bacteria that require minimum level of O2 for growth. (1-10%),
At atmospheric level of Oxygen, growth of these bacteria is
slowed/inhibited.
These bacteria have oxidative type of metabolism.
Example: Campylobacter