Listeriolysin S Is a Streptolysin S-Like Virulence Factor That Targets Exclusively Prokaryotic CellsIn Vivo

Abstract

Streptolysin S (SLS)-like virulence factors from clinically relevant Gram-positive pathogens have been proposed to behave as potent cytotoxins, playing key roles in tissue infection. Listeriolysin S (LLS) is an SLS-like hemolysin/bacteriocin present among Listeria monocytogenes strains responsible for human listeriosis outbreaks. As LLS cytotoxic activity has been associated with virulence, we investigated the LLS-specific contribution to host tissue infection. Surprisingly, we first show that LLS causes only weak red blood cell (RBC) hemolysis in vitro and neither confers resistance to phagocytic killing nor favors survival of L. monocytogenes within the blood cells or in the extracellular space (in the plasma). We reveal that LLS does not elicit specific immune responses, is not cytotoxic for eukaryotic cells, and does not impact cell infection by L. monocytogenes. Using in vitro cell infection systems and a murine intravenous infection model, we actually demonstrate that LLS expression is undetectable during infection of cells and murine inner organs. Importantly, upon intravenous animal inoculation, L. monocytogenes is found in the gastrointestinal system, and only in this environment LLS expression is detected in vivo. Finally, we confirm that LLS production is associated with destruction of target bacteria. Our results demonstrate therefore that LLS does not contribute to L. monocytogenes tissue injury and virulence in inner host organs as previously reported. Moreover, we describe that LlsB, a putative posttranslational modification enzyme encoded in the LLS operon, is necessary for murine inner organ colonization. Overall, we demonstrate that LLS is the first SLS-like virulence factor targeting exclusively prokaryotic cells during in vivo infections.

Hyperinvasiveness of Listeria monocytogenes sequence type 1 is independent of lineage I‐specific genes encoding internalin‐like proteins

“…Single CFU data are presented as dots, bars indicate the mean and error bars indicate the standard error of the mean (SEM). Statistical analysis (nonparametric Kruskal-Wallis test followed by Dunn's multiple comparison) did not reveal any significant difference between deletion mutants (LMJF5203_Δ00388, LMJF5203_Δ02767, LMJF5203_Δ02537, and LMJF5203_Δ01291) and parental strain were shown to be involved in in vivo infection, while no particular phenotype could be attributed to these virulence factors in in vitro infections of cell lines (Quereda, Andersson, Cossart, Johansson, & Pizarro-Cerda, 2018;Quereda et al, 2017;Rupp et al, 2017;Sabet et al, 2008). Therefore, we cannot fully rule out an impact of these four genes on the infectious process either in other cell types, which were not represented in our study, or in more complex physiological systems including the immune system and host barriers where such membrane proteins may exhibit moonlighting functions on the bacterial cell surface (Copley, 2012).…”

“…Certainly, in vitro infection assays are limited tools for the study of virulence factors as they do not reflect all aspects of the infectious process in vivo. Indeed, other virulence factors ( llsB , inlJ ) were shown to be involved in in vivo infection, while no particular phenotype could be attributed to these virulence factors in in vitro infections of cell lines (Quereda, Andersson, Cossart, Johansson, & Pizarro‐Cerda, ; Quereda et al, ; Rupp et al, ; Sabet et al, ). Therefore, we cannot fully rule out an impact of these four genes on the infectious process either in other cell types, which were not represented in our study, or in more complex physiological systems including the immune system and host barriers where such membrane proteins may exhibit moonlighting functions on the bacterial cell surface (Copley, ).…”

Bacteriocins of Listeria monocytogenes and Their Potential as a Virulence Factor

“…The genetic similarity of LLS to SLS, which is a major virulence factor of S. pyogenes, led to the intensive investigation of the effects of LLS on virulence using various animal models [23,25,30,[38][39][40]. The initial evidence of the contribution of LLS to L. monocytogenes virulence was obtained via mouse intraperitoneal infection model.…”

“…Later, the role of LLS in L. monocytogenes deep organ colonization was specifically examined with the mouse intravenous infection model, in which the intestinal lumen was not involved in infection and only inner organs were infected [40]. In mice intravenously infected with the llsA deletion strain, bacterial loads did not change in spleens or livers compared with the F2365 WT strain, suggesting that the role of LLS is limited to the intestine and is not essential for L. monocytogenes infection once this pathogen crosses the intestinal barrier [40].…”

Subcutaneous vaccination with a live attenuated Yersinia pseudotuberculosis plague vaccine

“…3). Although it seems trivial that skin is not directly connected to the intestine, the transit of bacteria via blood could have redirected it to unexpected organs, as was observed for Listeria monocytogenes moving from blood to the intestine [20]. The absence of vaccine in feces demonstrates that it did not reach the gut lumen, and that fecal dissemination of this GMO in nature is very unlikely.…”