Effect of neutral electrolyzed water and nisin on Listeria monocytogenes biofilms, and on listeriolysin O activity

All food processing surfaces are potential sites for biofilm formation of foodborne pathogens, which may result in increased virulence and better adaptation to survival in foods. This study was aimed to evaluate the effect of two antimicrobials, neutral electrolyzed water (NEW) and nisin, and their combination, on Listeria monocytogenes Scott A biofilms formed on glass and stainless steel surfaces. We also examined the effects of sub-lethal doses of NEW on listeriolysin O (LLO) activity from free and biofilm listerial cells. Coupons inoculated with L. monocytogenes cells were used to produce biofilms by incubation for four days at 37°C. An orthogonal experimental design with two replicates was used to test the effect of four factors on biofilm population. The factors were antimicrobial agents: NEW (65ppm), nisin (6976IU/per coupon), and their combination; temperature: 20°C and 37°C; contact time: 5, 10, 20 and 45min; and type of material: glass or stainless steel. Antimicrobial compounds and exposure time significantly affected L. monocytogenes populations in biofilms from both surfaces. A bactericidal effect was shown by NEW on free listerial cells at 30ppm for 0.5min of exposure, regardless treatment temperature. Same effect was observed on listerial biofilms at 65ppm or higher concentrations, after 10min contact time. A sub-lethal concentration of NEW acting on listerial biofilms resulted in an increased LLO activity, while non-treated biofilms exhibited a reduced activity, but higher than that found for free cells. The use of NEW as a sanitizer may be effective in reducing bacterial contamination. In addition because of its safety, which would benefit the food industry and its environmental friendliness, NEW may be of significant use in the food industry.