Bacteria are a cellular (unicellular) small creatures which can be seen only through a microscope. They are found in air, seas, lakes and in the intestine of men and other creatures. They are in the shape of Coccus, Bacillus and Spirillium. They multiply quite rapidly through simple fission (division of cell) or sexual or asexual reproduction. Lakhs of bacteria are produced within an hour.

Advantages of Bacteria:

(i) Some bacteria are useful in agriculture. It increases soil fertility. These bacteria are capable of converting atmospheric free nitrogen into nitrogenous compounds which is essential for all plants. Some Bacteria attack dead bodies of plants and animals and convert their complex compound into simpler substances, e.g. carbondioxide (CO₂), Water(H₂O), Nitrates(NO₃), Sulphate(SO₄) etc.

(ii) Bacterium lactici acidi and B. acidi lactici are found in milk. These bacteria ferment lactose sugar found in milk to form lactic acid by which milk becomes sour. Lactic acid bacteria help in the formation of curd.

(iii) Some bacteria are very much useful in industries. Vinegar is manufactured from sugar solution in the presence of Acetobacter aceti. Clostridium acetobutylicum takes part in the manufacture of butyl alcohol and acetone. Bacteria helps in fibre ratting. The process of fibre ratting is carried out by clostridium butyricum inhabiting in the water. In tobacoo industries Bacillus megathenium mycococcus is used for its fermentative capacity for developing flavour and taste in tobacco leaves. In tea industries by fermentative action of Mycocccus candisdans, special taste is developed in the tea leaves. Some bacteria are useful in tanning of leather. They decompose fats which are found in skin of animals with the result that skin and hairs are separated from each other and this leather ready for use.

(iv) Some bacteria are useful in manufacturing medicine. Some of the antibiotics are manufactured by bacterial actions. a.g. Bacillus brevis-antibotic thyrothricin, B subtilis-abtibiotic subtelin. Vitamin B₂ is manufactured by ferentative action of clostridium acetobutylicum. Some of the antibiotic are obtained from different species of streptomyces. Such as streptomycin (from Bacteria streptomyces grisieus), chloromycetin (from S. Venezuelae), Auremycin (from aureofaciens), Terramycin (from rimosu) Neomycin (from fradiae). etc.