Short communication: Screening inhibition of dairy-relevant pathogens and spoilage microorganisms by lactose oxidase.

The inhibitory effect of lactose oxidase on the growth of foodborne pathogens and spoilage microorganisms associated with dairy products was evaluated through an overlay inhibition assay. Lactose oxidase generates hydrogen peroxide via lactose oxidation into lactobionic acid. Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica ser. Typhimurium, Staphylococcus aureus, Pseudomonas fragi, and Penicillium chrysogenum were used as indicators. A commercially available solution of lactose oxidase was applied at different concentrations (0, 0.12, 1.2, and 12 g/L) in 4 types of media [brain heart infusion agar (BHI), BHI + sodium thiocyanate (NaSCN), BHI + lactose, and BHI + NaSCN + lactose] to evaluate the effect of lactose and thiocyanate on microbial inhibition. Lactose oxidase inhibited the growth of all the indicators at a concentration of 12 g/L of the enzyme solution in the presence of lactose alone and in combination with NaSCN. However, supplementation with NaSCN had no effect on the magnitude of microbial inhibition. Staphylococcus aureus was the most sensitive pathogen, and Ps. fragi was the most sensitive of all the indicators in general to lactose oxidase. Listeria monocytogenes and Ps. fragi showed higher susceptibility to the antimicrobial effect of lactose oxidase at 6°C than at their corresponding optimum growth temperature. The inhibitory effect was attributed to the generation of hydrogen peroxide from the oxidation of lactose. Findings from this study demonstrate that lactose oxidase could be used as a novel approach to inhibit the growth of mold and bacteria. It could also be applied as a label-friendly preservative in dairy foods.