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any of a group of chemically similar substances obtained from molds of the genus Penicillium that were the first antibioticantibiotic,
any of a variety of substances, usually obtained from microorganisms, that inhibit the growth of or destroy certain other microorganisms. Types of Antibiotics
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 agents to be used successfully in the treatment of bacterial infections in humans. The antagonistic effect of penicillin on bacteria was first observed by the Scottish biologist Sir Alexander FlemingFleming, Sir Alexander,
1881–1955, Scottish bacteriologist, discoverer of penicillin (1928) and lysozyme (1922), an antibacterial substance found in saliva and other body secretions. Educated at St. Mary's Hospital Medical School, Univ.
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 in 1928. Although he recognized the therapeutic potential of penicillin, it was not until 1941 that a group of biologists working in England, including Oxford's Sir H. W. FloreyFlorey, Howard Walter Florey, Baron,
1898–1968, British pathologist, b. Australia. He was educated at Adelaide Univ. and at Cambridge and Oxford and returned to Oxford as professor of pathology in 1935.
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 and E. B. ChainChain, Ernst Boris,
1906–79, English biochemist, b. Berlin, Germany. In 1933 he left Germany and went to England, where he conducted research at Cambridge from 1933 to 1935 and at Oxford from 1935; he lectured (1936–48) in chemical pathology at Oxford.
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, purified the substance and established its effectiveness against infectious organisms and its lack of toxicity to humans. The first successful treatment of a patient with penicillin occurred in New Haven, Conn., in 1942. Despite the development of hundreds of different antibiotics in recent decades, penicillin remains important in antibiotic therapy.

Small amounts of the antibiotic were first obtained from strains of the mold species P. notatum grown in fermentation bottles. During World War II need for the drug spurred development of better production methods; in the current method highly productive strains of Penicillium are grown in a cornsteep liquor medium in fermentation vats. The main form of penicillin produced by this method is benzylpenicillin, which, like all penicillins, is a derivative of 6-aminopenicillanic acid. Phenoxymethyl penicillin, which can be given orally because it is resistant to degradation by stomach acid, is produced by the species P. chrysogenum.


Penicillin is effective against many gram-positive bacteria (see Gram's stainGram's stain,
laboratory staining technique that distinguishes between two groups of bacteria by the identification of differences in the structure of their cell walls. The Gram stain, named after its developer, Danish bacteriologist Christian Gram, has become an important tool
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), including those that cause syphilis, meningococcal meningitis, gas gangrene, pneumococcal pneumonia, and some staphylococcal and streptococcal infections. Most gram-negative bacteria are resistant to the antibiotic, but some, such as the bacteria that cause gonorrhea, are susceptible, and others are responsive to high penicillin concentrations or to only certain classes of penicillins. Tuberculosis bacteria, protozoans, viruses, and most fungi are not affected by penicillin. The class of penicillins that includes ampicillinampicillin
, a penicillin-type antibiotic that is effective against both gram-negative microorganisms and gram-positive microorganisms such as Escherichia coli. It is often used in the treatment of urinary tract infections, but resistant organisms are increasingly common
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 and amoxicillin with clavulanate (Augmentin) is active against gram-positive and gram-negative bacteria such as Haemophilus influenzae and Escherichia coli. All penicillins act by interfering with synthesis of the cell wall.

Drug Resistance and Sensitivity

Use of penicillin is limited by the fact that, although it causes fewer side effects than many other antibiotics, it causes allergic sensitivity in many individuals, including skin reactions and allergic shock. In addition, many microorganisms have developed resistance to the penicillins, and serious hospital epidemics involving infants and surgical patients have been caused by penicillin-resistant staphylococci (see drug resistancedrug resistance,
condition in which infecting bacteria can resist the destructive effects of drugs such as antibiotics and sulfa drugs. Drug resistance has become a serious public health problem, since many disease-causing bacteria are no longer susceptible to previously
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). Some organisms are resistant because they produce an enzyme, penicillinase, that destroys the antibiotic. Synthetically produced penicillins such as methicillin and oxacillin have been developed that are not degraded by the penicillinase enzyme, but these new penicillins have no effect on bacteria that have developed resistance by other means, e.g., by altered cell wall structure. Other antibiotics, such as erythromycinerythromycin
, any of several related antibiotic drugs produced by bacteria of the genus Streptomyces (see antibiotic). Erythromycin is most effective against gram-positive bacteria such as pneumococci, streptococci, and some staphylococci (see Gram's stain).
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, have become important in treating infections by microorganisms resistant to penicillin.


See E. Lax, The Mold in Dr. Florey's Coat: The Story of the Penicillin Miracle (2004).


The collective name for salts of a series of antibiotic organic acids produced by a number of Penicillium and Aspergillus species; active against most gram-positive bacteria and some gram-negative cocci.


any of a group of antibiotics with powerful bactericidal action, used to treat many types of infections, including pneumonia, gonorrhoea, and infections caused by streptococci and staphylococci: originally obtained from the fungus Penicillium, esp P. notatum. Formula: R-C9H11N2O4S where R is one of several side chains