What Are The Different Classifications Of Bacteria?

The many different types of bacteria may be categorized according to the way the air affects them: the aerobic depend upon air, the anaerobic upon the lack of air or oxygen. By and large, bacteria prefer darkness to light.
Bacteria are also classified according to their basic shapes. There is the spherical kind known as "cocci," which grow in pairs, clusters, or chains. Then there are the rod-shaped bacteria, and therefore termed "bacilli," an example of which is the typhoid bacillus. Still another kind is the "spirelli," the spiral-shaped bacteria, of which the Asiatic cholera germ is an example. And there is a subdivision of the latter, known as the "spirochetes." The germ that spreads syphilis is one of these.

Smaller than bacteria are rickettsia, named after their discoverer, H. T. Ricketts. And much smaller than even the rickettsia are the viruses, their name coming from a root meaning "poison."