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nisin Antibiotic isolated in 1944 from lactic streptococci group N; inhibits some but not all clostridia; not used medically. Used as a food preservative. It is naturally present in cheese, being produced by a number of strains of cheese starter organisms. It lowers the resistance of many thermophilic bacteria to heat and so permits a reduction in the time and/or temperature of heating when processing canned vegetables.