Classification of Bacteria
Bacteria, singular bacterium, any of a group of microscopic single-celled organisms that live in enormous numbers in almost every environment on Earth, from deep-sea vents to deep below Earth’s surface to the digestive tracts of humans.
A Classification of bacteria on the basis of mode of nutrition
Phototrophs are bacteria which gain energy from light
They are further divided into two groups on the basis of source of electron.
i) Photolithotrops: these bacteria gain energy from light and uses reduced inorganic compounds such as H2S as electron source.
ii)Photoorganotrops: these bacteria gain energy from light and uses organic compounds such as succinate as electron source.
Chemotrophs are 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.
i) Chemolithotrops: they gain energy from oxidation of chemical compound and reduces inorganic compounds such as NH3 as electron source. Eg. Nitrosomonas
ii) Chemoorganotrops: they gain energy from chemical compounds and uses organic compound such as glucose and amino acids as source of electron. eg. Pseudomonas
Those bacteria which uses carbondioxide as sole source of carbon to prepare its own food.
Autotrops are divide into two types on the basis of energy utilized to assimilate carbondioxide. Ie. Photoautotrops and chemoautotrops
Photoautotrops: they utilized light to assimilate CO2. They are further divided into two group on the basis of electron sources. Ie. Photolithotropic autotrops and Photoorganotropic autotrops
Chemoautotrops: they utilize chemical energy for assimilation of CO2
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
Some heterotrops are simple, because they have simple nutritional requirement. However there are some bacteria that require special nutrients for their growth; known as fastidious heterotrops.
B Classification of bacteria on the basis of optimum temperature of growth
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, Psychroflexus
2. Psychrotrops (facultative psychrophiles):
Those bacteria that can grow even at 0°C but optimum temperature for growth is (20-30)°C
Those bacteria that can grow best between (25-40)C but optimum temperature for growth is 37C
Most of the human pathogens are mesophilic in nature
Examples: coli, Salmonella, Klebsiella, Staphalococci
Those bacteria that can best grow above 45C.
Thermophiles capable of growing in mesophilic range are called facultative thermophiles.
True thermophiles are called as Stenothermophiles, they are obligate thermophiles,
Thermophils contains saturated fattyacids in their cell membrane so their cell membrane does not become too fluid even at higher temperature.
Examples: Streptococcus thermophiles, Bacillus, Thermus aquaticus,
Those bacteria that have optimum temperature of growth above 80C.
Mostly Archeobacteria are hyperthermophiles.
Monolayer cell membrane of Archeobacteria is more resistant to heat and they adopt to grow in higher remperature.
C Classification of bacteria on the basis of optimum pH of growth
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
Those bacteria that grow best at alkaline pH
Example: vibrio cholerae: optimum ph of growth is 8.2
Those bacteria that grow best at neutral pH (6.5-7.5)
Most of the bacteria grow at neutral pH
Example: E. coli
D Classification of bacteria on the basis of salt requirement
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 (-Ve) charged glutamic acid and aspartic acids. So high concentration of Na+ ion concentration is required to shield the –ve charge.
Example: Archeobacteria, Halobacterium, Halococcus
Most of the bacteria do not require NaCl but can tolerate low concentration of NaCl in growth media are called halotolerant
E 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
2. Facultative anaerobes:
Those bacteria that do not require O2 but can use it if available.
Growth of these bacteria become batter in presence of O2
These bacteria carryout both oxidative and fermentative type of metabolism
Examples: Ecoli, Klebsiella, Salmonella
3. Aerotolerant anaerobes;
Those bacteria do not require O2 for growth but can tolerate the presence of O2.
Growth of these bacteria is not affected by the presence of O2.
These bacteria have only fermentative type of metabolism.
Those bacteria that do not require O2 for growth but can tolerate low concentration of O2.
At atmospheric level of Oxygen growth of these bacteria is inhibited.
These bacteria only have oxidative type of metabolism
5. Obligate anaerobes:
Those bacteria that can grow only in absence of Oxygen.
Oxygen is harmful to obligate anaerobes
These bacteria have only fermentative type of metabolism
Examples: Peptococcus, methanococcus
F Classification of bacteria on the basis of Morphology
These bacteria are spherical or oval in shape
On the basis of arrangement, cocci are further classified as-
i) Diplococcus: coccus in pair. Eg, Neissseria gonorrhoae, Pneumococcus
ii) Streptococcus: coccus in chain. Eg. Streptococcus salivarius
iii) Staphylococcus: coccus in bunch. Eg. Staphylococcus aureus
iv) Tetrad: coccus in group of four.
v) Sarcina: cocus in cubical arrangement of cell. Eg. Sporosarcina
These are rod shaped bacteria
On the basis of arrangement, bacilli are further classified as-
i) Coccobacilli: Eg. Brucella
ii) Streptobacilli: chain of rod shape bacteria: Eg. Bacillus subtilis,
iii) Comma shaped: Eg. Vibrio cholarae
They are cell wall lacking bacteria
Also known as PPLO (Pleuropneumonia like organism)
They are spiral shaped bacteria
5. Rickettsiae and Chlamydiae;
They are obligate intracellular parasites resemble more closely to viruses than bacteria
They have filamentous or branching structure
They resemble more closely to Fungi than bacteria
G Classification of bacteria on the basis of Gran staining
1. Gram positive bacteria:
cell wall of these bacteria is composed of peptidoglycan layer only.
