Understanding virulence variability among listeria monocytogenes clonal complexes

Introduction and objective: Listeria monocytogenes is a foodborne pathogen that causes human listeriosis. This pathogen is characterized by an intra-species heterogeneity so, its strains can be grouped into clonal complexes (CCs) defined either as hypervirulent – mainly associated with clinical cases, or hypovirulent – associated with food or food processing environments and causing infections mainly in highly susceptible individuals. This pathogen has a complex infection mechanism represented in Figure 1. A crucial step for infection is the internalization of the pathogen into host cells, which is accomplished by the linkage of internalins (A and B) to host receptors. The differences in virulence potential may be explained by the presence of mutations in key virulence genes. The purpose of this study was to explore the virulence capacity based of L. monocytogenes CCs through in vitro infection assays and to further assess mutations in the inlA gene, which is crucial for the invasion into intestinal epithelial cells by the pathogen. Methodology: Eleven isolates were selected based on the top five hypervirulent CCs occurring in Portugal and one strain from CC4 was also included, as this hypervirulent CC is one of the best characterized CCs around Europe. Five isolates from hypovirulent CCs were also used. Invasion assays were performed in Caco-2 cells and subsequently the presence of PMSC mutations in the inlA gene were assessed, using the MEGA software (version 10.1.8). Results: Our results show a clear-cut difference on invasion capacity between isolates from hyper- and hypovirulent CCs, with the latter showing significantly reduced efficiencies. Sequence analysis of the inlA gene, revealed that only hypovirulent CCs carried PMSC mutations, leading to truncated forms of the InlA protein. A 3-codon deletion was detected in isolates from CC6.