Bacteria are wide-spread in distribution and are present in almost all of the biosphere. They have vital importance on earth and hence studied extensively in biology.
They are unicellular (single celled) organisms and are of microscopic in size invisible to naked eye.
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
Bacterial cell wall is another feature for classification. This can also be called as classification of bacteria by gram staining. 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, in bacterial cell wall, grams staining helps inclassification of bacteria as gram positive & gram negative bacteria.
The gram stain consists of colouring dyes like crystal violet and saffron. When a bacteria culture is added with gram’s stain, gram positive bacteria show violet colour, while gram negative bacteria show saffron colour. This difference is due to the cell wall content in them.
So, the bacterial species which take up blue colour on grams stain is termed gram +ve bacteria and the one which takes up red or orange colour is called gram -ve bacteria.
Bacterial species showing gram +ve character is considered rigid cell bacteria while other is less rigid.
Bacterial species are differentiated based on shapes. They 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 in shape and called as spiral shaped bacteria. Ex: syphilis causing bacteria.
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.
See the figure above for details.
Motile bacteria have flagella of different numbers and at different and definite places (points) on the cell wall.
Atrichous bacteria: With no flagella on the bacterial cell wall.
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. The flagella are present all over the cell wall.
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 synthesize 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.
Bacteria can grow in different temperature and so they are classified even based on the temperature they can survive at. Like Thermophilic, mesothermic, Hypoctermic bacteria.
Thermophylic bacteria are those which can survive at high temperature of 45 to 60 degree temperature while mesothermic can survive at 25 to 45 degrees and hypothermic survive at 8 degree or even less temperature.
Bacteria are classified based on the oxygen requirement. Like aerobic, anaerobic and further as obligate aerobic, facultative aerobic, obligate anaerobic and facultative anaerobes.
Aerobic bacteria survive only in the presence of high or sufficient oxygen levels while facultative obligate anaerobes survive only in absence of oxygen while facultative anaerobes survive in low and also lack of oxygen conditions.
Classification of bacteria into different phyla is done as per taxonomy i.e. botanical nomenclature
See the importance of bacteria.