Eg. Staphylococcus, Streptococcus, micrococcus
2. Gram negative bacteria:
cell wall of these bacteria is composed of Peptidoglycan and outer membrane.
Eg. E. coli, Salmonella
H Classification of bacteria on the basis of Flagella
1. Monotrichous bacteria:
bacteria having single flagella in one end of cell.
eg. Vibrio cholera, Pseudomonas aerogenosa
2. Lophotrichous bacteria:
bacteria having bundle of flagella in one end of cell.
3. Amphitrichous bacteria:
bacteria having single or cluster of flagella at both end of cell.
4. Peritrichous bacteria:
bacteria having flagella all over the cell surface.
Eg. E.coli, Salmonella, Klebsiella
5. Atrichous bacteria:
bacteria without flagella.
I Classification of bacteria on the basis of Spore
1. Spore forming bacteria:
Those bacteria that produce spore during unfavorable condition.
These are further divided into two group
i) Endospore forming bacteria:
Spore produced within the bacterial cell.
Bacillus, Clostridium, Sporosarcina etc
ii) Exospore forming bacteria: Spore produced outside the cell
2. Non sporing bacteria:
those bacteria which do not produce spore.
Eg. E. coli, Salmonella
-Bergey’s Manual of Systematic Bacteriology
Bergey’s Manual of Systematic Bacteriology. First published in 1923, the Manual is an effort to classify known bacteria and to make this information accessible in the form of a key. A companion volume, Bergey’s Manual of Determinative Bacteriology, serves as an aid in the identification of those bacteria that have been described and cultured.
Description of the Major Categories & Groups of Bacteria
There are two different groups of prokaryotic organisms: eubacteria and archaebacteria. Eubacteria contain the more common bacteria, ie, those with which most people are familiar. Archaebacteria do not produce peptidoglycan, a major difference between them and typical eubacteria. They also differ from eubacteria in that they live in extreme environments (eg, high temperature, high salt, or low pH) and carry out unusual metabolic reactions, such as the formation of methane. A key to the four major categories of bacteria and the groups of bacteria comprising these categories is presented in Table 2. The four major categories are based on
the character of the cell wall: gram-negative eubacteria that have cell walls, gram-positive eubacteria that have cell walls, eubacteria lacking cell walls, and the archaebacteria.
Table 2. Major Categories and Groups of Bacteria That Cause Disease in Humans Used As an Identification Scheme in Bergey’s Manual of Determinative Bacteriology, 9th Ed.
I. Gram-negative eubacteria that have cell walls
Group 1: The spirochetes
Group 2: Aerobic/microaerophilic, motile helical/vibroid gram-negative bacteria
Group 3: Nonmotile (or rarely motile) curved bacteria
Group 4: Gram-negative aerobic/microaerophilic rods and cocci
Bacteroides (some species)
Group 5: Facultatively anaerobic gram-negative rods
Escherichia (and related coliform bacteria)
Group 6: Gram-negative, anaerobic, straight, curved, and helical rods
Group 7: Dissimilatory sulfate- or sulfur-reducing bacteria
Group 8: Anaerobic gram-negative cocci
Group 9: The rickettsiae and chlamydiae
Group 10: Anoxygenic phototrophic bacteria
Group 11: Oxygenic phototrophic bacteria
Group 12: Aerobic chemolithotrophic bacteria and assorted organisms
Group 13: Budding or appendaged bacteria
Group 14: Sheathed bacteria
Group 15: Nonphotosynthetic, nonfruiting gliding bacteria
Group 16: Fruiting gliding bacteria: the myxobacteria
II. Gram-positive bacteria that have cell walls
Group 17: Gram-positive cocci
Group 18: Endospore-forming gram-positive rods and cocci
Group 19: Regular, nonsporing gram-positive rods
Group 20: Irregular, nonsporing gram-positive rods
Group 21: The mycobacteria
Groups 22–29: Actinomycetes
III. Cell wall-less eubacteria: The mycoplasmas or mollicutes
Group 30: Mycoplasmas
Group 31: The methanogens
Group 32: Archaeal sulfate reducers
Group 33: Extremely halophilic archaebacteria
Group 34: Cell wall-less archaebacteria
Group 35: Extremely thermophilic and hyperthermophilic sulfur metabolizers