Size and Habitat

Thirty trillion bacteria of average size weigh about 28 g (1 oz). Bacteria are measured in microns (0.001 micrometer, about 0.00004 in) and most types range from 0.1 to 4.0 microns in width and 0.2 to 50 microns in length. Bacteria are found every where. Approximately 2,000 species have been identified, many of them living in conditions that would destroy other organisms. They have been found in the almost airless reaches of the upper atmosphere, 10 km (6 mi) below the surface of the ocean, in frozen soil, and on rocks in hot springs. Some bacteria produce a resting stage, the endospore, which is the most resistant living thing known and can be killed only by boiling in steam under pressure for many hours.

Classification
Bacteria are neither plant nor animal. Both bacteria and plants have rigid cell walls, but unlike plants, most kinds of bacteria move about and use organic foods for energy and growth; only a few use photosynthesis.

On the basis of their shapes, bacteria may be grouped into three main type; the rod-shaped bacilli, which often have small whiplike structures known as Flagella that propel the organism; the spherical cocci (singular coccus), which may grow in chains (streptococci, or strep germs," as in strep throat) or which may clump together like a bunch of grapes Istrphylococci); and the comma or spiral shape spirilla and prirochetes (one of which is the cause of syphilis). Another kind of bacteria, the mycoplasmas, have no rigid cell walls and consequently are called pleuropneumonialike organisms, because they cause a contagious pneumonia in cows and human beings.