CLASSIFICATION OF BACTERIA
Many scientists before the discovery of microbes had suggested that invisible organisms were responsible for spreading diseases but they were unable to provide proof for it. The proof was given by Leeuwenhoek who observed and described microorganisms. After him Pasteur described that microbes inhabit air and are the cause of contamination of food .
But it was the dutch scientist ,Otto Muller who first proposed classification of bacteria. After him many different types of bacteria were discovered and hence the classification change and expanded.
Earlier classification was based only on the phenotypic appearance , with the advancement in chemical studies; chemotaxonomic characteristics were also included. The current system is based on phenotypic, chemotaxonomic and genotypic along with phylogeny.
I. On the basis on mode of nutrition
There are 4 types of bacteria based on their mode of nutrition i.e. from where they get their energy .
1. Phototrophs: Gain their energy from the sunlight. They are further divided on the basis of their source of electron as
• Photolithotrops: they use reduced inorganic compounds such as hydrogen sulphide as their electron source. Eg. Chromatium okenii
• Photoorganotrops: they use organic compounds such as succinate as their electron source. Eg. Spirulina
2.Chemotrophs : Gain energy from chemical compounds. They are further divided on the basis of their source of electron as
• Chemolithotrops: they use energy from oxidation of compound and inorganic compounds such as ammonia as electron source. Eg. Nitrosomonas
• Chemoorganotrops: they use energy from chemical compounds and uses organic compound such as glucose and amino acids as electron source.
3. Heterototrophs : Can use organic compounds as their energy source but can’t utilize carbon dioxide.
4. Autotrophs: They use carbon dioxide(CO2) as their energy source and are further divided on the basis of how they utilize this CO2
II. On the basis of Morphology
1.Coccus: These are spherical in shape .They can be further classified on the basis of their arrangement such as :
• Monococcus – occurs individually or singular. Example: Micrococcus flavus.
• Diplococcus- occur in pairs . Example: Diplococcus pneumonia.
• Streptococcus- occurs in chains. Example: – Streptococcus pyogenes.
• Staphylococcus – occurs in cluster or bunch Example: Staphylococcus aureus
2. Bacilli: These bacteria are rod-like in appearance.
• i) Coccobacilli: Eg. Brucella
• ii) Streptobacilli: chain of rod shape bacteria: Eg. Bacillus subtilis,
• iii) Comma shaped: Eg. Vibrio cholarae
• iv) Chinese letter shaped: Corynebacterium dephtherae
3. Spirochaetes : These are spiral in shape.
III. On the basis of temperature optimum for growth
1.Mesophiles : These can grow between 25-40C but optimum temperature is 37C. Examples: Salmonella, Staphylococci
2. Psychrophiles: These can grow at 0°C or below but optimum temperature for growth is 15 °C or below and maximum temperature is 20°C . Examples: Vibrio psychroerythrus, Psychroflexus
