[priyasaravanan_1406]

Bacteria a part of the microbial flora is omnipresent either being beneficial or harmful or harmless to other living forms like plants, animals and humans. With over millions of bacteria present in the planet, it is not an easy job to identify, isolate and study a particular species or particular bacteria as such. But thanks to the microbiologists who categorized bacteria based on basic and important factors making all the bacteria fall under any one of the categories and thus making the process of isolation and identification much easier.

Bacteria are classified based on various factors like shape (morphology), Cell wall structure, Respiration (metabolism), type of nutritional source, characteristic and environmental factor.

Shape
Bacteria are of different shapes and are classified based on their morphology into Bacillus, Coccus, spiral, comma shaped and flagellated.
Bacilli: Rod shaped bacteria. Diplobacilli (2 cells), tetrad (4 cells), palisade (two cells arranged parallel) or sterptobacilli (chain arrangement).
Coccus: Spherical shaped bacteria which is further classified as monococcus (a cell), diplococci (2 cells), streptococci (cells arranged like chain) and cells in bunch (Staphylcocci).
Spiral: Spiral shaped bacteria sub divided into spirilla and spirochetes.
Comma shaped: Vibrio cholera is an example of comma shaped bacterium.
Flagellated: Some bacteria have flagella projecting from the cell membrane which is used for mobility and are classified under the group flagellated bacterium. Atrichous (no flagella), monotrichous (uni flagella), amphitrichous (bi flagella) and polytrichous (more flagella).

Cell wall structure
Gram’s staining is the procedure introduced by a Danish scientist, Cristian Gram in the year 1884 which is used till date to differentiate the bacteria based on their cell wall structure. The cell wall of bacteria is composed of peptidoglycans and lipopolysachharide and the varying amount of these two components is used to classify the bacteria by Gram staining into Gram positive and Gram Negative bacteria. Gram staining procedure involves step by step treatment of the cells with dyes.
1. Smear bacteria on a slide
2. Add Drops of Crystal violet dye to the smeared slide
3. Add a mordant solution
4. Wash with ethanol
5. Add Safranin dye and once the slide is ready observe under microscope
Gram positive bacteria appears violet as it retains the violet dye which is related to the thick layer composed of peptidoglycans and Gram negative bacteria appears pink as the treatment with ethanol washes off the violet dye by disrupting the external lipopolysaccharide layer and hence the secondly added red dye is retained. Gram negative bacteria have thin walls.

Respiration
Respiration of bacteria is generally seen as a metabolic process which involves production of energy by breaking down the sugar molecule. Some bacteria require oxygen for this metabolic process and are called as aerobes and bacteria which do not require oxygen for this metabolic process are classified as anaerobes. There is another category called as facultative anaerobes which metabolize either way (both in the presence and absence of oxygen).

Nutritional Source
Like herbivores, carnivores and omnivores in animals and humans bacteria are also classified based on the type of energy source utilized by them for survival. Accordingly they are grouped as, photoautotrophs, chemoautotrophs, heterotrophs, symbiotic, saprophytes and parasites.

Characteristic
Bacteria thrive by being either harmless or harmful or beneficial to the other life forms. Accordingly they are classified as beneficial and pathogenic bacteria. There are many bacteria being isolated and identified in the area of biodegradation and are considered as beneficial microbes for its potential to degrade the waste and can also be called environmental friendly. Another example for the beneficial bacteria is the bacterial flora present in the gut which helps in proper degradation of the food molecules thus enhancing the absorption and hence is considered as beneficial. The bacteria classified as pathogens are known to cause infections and disease in the host organism.

Environment
All bacteria do not survive in common environmental condition. Different bacteria survives different environmental conditions and based on this factor bacteria are classified as Mesophiles, neutrophiles, extremophiles, acidophiles, alkaliphiles, thermophiles, psychroplhiles, halophiles and osmophiles.

Spore Formation
Different species of bacteria under unfavorable conditions form spores and remains dormant and revert back to active vegetative state under favorable environmental conditions and based on this bacteria is classified as spore forming and non-spore forming bacteria.

All the factors used to classify the bacteria acts as an important tool in identification of a bacterium and ensures better understanding while studying different species of bacteria.