Listeria monocytogenes is a food-borne pathogen capable of adhering to a range of surfaces utilized within the food industry, including stainless steel. The factors required for the attachment of this ubiquitous organism to abiotic surfaces are still relatively unknown. In silico analysis of the L. monocytogenes EGD genome identified a putative cell wall-anchored protein (Lmo0435 [BapL]), which had similarity to proteins involved in biofilm formation by staphylococci. An insertion mutation was constructed in L. monocytogenes to determine the influence of this protein on attachment to abiotic surfaces. The results show that the protein may contribute to the surface adherence of strains that possess BapL, but it is not an essential requirement for all L. monocytogenes strains. Several BapL-negative field isolates demonstrated an ability to adhere to abiotic surfaces equivalent to that of BapL-positive strains. BapL is not required for the virulence of L. monocytogenes in mice.Listeria monocytogenes is a food-borne pathogen that causes serious illness, including meningitis, septicemia, and stillbirth, with a mortality rate of up to 30% (37). More recently, there have been reports of listerial gastroenteritis following the consumption of several different food types (16,34). A number of studies have demonstrated that this organism is able to persist in the food-processing environment for several months and even up to 10 years (23, 29). One of the major causes for concern about L. monocytogenes in these environments is its ability to attach to many different surfaces (2). Indeed, there is recent evidence to show that listerial biofilms formed inside the lumens of stainless steel tubes are able to withstand the shears generated by high-Reynolds-number flows (31). Biofilms, including those produced by L. monocytogenes, are more resistant to detergents and disinfectants (33) and also are a potential source of contamination within food-processing plants; hence, they pose a risk to the maintenance of product safety (27). Consequently, there is considerable interest in determining the mechanisms of attachment and biofilm formation.Our in silico analysis of the genome sequence of L. monocytogenes identified an open reading frame (lmo0435) for a protein with similarity to biofilm-associated proteins (Bap) believed to be important for the binding of staphylococci to abiotic surfaces (10). This Bap protein also has been implicated in the virulence of Staphylococcus aureus (10, 11). Thus, the aim of the current study was to establish if this protein (Lmo0435 [BapL]) of L. monocytogenes influenced biofilm formation and virulence and to determine the prevalence of the lmo0435 (bapL) gene within a selection of field isolates. MATERIALS AND METHODS Bacterial strains and plasmids.A list of the L. monocytogenes isolates and plasmids used in this study is given in Table 1. The strains were cultured in tryptone soya broth (TSB; Oxoid) or brain heart infusion agar (Oxoid) with shaking at 37°C unless otherwise stated. Escherichia coli JM...

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We show in this report that traces of juices released from salad leaves as they become damaged can significantly enhance colonization of salad leaves by Salmonella enterica. Salad juices in water increased Salmonella growth by 110% over the level seen with the unsupplemented control and in host-like serum-based media by more than 2,400-fold over control levels. In serum-based media, salad juices induced growth of Salmonella via provision of Fe from transferrin, and siderophore production was found to be integral to the growth induction process. Other aspects relevant to salad leaf colonization and retention were enhanced, such as motility and biofilm formation, which were increased over control levels by >220% and 250%, respectively; direct attachment to salad leaves increased by >350% when a salad leaf juice was present. In terms of growth and biofilm formation, the endogenous salad leaf microbiota was largely unresponsive to leaf juice, suggesting that Salmonella gains a marked growth advantage from fluids released by salad leaf damage. Salad leaf juices also enhanced pathogen attachment to the salad bag plastic. Over 5 days of refrigeration (a typical storage time for bagged salad leaves), even traces of juice within the salad bag fluids increased Salmonella growth in water by up to 280-fold over control cultures, as well as enhancing salad bag colonization, which could be an unappreciated factor in retention of pathogens in fresh produce. Collectively, the study data show that exposure to salad leaf juice may contribute to the persistence of Salmonella on salad leaves and strongly emphasize the importance of ensuring the microbiological safety of fresh produce.IMPORTANCE Salad leaves are an important part of a healthy diet but have been associated in recent years with a growing risk of food poisoning from bacterial pathogens such as Salmonella enterica. Although this is considered a significant public health problem, very little is known about the behavior of Salmonella in the actual salad bag. We show that juices released from the cut ends of the salad leaves enabled the Salmonella cells to grow in water, even when it was refrigerated. Salad juice exposure also helped the Salmonella cells to attach to the salad leaves so strongly that washing could not remove them. Collectively, the results presented in this report show that exposure to even traces of salad leaf juice may contribute to the persistence of Salmonella on salad leaves as well as priming it for establishing an infection in the consumer.