SENSITIVITY AND RESISTANCE OF LISTERIA-MONOCYTOGENES ATCC-19115, SCOTT-A, AND UAL500 TO NISIN

Listeria monocytogenes ATCC 19115, Scott A, and UAL500 were evaluated for sensitivity to nisin (0 to 50-mu-g/ml) using a direct plating method. Nisin (10-mu-g/ml) decreased an initial population of L. monocytogenes (10(9) CFU per ml) by 6- to 7-log cycles. Sensitivity to nisin was enhanced by addition of 2% NaCl or by reduction of the medium pH from 6.5 to 5.5 with either hydrochloric or lactic acid. Mutants resistant to 50-mu-g/ml nisin were detected at frequencies of 10(-6) to 10(-8). Nisin-resistant L. monocytogenes mutants should be expected to arise when nisin is used as an antimicrobial in food systems.