Bacteria are classified and identified to distinguish one organism from another and to group similar organisms by criteria of interest to microbiologists or other scientists.
The classification of bacteria serves a variety of different functions. Because of this variety, bacteria may be grouped using many different typing schemes.
The grounds for the classification commonly used may be:
- Both wet-mounted and properly stained bacterial cell suspensions can yield a great deal of information.
- These simple tests can indicate the Gram reaction of the organism; whether it is acid-fast; its motility; the arrangement of its flagella; the presence of spores, capsules, and inclusion bodies; and, of course, its shape.
- This information often can allow identification of an organism to the genus level, or can minimize the possibility that it belongs to one or another group.
- A primary distinguishing characteristic is whether an organism grows aerobically, anaerobically, facultatively (i.e., in either the presence or absence of oxygen), or microaerobically (i.e., in the presence of a less than atmospheric partial pressure of oxygen). The proper atmospheric conditions are essential for isolating and identifying bacteria.
- Other important growth assessments include the incubation temperature, pH, nutrients required, and resistance to antibiotics. For example, one diarrheal disease agent, Campylobacter jejuni, grows well at 42° C in the presence of several antibiotics; another, Y. enterocolitica, grows better than most other bacteria at 4° C. Legionella, Haemophilus, and some other pathogens require specific growth factors, whereas E. coli and most other Enterobacteriaceae can grow on minimal media.
Antigens and Phage Susceptibility
- Cell wall (O), flagellar (H), and capsular (K) antigens are used to aid in classifying certain organisms at the species level, to serotype strains of medically important species for epidemiologic purposes, or to identify serotypes of public health importance.
- Serotyping is also sometimes used to distinguish strains of exceptional virulence or public health importance, for example with V. cholerae (O1 is the pandemic strain) and E. coli (enterotoxigenic, enteroinvasive, enterohemorrhagic, and enteropathogenic serotypes).
- Phage typing (determining the susceptibility pattern of an isolate to a set of specific bacteriophages) has been used primarily as an aid in epidemiologic surveillance of diseases caused by Staphylococcus aureus, mycobacteria, P. aeruginosa, V. cholerae, and S. Typhi. Susceptibility to bacteriocins has also been used as an epidemiologic strain marker. In most cases recently, phage and bacteriocin typing have been supplanted by molecular methods.
- Most bacteria are identified and classified largely on the basis of their reactions in a series of biochemical tests.
- Some tests are used routinely for many groups of bacteria (oxidase, nitrate reduction, amino acid degrading enzymes, fermentation or utilization of carbohydrates); others are restricted to a single family, genus, or species (coagulase test for staphylococci, pyrrolidonyl arylamidase test for Gram-positive cocci).
1. Classification on the basis of Gram Stain and Bacterial Cell Wall
- Of all the different classification systems, the Gram stain has withstood the test of time. Discovered by H.C. Gram in 1884 it remains an important and useful technique to this day.
- It allows a large proportion of clinically important bacteria to be classified as either Gram positive or negative based on their morphology and differential staining properties.
- Slides are sequentially stained with crystal violet, iodine, then destained with alcohol and counter-stained with safranin. Gram positive bacteria stain blue-purple and Gram negative bacteria stain red.
- The difference between the two groups is believed to be due to a much larger peptidoglycan (cell wall) in Gram positives. As a result the iodine and crystal violet precipitate in the thickened cell wall and are not eluted by alcohol in contrast with the Gram negatives where the crystal violet is readily eluted from the bacteria.
- As a result bacteria can be distinguished based on their morphology and staining properties.
- Some bacteria such as mycobacteria are not reliably stained due to the large lipid content of the peptidoglycan. Alternative staining techniques (Kinyoun or acid fast stain) are therefore used that take advantage of the resistance to destaining after lengthier initial staining.
2. Classification of Bacteria on the Basis of Shape
In the year 1872 scientist Cohn classified bacteria to 4 major types depending on their shapes are as follows:
A) Cocci:These types of bacteria are unicellular, spherical or elliptical shape. Either they may remain as a single cell or may aggregate together for various configurations. They are as follows:
- Monococcus:– they are also called micrococcus and represented by single, discrete round Example: Micrococcus flavus.
- Diplococcus:– the cell of the Diplococcus divides ones in a particular plane and after division, the cells remain attached to each other. Example: Diplococcus pneumonia.
- Streptococcus: – here the cells divide repeatedly in one plane to form chain of cells. Example: – Streptococcus pyogenes.
- Tetracoccus: – this consists of four round cells, which defied in two planes at a right angles to one another. Example: – Gaffkya tetragena. Staphylococcus: – here the cells divided into three planes forming a structured like bunches of grapes giving and irregular configuration. Example: – Staphylococcus aureus.
- Sarcina: -in this case the cells divide in three planes but they form a cube like configuration consisting of eight or sixteen cells but they have a regular shape. Example: –Sarcina lutea.
B) Bacilli: – These are rod shaped or cylindrical bacteria which either remain singly or in pairs. Example: –Bacillus cereus.
C) Vibro: – The vibro are the curved, comma shaped bacteria and represented by a single genus. Example: – Vibro cholerae.
