Bacteria are wide-spread in distribution and are present in almost all of the biosphere. They are unicellular i.e. single celled organisms. Yet they are of different types and some are useful to man, environment and some are harmful. In microbiology classification of bacteria is based on many factors like their shape, nutrition requirement, cell wall staining, the cell appendages etc. Of these bacteria, those considered useful to humans and also those pathogenic bacteria which cause disease to plants and animals are given more importance and widely studied. Some features of bacteria classification also help in identification bacteria
Classification of bacteria| based on cell wall.
Bacterial cell wall is another feature for classification. Cell wall is present in bacteria unlike other micorbial cells and animal cells. This bacterial cell wall is made of protein (peptidoglycan) & lipid (lipopolysacharide). Based on quantity of peptidoglycan and lipopolysaccharides ratio, grams staining helps in classification of bacteria as gram positive & gram negative bacteria.
These bacteria have differences in colour when stained with crystal violet and saffron. Gram positive (G+ve) bacteria show violet colour while gram negative bacteria (G-ve) show saffron colour. This difference is due to the cell wall content in them.
Classification of Bacteria| Based on shapes or cell structure:
Bacteria have different cell structure but most of them come under two basic shapes like
a) Bacillus (stick or filament or rod like shape)- Among them if a bacterium is
Single cell then it is bacillus, if more it is bacilli like
Two celled: Diplo bacilli, four celled – tetrad etc.
Palisade: Here two cells of bacillus are arranged side by side like sticks in match box
Streptobacilli: Bacilli bacteria arranged like a chain.
b) Coccus (These are spherical)- Of them single-celled is called is called Monococcus,
Two celled: Diplococci,
Chain like arrangement: Strepto bacilli.
Bunch of grapes like: Staphylococci.
c) Comma shaped bacteria, Ex: Vibrio cholera bacteria causing cholera.
d) Spiral bacteria: Also called as spirochetes. These are spiral or hair like bacterial cells. Ex: syphilis causing bacteria.
Classification of bacteria based on flagella presence:
Flagella are appendages of locomotion for the bacterial cell. They emerge from the cell membrane. Not all bacteria have flagella but those motile bacteria have flagella.
Atrichous bacteria: With no flagella
Motile bacteria have flagella of different numbers and at different and definite places (points) on the cell.
Monotrichous bacteria: With one single flagella
Amphitrichous bacteria: Two flagella on both sides of cell:
Polytrichous bacteria: Many flagella at different points:
Lopotrichous bacteria: Flagella at one pole or point of the cell:
Flagella around the cell: Peritrichous bacteria.
Classification of bacteria based on nutrition requirements:
Bacteria obtain nutrition in different forms like
Autotrophs: These are bacteria which prepare their own food by using light as energy sources besides CO2 and organic matter.
Chemoautotrophs: These are bacteria which synthesise their own food by use of energy obtained from chemical sources.
Heterotrophs: These are bacteria which do not synthesize their own food but obtain it from other sources.
Symbiotic bacteria: These are bacteria which obtain food by being in mutual beneficial support with other organisms. Ex: Rhizobium bacteria in leguminous plants fix nitrogen in roots by absorbing it from air while in return take nutrition from the same plant. Another example is Entero bacteria in the intestine.
Saprophytic bacteria: These are bacteria which get their nutrition by consuming dead and decaying material. These there by help in cleaning the environment from accumulation of waste.
Pathogenic bacteria: These are bacteria which grow only in the body of other animals or plants. They get their nutrition from the host and besides induce disease to them.
This is a simple classification of bacteria based on appearance and other features.They are also classified on other ways like botanical nomenclature and other aspects.