Question L. monocytogenes is one of the most important foodborne pathogens because of the high hospitalization and mortality rate. Little is known about how exactly L. monocytogenes attaches to corn salad, one of the most prevalent fresh produce. Pathogenicity is highly serovar dependent giving rise to the question if there is a link between colonization and virulence. After contamination, attachment and biofilm formation poses a serious risk because it allows the bacteria to persist over long time periods. Therefore, the aim of this study is to investigate the attachment and colonization of L. monocytogenes on corn salad as well as its ability to form biofilm and to answer the question which genes are involved in these processes.
Methods Three strains of L. monocytogenes strains isolated from plant foods as well as from clinical samples were first characterized genotypically using whole genome sequencing (WGS). Strain NCTC 10587 was fluorescently labelled using the chromosomally integrating plasmid vector pAD1. Confocal laser scanning microscopy of the fluorescently labelled strain was used to confirm attachment to leaf surfaces, scanning electron microscopy (SEM) was used to track attachment at the single cell level. To quantify the biofilm formation, in vitro crystal violet assays were performed in triplicate in three different media at two temperatures for 72 h.
Results WGS of the three L. monocytogenes strains confirmed the presence of major virulence factors. Fluorescence microscopy was performed with infected corn salad leaves 48 h post infection. SEM imaging showed L. monocytogenes mainly colonizing the stomata of corn salad and no biofilm formation was confirmed on the surrounding areas. Contrary, crystal violet assays showed that biofilm formation was highest in Luria Bertani medium at 22°C after 12 to 14 h.
Conclusion The results of this study suggest a colonization of the corn salad stomata without biofilm formation on the leaf surfaces, even though the strains are capable of biofilm formation in vitro. Further studies need to elucidate the relevance of these findings for consumer safety.