Bacteria is the common name for a vast group of one-celled microscopic organisms that encompasses the smallest, simplest, and perhaps first form of cell life that evolved. They constitute one of two divisions in the kingdom Monera. They are unicellular and furnish both the raw material and the chemical machinery for their own reproduction. The oldest sign of life is a fossilized bacterial cell discovered in a rock in Africa and estimated at about 3.5 billion years old. The study of bacteria is called bacteriology, which belongs to the broader science of microbiology, or the study of all types of microorganisms, including one-celled Protozoa, yeasts, and algae. Medical microbiology is concerned with the behavior and control of pathogens, which are microorganisms that cause infectious diseases in humans and other animals. SIZE AND HABITAT Thirty trillion bacteria of average size weigh about 28 g (1 oz). Bacteria are measured in microns (0.001 mm, 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. They are found everywhere. Thousands of species are already known, many living in conditions that would destroy other organisms´┐Żin the almost airless reaches of the upper atmosphere, below the ocean surface to depths of 10 km (6 mi) and in sulfurous hydrothermal vents, in frozen soil, and on rocks in hot springs. Some bacteria produce a resting stage, the endospore. The most resistant living thing known, it can be killed only by boiling in steam under pressure for many hours.
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