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.
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