Biofilm formation is a matter of concern in food industries because biofilms facilitate the survival of pathogenic bacteria such as Listeria monocytogenes, which may contaminate food-processing equipment and products. In this study, nisin and two Enterococcus faecium strains were evaluated for their effect on biofilm formation by L. monocytogenes cultured in brain heart infusion broth and on stainless steel coupons. Elimination of preformed L. monocytogenes biofilms by peroxyacetic acid also was tested. Adhesion control experiments were performed with pure cultures of L. monocytogenes after swab collection of adhered cells, which were then enumerated on PALCAM agar plates and visualized by scanning electron microscopy. Formation of a biofilm was recorded when the number of adhered cells was at least 10(3) CFU/cm2. When L. monocytogenes was cocultured with E. faecium bac-, the number of adhered L. monocytogenes cells was 2.5 log lower (P = 0.002) when initially compared with the control culture, but after 6 h of incubation a biofilm was again detected. However, in coculture on stainless steel coupons, E. faecium bac+ inhibited L. monocytogenes adherence and did not allow biofilm formation for up to 48 h (P < 0.001). In the presence of nisin or after treatment with peroxyacetic acid, bacterial growth was reduced (P < 0.001) up to 4.6 and 5.6 log CFU/cm2, respectively, when compared with L. monocytogenes cultures on untreated coupons. However, after these treatments, cells were still present, and after 24 h of incubation, a renewed biofilm was detected in L. monocytogenes cultures treated with nisin. Although all tested conditions reduced L. monocytogenes growth to some extent, only coculture with E. faecium bac+ efficiently reduced biofilm formation, suggesting a potential control strategy for this pathogen.

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