Size : Typical 0.5-1.5 μm by 1.0- 3.0 μm
Range 0.2 by 100 μm.
Characteristics :
Bacteria are very small, relatively simple, single celled organisms whose genetic material is not enclosed in a special nuclear membrane. For this reason, bacteria are called prokaryotes, form greek word meaning prenucleus. Bacteria make the kingdom Monera. They include the true bacteria ( Eubacteria ) and related organisms ( Cyanobacteria, Actinomycetes, Mycoplasmas, Rickettsia, Chlamydia and others ) as well as archaebacteria. Bacterial cells generally have one of several shapes viz. Bacillus ( rod like ), Coccus ( spherical ), and spiral ( corkscrew ) are the most common shapes but some bacteria may be comma shaped ( Vibrio ), filamentaous ( actinomycetes ) and more recently known square shaped ( arcula ). Individual bacteria may from pairs, chains, clusters or other groupings. Such formations or cell groupings are usually characteristics of a particular genera and species. Bacteria are enclosed in cell walls that are largely composed of a substance called peptidoglycon or murein. Bacteria generally produce by dividing into equal daughter cells, this process is called binary fission. For nutrition, most bacteria use organic or from a living host. Some bacteria can manufacture their own food by photosynthesis, and some can derive nutrition from inorganic substances. They are abundant is soil, water and air and also major inhabitants of our skin, mouth nd intestine. Some bacteria live in environment that have extreme temperatures, pH or salinity. Although some bacteria cause disease, many play more beneficial roles such as cycling elements in the biosphere. In short the characteristics of bacteria can be summarized as;
* Bacteria are unicellular prokaryotic organisms.
* They are simple associations of similar cells.
* They possess simple internal structure.
* They grow on artificial laboratory media.
* Reproduction in bacteria is asexual, characteristically by simple cell division.
Practical Significance :
a. Some bacteria cause disease eg. Salmonella typhi -- Typhoid Mycobacterium tuberculosis -- Tuberculosis ( T.B.), Vibrio cholerae -- Cholera etc.
b. Bacteria perform important role in natural cycling of elements which contribute to soil fertility eg. Azotobacter and Rhizobium show nitrogen fixation activity.
c. Bacteria are useful in industry for manufacture of valuable compounds. eg. L-lysine production ; E. coil.
d. Some bacteria spoil foods eg. Clostridium botulinum in meat products.
e. Some bacteria make food eg. Lactobacillus sp. in dairy products and in preparation of idli.