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Inhibitory Effects of Lactococcus lactis and its Supernatant on Listeria monocytogenes.

BACKGROUND
The application of biological compounds generated by lactic acid bacteria, especially Lactococcus lactis, is recently considered to be a natural preservative for improving quality and health of food. The purpose of this study is to investigate the inhibitory potential of L. lactis supernatant on the expression of inlA, plc, and hly genes related to L. monocytogenes virulence capacity.


METHODS
L. lactis was cultured under anaerobic conditions for 16 - 18 hours. The supernatant and live bacteria were then separated by centrifuge. The antilisteria effects of L. lactis and supernatant were measured using the agar diffusion technique, and the effect on the expression of the virulence-related genes was calculated by real-time PCR. Also, the effects of live bacteria and its supernatant on the microbial count of milk and sausage infected by L. monocytogenes was evaluated by the colony count assay.


RESULTS
After 24 hours, the highest non-growing hole diameter was obtained in the presence of acidic supernatant (pH = 3.5). The microbial count showed the inhibitory effect on the eighth day after incubation with L. lactis. qPCR data revealed a down-regulation of virulence-related genes of inlA (8 fold), hly (6 fold), and plc (1 fold) in L. monocytogenes after 24-hour incubation with the supernatant.


CONCLUSIONS
Our findings showed that the supernatant of L. lactis has an effective inhibitory role in the growth of L. monocytogenes. In the presence of supernatant, among plc, inlA and hly genes, the expression of inlA and hly genes decreased after 2 hours, which could indicate the molecular inhibitory mechanism of L. lactis in L. monocytogenes.