D) Spirilla: – These type of bacteria are spiral or spring like with multiple curvature and terminal flagella. Example: –Spirillum volutans.
Actinomycetes are branching filamentous bacteria, so called because of a fancied resemblance to the radiating rays of the sun when seen in tissue lesions (from actis meaning ray and mykes meaning fungus).
Mycoplasmas are bacteria that are cell wall deficient and hence do not possess a stable morphology. They occur as round or oval bodies and as interlacing filaments.
3. Classification of Bacteria on the Basis of Mode of Nutrition
- Those bacteria which gain energy from light.
- Phototrops are further divided into two groups on the basis of source of electron.
- Photolithotrophs: these bacteria gain energy from light and uses reduced inorganic compounds such as H2S as electron source. Eg. Chromatium okenii.
- Photoorganotrophs: these bacteria gain energy from light and uses organic compounds such as succinate as electron source.
- 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.
- Chemolithotrophs: they gain energy from oxidation of chemical compound and reduces inorganic compounds such as NH3 as electron source. Eg. Nitrosomonas.
- Chemoorganotrophs: they gain energy from chemical compounds and uses organic compound such as glucose and amino acids as source of electron. eg. Pseudomonas pseudoflava.
- Those bacteria which uses carbondioxide as sole source of carbon to prepare its own food.
- Autotrophs are divided into two types on the basis of energy utilized to assimilate carbondioxide. ie. Photoautotrophs and chemoautotrophs.
- Photoautotrophs: they utilized light to assimilate CO2. They are further divided into two group on the basis of electron sources. Ie. Photolithotropic autotrophs and Photoorganotropic autotrophs
- Chemoautotrophs: 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 heterotrophs.
4. Classification of Bacteria on the Basis of Temperature Requirement
Bacteria can be classified into the following major types on the basis of their temperatures response as indicated below:
- 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 fatty acids in their cell membrane which gives fluid nature to the cell membrane even at lower temperature.
- Examples: Vibrio psychroerythrus, vibrio marinus, Polaromonas vaculata, Psychroflexus.
- 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)o C but optimum temperature for growth is 37C
- Most of the human pathogens are mesophilic in nature.
- Examples: E. coli, Salmonella, Klebsiella, Staphylococci.
- 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.
- 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.
5. Classification of Bacteria on the Basis of Oxygen Requirement
- Require oxygen to live.
- Example: Pseudomonas, common nosocomial pathogen.
- Can use oxygen, but can grow in its absence.
- They have complex set of enzymes.
- Examples: E. coli, Staphylococcus, yeasts, and many intestinal bacteria.
- Cannot use oxygen and are harmed by the presence of toxic forms of oxygen.
- Examples: Clostridium bacteria that cause tetanus and botulism.
- Cannot use oxygen, but tolerate its presence.
- Can break down toxic forms of oxygen.
- Example: Lactobacillus carries out fermentation regardless of oxygen presence.
- Require oxygen, but at low concentrations.
- Sensitive to toxic forms of oxygen.
- Example: Campylobacter.
6. Classification of Bacteria on the Basis of pH of Growth
- These bacteria grow best at an 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
- These bacteria grow best at an alkaline pH.
- Example: Vibrio cholerae optimum ph of growth is 8.2.
- These bacteria grow best at neutral pH (6.5-7.5).
- Most of the bacteria grow at neutral pH.
- Example: E. coli
7. Classification of Bacteria on the Basis of Osmotic Pressure Requirement
- Require moderate to large salt concentrations.
- 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.
- Ocean water contains 3.5% salt. Most such bacteria are present in the oceans.
- Archeobacteria, Halobacterium, Halococcus.
Extreme or Obligate Halophiles:
- Require a very high salt concentrations (20 to 30%).
- Bacteria in Dead Sea, brine vats.
- Do not require high salt concentrations for growth, but tolerate upto 2% salt or more.
8. Classification of Bacteria on the Basis of Number of Flagella
On the basis of flagella the bacteria can be classified as:
- Atrichos: – These bacteria has no flagella. Example: Corynebacterium diptherae.
- Monotrichous: – One flagellum is attached to one end of the bacteria cell. Example: – Vibro cholerae.
- Lophotrichous: – Bunch of flagella is attached to one end of the bacteria cell. Example: Pseudomonas.
- Amphitrichous: – Bunch of flagella arising from both end of the bacteria cell. Example: Rhodospirillum rubrum.
- Peritrichous : – The flagella are evenly distributed surrounding the entire bacterial cell. Example: Bacillus.
9. Classification of Bacteria on the basis of Spore Formation
- Spore forming bacteria:
- Those bacteria that produce spore during unfavorable condition.
- These are further divided into two groups:
i) Endospore forming bacteria: Spore is produced within the bacterial cell.
Examples. Bacillus, Clostridium, Sporosarcina etc
ii) Exospore forming bacteria: Spore is produced outside the cell.
- Non sporing bacteria:
- Those bacteria which do not produce spores.
Eg. E. coli, Salmonella.
- Trivedi P.C., Pandey S, and Bhadauria S. (2010). Textbook of Microbiology. Pointer Publishers; First edition
- Tortora, Gerard J., Funke, Berdell R.Case, Christine L.. (2013) Microbiology :an introduction Boston : Pearson,