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The multidisciplinary science of microorganisms. The prefix micro generally refers to an object sufficiently small that a microscope is required for visualization. In the seventeenth century, Anton van Leeuwenhoek first documented observations of bacteria by using finely ground lenses. Bacteriology, as a precursor science to microbiology, was based on Louis Pasteur's pioneering studies in the nineteenth century, when it was demonstrated that microbes as minute simple living organisms were an integral part of the biosphere involved in fermentation and disease. Microbiology matured into a scientific discipline when students of Pasteur, Robert Koch, and others sustained microbes on various organic substrates and determined that microbes caused chemical changes in the basal nutrients to derive energy for growth. Modern microbiology continued to evolve from bacteriology by encompassing the identification, classification, and study of the structure and function of a wide range of microorganisms including protozoa, algae, fungi, viruses, rickettsia, and parasites as well as bacteria. The comprehensive range of organisms is reflected in the major subdivisions of microbiology, which include medical, industrial, agricultural, food, and dairy.

Throughout much of the twentieth century, American microbiologists led an age of scientific triumph. In the battle against infectious disease, researchers and pharmaceutical firms improved vaccines and therapeutic sera to control measles, diphtheria, mumps, rubella, and whooping cough. At Vanderbilt University and the Rockefeller Institute, Ernest W. Goodpasture (1886–1960), Thomas M. Rivers (1888–1962), and Richard E. Shope (1901– 1966) transformed the study of influenza, herpes, and encephalitis, developing methods of culturing these viruses in chick embryos. John F. Enders (1897–1985) and his colleagues at Harvard University devised a technique in 1949 for growing polio virus in cultures of tissue cells. With in a decade, Jonas E. Salk (1914–1995) and Albert B. Sabin (1906–1993) introduced two separate polio vaccines, largely eliminating the scourge of infantile paralysis. While penicillin was introduced as a therapeutic agent in the early 1940s by the British researchers Alexander Fleming, Howard Florey, and Ernst Chain, American scientists also studied the antagonisms between different microbes. In 1939, Rockefeller Institute researcher René Dubos (1901–1981) isolated a crystalline antibiotic, gramicidin, from a soil organism. Unfortunately, gramicidin proved too toxic for internal use. Dubos's former teacher, Selman Waksman (1888–1973), found another group of soil organisms showing anti-germicidal properties. Waksman and his students at Rutgers University identified streptomycin, the first effective antibiotic in the treatment of tuberculosis.
Microbiologists equally contributed to the development of molecular biology. In the 1930s and 1940s, Oswald T. Avery (1877–1955) and his colleagues at the Rockefeller Institute showed that DNA played a role in transforming non-virulent pneumococci into virulent forms, intimating that this substance might be generally involved in heredity. Employing a bacteriophage of E. coli, Max Delbruck (1906–1981) and Salvador Luria (1912– 1991) revealed that bacteria and viruses followed normal principles of replication and mutation, there by establishing phage as a model organism for genetic research. Joshua Lederberg (1925–) and Edward L. Tatum (1909– 1975) showed that bacteria can exchange genes when cultured in direct contact. In 1952, Lederberg and Norton D. Zinder (1928–) elucidated the phenomenon of bacterial transduction, where a phage carries DNA from one bacterium to another. Their research suggested a mechanism for introducing genes into new cells, a technique now common in genetic engineering. In the 1960s and early 1970s, Matthew S. Meselson (1930–), David Baltimore (1938–), and Howard M. Temin (1934–1994) employed bacteriophages and other viruses to delineate the relationship among DNA, RNA, and protein synthesis.