Interactions between Listeria monocytogenes and food-associated or environmental bacteria are critical not only for the growth but also for a number of key biological processes of the microorganism. In this regard, limited information exists on the impact of other microorganisms on the virulence of L. monocytogenes. In this study, the growth of L. monocytogenes was evaluated as single culture or in co-culture with L. innocua, Bacillus subtilis, Lactobacillus plantarum and Pseudomonas aeruginosa in tryptic soy broth (10°C/10 days and 37°C/24 hours). Transcriptional levels of 9 key virulence genes (inlA, inlB, inlC, inlJ, sigB, prfA, hly, plcA, plcB) and invasion efficiency and intracellular growth in Caco-2 cells, were determined for L. monocytogenes following grown in mono- or in co-culture for 3 days at 10°C or 9 hours at 37°C. The growth of L. monocytogenes was negatively affected by the presence of L. innocua and B. subtilis, while the effect of cell-to-cell contact on L. monocytogenes growth was dependent on the competing microorganism. Co-cultivation affected the in vitro virulence properties of L. monocytogenes in a microorganism-specific manner, with L. innocua mainly enhancing and B. subtilis reducing the invasion of the pathogen in Caco-2 cells. Assessment of the mRNA levels of L. monocytogenes virulence genes in the presence of the four tested bacteria revealed a complex pattern in which the observed up- or down-regulation was only partially correlated with growth or in vitro virulence and mainly suggested that L. monocytogenes may display a microorganism-specific transcriptional response. Importance Listeria monocytogenes is the etiological agent of the severe foodborne disease listeriosis. Important insight regarding the physiology and the infection biology of this microorganism has been acquired the past twenty years. However, despite the fact that L. monocytogenes co-exists with various microorganisms throughout its lifecycle and transmission from the environment to foods and then to the host, there is still limited knowledge related to the impact of surrounding microorganisms on L. monocytogenes biological functions. In this study we showed that L. monocytogenes modulates specific biological activities (i.e. growth and virulence potential) as a response to co-existing microorganisms and differentially alters the expression of virulence-associated genes when confronted with different bacterial genus and species. Our work suggests that the interaction with different bacteria plays a key role in the survival strategies by L. monocytogenes and supports the need to incorporate biotic factors in the research conducted for the identification of mechanisms deployed by this organism to be established in different environments.
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