3. Thermophiles: These can best grow above 45C. True thermophiles are known as Stenothermophiles.
Examples: Streptococcus thermophile, Thermus aquaticus
4. Hypethermophiles: These have optimum temperature of growth above 80C. Examples: Aquifex, Thermotoga
IV. On the basis of optimum pH of growth
1. Alkaliphiles: These grow best at alkaline pH. optimum pH of growth is 8.2. Example: vibrio cholera.
2. Acidophiles: These grow best at acidic pH. Examples: Thiobacillus, Thermoplasma
3. Neutrophiles: These bacteria can grow best at neutral pH. Optimum pH of growth is 6.5-7.5 . Example: E. coli
V. On the basis of oxygen requirement
1.Obligate Aerobes: These bacteria require oxygen for their growth and can’t survive without it. Example: Mycobacterium, Bacillus
2.Faculatative anaerobes: These bacteria do not require oxygen for their survival .These bacteria can carry out both oxidative and fermentative metabolism.Example: E.coli and salmonella
3.Aerotolerant Anaerobes: These bacteria do not require oxygen but can withstand its presence.These only use fermentative metabolism. Example:lactobacillus
4.Microaerophiles: These bacteria can grow only in low concentrations of oxygen. They have only oxidative metabolism pathway.Example: campylobacter
VI. On the basis of Flagella
1.Monotrichous: These bacteria have only a single flagella at one end. Example: Vibrio cholera
2.Amphitrichous: These bacteria have flagella at both the ends . It can be singular or in clusters. Example: Rhodospirillum rubum
3.Peritrichous : These bacteria have flagella all over their surface. Example: Klebsiella
4. Lophotrichous: These bacteria have a tuft of flagella at one end . Example: Pseudomonas fluroscence
5. Atrichous: These bacteria do not flagella.Example: Shigella
VII. On the basis of spore production
1.Spore forming : These bacteria produce spores during unfavourable periods or under stress. These can be of two types depending on where the spore is formed.
• Endospore forming: These bacteria produce spore within the cell itself. Example: clostridium
• Exospore forming : These bacteria produce spore outside the cell. Example: methylosinus
2.Non spore forming : These bacteria do not have the ability to form spores. Example: E.coli
The Bergey Classification of Bacteria is a resource used for determining the identity of bacterial species by utilizing every characterizing aspect. It was First published in 1923 by David Hendricks Bergey, it is used for classification of bacteria on the basis of their structural and functional attributes and subsequently arranging them into specific familial orders.
It was published in four volumes and divided into 19 parts.
Volume 1 : Includes information on all types of Gram-negative bacteria considered to be of “medical and industrial importance.”
Volume 2 : Included information on all types of Gram-positive bacteria.
Volume 3 : It deals with the remaining, slightly different Gram-negative bacteria, along with archaea.
Volume 4 : It consists of filamentous actinomycetes and other bacteria.
Bergey Division I = The Cyanobacteria (also known as the blue-green alga) – These can use light as their energy source under aerobic conditions.
Bergey Division II = Includes photobacteria and all other classical bacteria – It consists of 19 parts
1. Archeobacteria- the archeobacteria were mixed within the 19 parts of the book in the 8th Edition (1974)
2. Phototrophic Bacteria: Rhodospirillum , Rhodopseudomonas
3. Gliding Bacteria: Myxococcus, Simonsiella
4. Sheathed Bacteria: Sphaerotilus, Leptothrix
5. Budding Bacteria: Caulobacter ,Gallionella
6. Spirochetes: Spirochaeta,Treponema
7. Spiral and Curved Bacteria: Spirillum, Auqaspirillum Oceanospirillum
8. Gram-negative Aerobic Rods and Cocci: Pseudomonas, Azotobacter, Rhizobium ,Agrobacterium
9. Gram-Negative Facultative Anaerobic Rods: Escherichia, Salmonella,Shigella,Klebsiella,Enterobacter, Yersinia,Chromobacterium,Flavobacterium
10. Gram-negative anaerobes: Bacteriodes, Desulfovibrio
11. Gram-Negative cocci: Nisseria, Acinetobacter, Paracoccus
12. Gram-negative anaerobic cocci: Veillonella, Acidaminococcus
13. Gram-Negative Chemolithotrophic: Nitrobacter,Thiobacillus
14. Gram-Positive Cocci: Micrococcus,Staphylococcus, Streptococcus, Leuconostoc
15. Endospore-forming Rods and cocci: Bacillus,Clostridium
16. Gram-positive, non-sporing rods: Lactobacillus, Listeria
17. Actinomycetes : Corynebacterium ,Arthobacter, Eubacterium,Actinomyces,Mycobacterium,Frankia,Nocardia,Streptomyces
18. Rickettsias: Rickettsia,Erhlichia, Bartonella,Chlamydia
19. Mycoplasmas: Mycoplasma,Thermplasma,Spiroplasma