Isolation and characterization of nisin-producing Lactococcus lactis subsp. lactis from bean sprouts.

Bacterial isolates from bean-sprouts were screened for anti-Listeria monocytogenes bacteriocins using a well-diffusion method. Thirty-four isolates inhibited the growth of L. monocytogenes. Ten strains, which produced the biggest zones of inhibition, were characterized. One strain, isolate 80, which exhibited the strongest inhibition against L. monocytogenes, was selected for subsequent in-depth analysis. Both ribotyping and DNA sequencing of 16S ribosomal RNA demonstrated that the isolate was Lactococcus lactis subsp. lactis. Primary characterization showed that the anti-L. monocytogenes compound was proteinaceous and the antimicrobial effects were not caused by pH, a phage or $\rm H\sb2O\sb2,$ indicating that it was a bacteriocin. A plasmid cured derivative of isolate 32 was obtained. This plasmid-free strain still produced a bacteriocin, and had the same antibiotic resistance pattern as the wild type strain, indicating that the antibiotic resistance and bacteriocin genes were most likely to be chromosomally mediated. These bacteriocin-producing isolates produced nisin Z. Tests indicate that lactic acid starter cultures, possibly in combination with other hurdles, can be used to control the growth of L. monocytogenes in fresh-cut vegetables. (Abstract shortened by UMI.