The role of the Rhs insertion in Listeria monocytogenes

Listeria (L.) monocytogenes is one of the most concerning human foodborne pathogens as it can cause listeriosis, a rare but severe disease associated with high mortality rates. Contaminated food is the main source of listeriosis. L. monocytogenes can cause self-limiting gastroenteritis in healthy individuals, whereas in immunocompromised and elderly individuals it results in an invasive and systemic infection, leading to meningoencephalitis or septicaemia. In addition, L. monocytogenes can infect pregnant women and the unborn leading to abortion. Since L. monocytogenes can survive multiple stress conditions, it is found in diverse habitants e.g. soil, the food processing environment, food and the host. Not all L. monocytogenes strains are equal. There is variability regarding prevalence, virulence, pathogenicity and stress response. In this project, we focus on one clonal complex, namely CC121, which is the most abundant clonal complex, mainly found in food and food processing environment. We aim to understand why this clonal complex is so abundant in the food producing environment and able to survive for months and years in the same food plant. Our hypothesis is that CC121 strains harbour specific genetic elements, which enable or support their survival under stress conditions in the food producing environment, like cleaning and disinfection. In recent years, we were able to characterize two of these genetic elements, a transposon, which is a mobile genetic element, and a new stress survival islet. In addition, we discovered a large region in the genome of L. monocytogenes CC121 strains, harbouring a protein with rearrangement hotspot (Rhs) repeats, which are conserved motifs. Rhs proteins are known to be involved in bacterial c ompetition and pathogenicity, and function like toxin-antitoxin systems. Our hypothesis is that two toxin-antitoxin systems are located in this genomic region, which we aim to characterize. We speculate that these toxin-antitoxin systems mediate bacterial competition leading to the dominance of the CC121 strains, and support virulence. Although one of most essential virulence factors is nonfunctional in CC121 strains, they are able to cause listeriosis. The reason for this observation is unknown. We hypothesize that these novel toxin-antitoxin systems support infection in this specific strain type.