Abstract
Nisin produced by certain Lactococcus lactis strains is commercially used in meat and dairy industries because of its effective antibacterial activity and food safety characteristics. It has been proved that the antibacterial activity could be enhanced when combined with other antimicrobial agents. In this study, we demonstrated that nisin and 3-phenyllactic acid (PLA) in combination displayed excellent combinational antibacterial activity against foodborne pathogens including S. xylosus and M. luteus. The potential application in food preservation was further verified via microbial analysis during the storage of meat and milk, and determination of strawberry rot rate. Scanning electron microscopy observation indicated a distinct mode of PLA with nisin, which may target at the dividing cell, contributing to their combinational antibacterial effect of nisin and PLA. Considering the positive results, a nisin-PLA co-producing strain was constructed based on the food-grade strain L. lactis F44, a nisin Z producer. By the knockout of two L-lactate dehydrogenase (LDH) and overexpression of D-LDHY25A, the yield of PLA was significantly increased 1.77-fold in comparison with the wild type. Anti-bacterial assays demonstrated that the fermentation product of the recombinant strain performed highly effective antibacterial activity. These results provided a promising prospect for the nisin-PLA co-expressing L. lactis in food preservation on account of its considerable antibacterial activity and cost-effective performance.
Highlights
Food safety is generally recognized as an essential public safety issue worldwide
To verify whether the combination of nisin and phenyllactic acid (PLA) possessed a positive effect in antimicrobial activity, pathogens of M. luteus and S. xylosus were used as indictors
The combined nisin-PLA performed a stronger bacteriostatic efficacy against M. luteus. All these results demonstrated that the antibacterial efficacy of nisin was substantially enhanced when combined with PLA
Summary
Food safety is generally recognized as an essential public safety issue worldwide. Microbial contamination of food products is of great importance effecting food safety, which can bring about great economic losses and prevalent foodborne diseases (Chatterjee and Abraham, 2017). Nisin, produced by certain strains of Lactococcus lactis, is a natural antimicrobial peptide exhibiting broad-spectrum antimicrobial activity against a majority of Gram-positive foodborne bacteria and some Gram-negative pathogens when combined with EDTA or physical treatments (Gharsallaoui et al, 2015). It exerts its antimicrobial activity by both pore formation on the surface of cells and inhibition of cell wall biosynthesis (Chatterjee et al, 2005). It has less significance for fruit or bread preservation due to its weak antifungal activity
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