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 of 72 isolates inhibited the growth of L. monocytogenes Scott A. One, HPB 1688, which had the biggest inhibition zone against L. monocytogenes Scott A, was selected for subsequent analysis. Both ribotyping and DNA sequencing of 16S ribosomal RNA gene demonstrated that the isolate was Lactococcus lactis subsp. lactis. Polymerase chain reaction and nucleotide sequencing revealed that the genomic DNA of the bean-sprout isolates contained a nisin Z structural gene. In MRS broth, bean-sprout isolate HPB 1688 survived at 3-4.5 degrees C for at least 20 d, grew at 4 degrees C and produced anti-listerial compounds at 5 degrees C. When co-cultured with L. monocytogenes in MRS broth, the isolate inhibited the growth of L. monocytogenes at 4 degrees C after 14 d and at 10 degrees C after 2 d. When co-inoculated with 10(2) cells g-1 of L. monocytogenes on fresh-cut ready-to-eat Caesar salad, L. lactis subsp. lactis (10(8) cells g-1) was able to reduce the number of L. monocytogenes by 1-1.4 logs after storage for 10 d at 7 zero and 10 degrees C. A bacteriocin-producing Enterococcus faecium was also able to reduce the numbers of L. monocytogenes on Caesar salad, but did not act synergistically when co-inoculated with L. lactis subsp. lactis.