They occur singly or in groups and are widely distributed in nature. They are present almost everywhere in water, in air, in soil and in all organic bodies, living or dead. They present mostly in the upper layer of cultivated fertile soil. Bacteria are also present in large numbers in the intestines of man and animals, e.g. Escherichia coli. Some of them can withstand extreme heat and cold. Most of them can live in the presence of oxygen and are said to be aerobic, e.g. Bacillus subtilis. But a few can live only in the absence of oxygen and are said to be anaerobic, e.g. Clostridium tetani.
Table: Comparison of the surface area/volume ratio of spherical organisms of different sizes.
|Diameter of Surface area μm
μm2 (4πr2 )
|Surface area/Volume μm-1 (3/r)
*For a given volume, the geometrical shape that has the smallest surface area/volume ratio is a sphere; i.e., if two organisms have the same volume, one being spherical and the other cylindrical, the cylindrical organism has the greater surface area/volume ratio.