The competitive growth in biofilms of Listeria monocytogenes with cultures isolated from dairy and meat plant environment

The growth of L. monocytogenes Scott A with competitive cultures obtained from dairy and meat processing plant environment was studied in biofilms and in broth using two low nutrient media (0.2 and 1% tryptic soy broth) and two temperatures (10 and 21{dollar}spcirc{dollar}C). Stainless steel slides (SSS) were periodically transferred to fresh media to produce the biofilm. Growth of L. monocytogenes in biofilm was observed in most experiments, even in the presence of antimicrobial substances produced by competitive cultures such as Bacillus (D18), Streptococcus (M7) and Pseudomonas (D9). Antimicrobial agents produced by competitive cultures could inhibit the growth of L. monocytogenes in broth and its attachment to SSS, but not in biofilm. Formation of biofilm on SSS might prevent antimicrobial substances produced by competitive cultures from reaching embedded cells resulting in lack of inhibition of growth of L. monocytogenes. Flavobacteria (C24) and Pseudomonas (M21) stimulated growth of L. monocytogenes in biofilms. High numbers of L. monocytogenes were observed in broth at every transferring period. This might be due to the release of a heavy inoculum of biofilm cells from the SSS to the liquid media and subsequent growth in the broth.; The occurrence of L. monocytogenes in the environment of meat processing plant was also observed. Twenty percent of collected sponge swab samples (6 among 30) contained Listeria spp. and most of Listeria spp. were L. monocytogenes (83%). L. monocytogenes was also found on both food contact and non-food contact surfaces.; The existence of L. monocytogenes in food contact and non-food contact areas, the growth of L. monocytogenes in biofilm, the resistance of imbedded L. monocytogenes to antimicrobial agents in biofilm, and the release of L. monocytogenes from biofilm into liquid media contribute to the difficulty in controlling postprocessing contamination of L. monocytogenes into food products.