Listeria monocytogenes faecal carriage is common and driven by microbiota

Listeria genus comprises two opportunistic pathogenic species, L. monocytogenes (Lm) and L. ivanovii, and several non-pathogenic species. All can thrive as saprophytes, whereas only pathogenic species cause systemic infections in human and cattle. Identifying Listeria species respective biotopes is critical to understand the ecological contribution of Listeria pathogenic potential. Here, we aimed at detecting Listeria in samples of diverse origins, to highlight ecological differences between pathogenic and non-pathogenic species. We retrieved 16S rDNA datasets from the metagenomics MG-RAST database and determined the prevalence and abundance of Listeria species in various sources. Overall, Listeria was detected in 14% of datasets. Lm was the most prevalent species, most abundant both in soil and host-associated environments, including in 5% of human stools. Lm was also detected in 10% of human stool samples from an independent cohort of 900 healthy asymptomatic donors. A specific microbiota signature was associated with Lm faecal carriage in human, as well as in experimentally inoculated mice, in which it preceded Lm long-term gut colonization, indicating that gut microbiota composition influences Lm faecal carriage. These results suggest that asymptomatic faecal carriage, rather than disease, exerts purifying selection on Lm "virulence genes".