Nisin-curvaticin 13 combinations for avoiding the regrowth of bacteriocin resistant cells of Listeria monocytogenes ATCC 15313

Nisin (25-100 IU/ml) and curvaticin 13 (160 AU/ml), a bacteriocin produced by Lactobacillus curvatus SB13, were shown to have a bactericidal effect against Listeria monocytogenes ATCC 15313 in TSB-YE broth (pH 6.5), but it was only transitory. Regrowth was not due to the loss of bacteriocin activity. Cells surviving nisin or curvaticin 13 were more resistant to the respective bacteriocin than the parental strain. Survivors to curvaticin 13 were resistant to the class IIa bacteriocins (camocin CP5, pediocin AcH) but remained sensitive to nisin. The frequencies of spontaneous nisin resistants decreased with increasing bacteriocin concentration and the presence of salts (NaCl, K2HPO4). The behaviour of nisin (1000 IU/ml) or curvaticin 13 (640 AU/ml) resistant variants (Nis1000, Curv645) was investigated in the presence of nisin (100 IU/ml) or curvaticin 13 (320 AU/ml) at 22 and 37 degrees C, and compared with that of the parental strain. The effectiveness of nisin was the same at both temperatures, whereas curvaticin 13 displayed a faster bactericidal action at 37 degrees C. Nis1000 cells were less sensitive to curvaticin 13 than the parental strain, whereas Curv640 cells were more sensitive to nisin than the parental strain. Simultaneous or sequential additions of nisin (50 IU/ml) and curvaticin 13 (160 AU/ml) were performed at 22 degrees C in broth inoculated with the parental strain. All combinations induced a greater inhibitory effect than the use of a single bacteriocin. Simultaneous addition of bacteriocins at t0 led to the absence of viable cells in the broth after 48 h.