Today, most bacterial diseases of humans and their etiologic agents have been identified, although important variants continue to evolve and sometimes emerge, e.g., Legionnaire's Disease, tuberculosis and toxic shock syndrome. During this period, great emphasis was placed on applying Koch's postulates to test proposed cause-and-effect relationships between bacteria and specific diseases. The initial advances in pathogenic bacteriology were derived from the identification and characterization of bacteria associated with specific diseases. The discipline of bacteriology evolved from the need of physicians to test and apply the germ theory of disease and from economic concerns relating to the spoilage of foods and wine. Because they are ubiquitous and have a remarkable capacity to adapt to changing environments by selection of spontaneous mutants, the importance of bacteria in every field of medicine cannot be overstated. These organisms exist widely in both parasitic and free-living forms. Many bacteria multiply at rapid rates, and different species can utilize an enormous variety of hydrocarbon substrates, including phenol, rubber, and petroleum. Superficially, bacteria appear to be relatively simple forms of life in fact, they are sophisticated and highly adaptable. Medically they are a major cause of disease.
Bacteria are single-celled microorganisms that lack a nuclear membrane, are metabolically active and divide by binary fission.
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