We are going to discuss on the basic and possible ways of classifying and identifying Bacteria.Before we start, it is necessary to define a bacteria in general:
Bacteria are single celled microbes. The cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles. Instead their control centre containing the genetic information is contained in a single loop of DNA.
Bacteria are classified and identified base on the following characteristics.
- Based on Mode of nutrition
- Based on it optimum temperature requirement for growth
- Based on it optimal pH for growth
- Based on it salt concentration
- on the basis of gaseous requirement
- Base on it morphology
- Type of gram staining
- on the basis of flagella
- on the basis of spore
Classification of bacteria based on their mode of nutrition
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| Classification of bacteria base on nutrition |
Phototrops Bacteria:
These are those bacteria which gain energy from light.They are further divided into 2 groups based on the source of energy or electrons.
We have the Photolitithrotrops and the photoorganotrops
Photolithotrops Are bacteria that gain energy from light and uses reduced inorganic compounds such as H2S as electron source. Eg. Chromatium okenii while the Photoorganotrops are those bacteria who gain energy from light and uses organic compounds such as succinate as electron source.
Chemotrophic Bacteria
Just from the name Chemo we understand they make uses of chemicals.
These are bacteria that gain energy from chemical compounds.These bacteria cannot carry out photosynthesis.
They are further divided into two groups based on the source of electron.
Chemolithotrops are those bacteria that gain energy from oxidation of chemical compound and reduces inorganic compounds such as NH3 as electron source. Eg. Nitrosomonas on the other hand
Chemoorganotrops bacteria gain energy from chemical compounds and uses organic compound such as glucose and amino acids as source of electron. eg. Pseudomonas pseudoflava
Autotrops:
These are bacteria that uses carbondioxide as sole source of carbon to prepare its own food.
Autotrops are also divided into 2 two types on the basis of energy utilized to assimilate carbondioxide. We have Photoautotrops and chemoautotrops
Photoautotrops are those that utilized light to assimilate CO2 and 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
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
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
Classification and Identification of bacteria on the basis of optimum temperature of growth
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| classification of bacteria base on optimum temperature |
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, Psychroflexus
2. Psychrotrops (facultative psychrophiles):
- Those bacteria that can grow even at 0°C but optimum temperature for growth is (20-30)°C
3. Mesophiles:
- 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, Staphulococci
4. Thermophiles:
- 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 stearothermophilus, Thermus aquaticus,
5. Hypethermophiles:
- 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.
- Examples: Thermodesulfobacterium, Aquifex, Pyrolobus fumari, Thermotoga
Classification and identification of bacteria on the basis of optimum pH of growth
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| classification base on PH |
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
2. Alkaliphiles:
- Those bacteria that grow best at alkaline pH
- Example: vibrio cholerae: oprimum 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
Classification and identification of bacteria on the basis of salt requirement
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| CLASSIFICATION OF BACTERIA BASED ON PH |
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 (-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
2. Halotolerant:
- Most of the bacteria do not require NaCl but can tolerate low concentration of NaCl in growth media are called halotolerant
Classification and identification of bacteria on the basis of gaseous requirement
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| Classification of bacteria base on aerobic or anaerobic |
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: coli, 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.
- Example: lactobacillus
4. Microaerophiles:
- 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
- Example: Campylobacter
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, Peptostreptococcus, Slostridium, methanococcus
6. Capnophiles:
- Those bacteria that require carbondioxide for growth.
- They are CO2 loving organism
- Most of the microaerophiles are capnophilic in nature.
- Example: Campylobacter, Helicobacter pylori, Brucella abortus
Classification and identification of bacteria on the basis of Morphology
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| classification of bacteria based on 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. 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
2. Bacilli:
- 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
- iv) Chinese letter shaped: Corynebacterium dephtherae
3. Mycoplasma
- They are cell wall lacking bacteria
- Also known as PPLO (Pleuropneumonia like organism)
- Mycoplasma pneumoniae
4. Spirochaetes:
- They are spiral shaped bacteria
- Spirochaetes
5. Rickettsiae and Chlamydiae;
- They are obligate intracellular parasites resemble more closely to viruses than bacteria
6. Actinomycetes
- They have filamentous or branching structure
- They resemble more closely to Fungi than bacteria
- Example: Streptomyces
Classification and identification of bacteria on the basis of Gran staining
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| classification of bacteria based on gram stain |
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
Classification and identification of bacteria on the basis of Flagella
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| classification of bacteria based on 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.
- eg. Pseudomanas fluroscence
3. Amphitrichous bacteria:
- bacteria having single or cluster of flagella at both end of cell.
- eg. Aquaspirillium
4. Peritrichous bacteria:
- bacteria having flagella all over the cell surface.
- Eg. E.coli, Salmonella, Klebsiella
5. Atrichous bacteria:
- bacteria without flagella.
- Eg. Shigella
Classification and Identification of bacteria on the basis of Spore
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| classification of bacteria based on spore |
1. Spore forming bacteria:
- These are bacteria that produce spore during unfavorable condition.They are further divided into two group below
i) Endospore forming bacteria:
- Spore produced within the bacterial cell.
- Bacillus, Clostridium, Sporosarcina etc
ii) Exospore forming bacteria:
- Spore produced outside the cell
- Methylosinus
2. Non sporing bacteria:
- those bacteria which do not produce spore.
- Eg. E. coli, Salmonella