The streptococcal phase-variable Type I Restriction-Modification system SsuCC20p dictates the methylome ofStreptococcus suisand impacts virulence

Abstract

AbstractPhase-variable Type I Restriction Modification (RM) systems are epigenetic regulatory systems that have been identified in numerous human bacterial pathogens. We previously showed that an emerging zoonotic lineage ofStreptococcus suisacquired a phase-variable Type I RM system named SsuCC20p. The SsuCC20p locus was present in the genome of disease-associated isolates from multiple streptococcal species. This indicates that it is not restricted toS. suisand can be acquired through horizontal gene transfer. We demonstrate that SsuCC20p phase-variability relies on a recombinase present within the locus.In vitro, only SsuCC20p is responsible for the genome methylation profiles that were detected in the representative zoonoticS. suisisolate 861160. In addition, we show that, contrary to previous observations,hsdSgenes located downstream of thehsdMgene and the recombinase gene, can contribute to the SsuCC20p genome methylation profile. SsuCC20p locked mutants expressing a singlehsdSeach showed unique genome methylation profiles. The differential genome methylation of the distinct locked mutants caused phase dependent differences in global gene expression in a growth condition dependent manner. We observed significant differences in virulence betweenhsdSlocked mutants in a zebrafish larvae infection model. These data indicate that the streptococcal phase-variable Type I RM system SsuCC20p can impact bacterial virulence via epigenetic regulation of gene expression and potentially contributes to the zoonotic potential ofS. suis.ImportancePhase-variation contributes to the virulence of bacterial pathogens as it allows a single strain to produce phenotypic diverse subpopulations. Phase-variable Restriction Modification (RM) systems are systems that allow for such phase-variation via epigenetic regulation of gene expression levels. The phase-variable RM system SsuCC20p was found in multiple streptococcal species and was acquired by an emerging zoonotic lineage ofStreptococcus suis. We show that the phase-variability of SsuCC20p is dependent on a recombinase encoded within the SsuCC20p locus. We characterized the genome methylation profiles of the different phases of SsuCC20p and showed that the differential genome methylation within the phases causes differences in gene expression levels and virulence. Altogether, we show that the acquisition of a phase-variable RM system impacts virulence and can potentially contribute to the zoonotic potential ofS. suis. Bacterial pathogens can increase their virulence through acquisition of mobile elements containing epigenetic regulatory systems such as RM systems.