Sensitivity of six strains of Listeria monocytogenes to nisin

The sensitivity of six strains of Listeria monocytogenes (serotype 1, 1a, 2, 4a, 4b, and a nonhemolytic strain) to nisin was compared at pH 5.0, 6.0, and 7.0. The effects of 240 and 600 IU/ml of nisin in broth on 10(3), 10(4), 10(6), and 10(8) CFU/ml of each of the strains were tested, and cell numbers were determined after incubation for 30 min, 60 min, and 24 h at 35 degrees C. Although populations decreased after a contact time of 30 min, survivors of 240 IU of nisin per mi reached high numbers after 24 h. Similar results were obtained for 600 IU of nisin per mi but there were no survivors after 24 h of incubation when inocula of less than 10(4) CFU/ml were tested irrespective of the strain. The sensitivity of L. monocytogenes to nisin at each pH was strain dependent, and it was enhanced at the low pH.