Streptococcus suis encodes multiple allelic variants of a phase-variable Type III DNA methyltransferase, ModS, that control distinct phasevarions

Streptococcus suis is a significant cause of bacterial meningitis in humans, particularly in S.E. Asia, and is a leading cause of respiratory and invasive disease in pigs. Phase-variable DNA methyltransferases, associated with Restriction-Modification (R-M) systems, are a source of epigenetic gene regulation, controlling the expression of multiple genes. These systems are known as phasevarions (phase-variable regulons), and have been characterised in many host-adapted bacterial pathogens. We recently described the presence of a Type III DNA methyltransferase in S. suis, ModS, which contains a simple sequence repeat (SSR) tract within the open reading frame of the modS gene, and which varied in length between individual strains. We also observed multiple allelic variants of the modS gene were present in a population of S. suis isolates. Here, we demonstrate that a biphasic ON-OFF switching of expression occurs in the two most common ModS alleles, ModS1 and ModS2, and that switching is dependent on SSR tract length. Further, we show that ModS1 and ModS2 are active methyltransferases in S. suis using Single-Molecule, Real Time (SMRT) sequencing. ON-OFF switching of each ModS allele results in the regulation of distinct phasevarions, with the ModS2 phasevarion impacting growth patterns and antibiotic resistance. This is the first demonstration of a phase-variable Type III DNA methyltransferase in a Gram-positive organism that controls a phasevarion. Characterising the phenotypic effects of phasevarions in S. suis is key to understanding pathogenesis and the development of future vaccines. ImportanceStreptococcus suis is a causative agent of meningitis, polyarthritis and polyserositis in swine, and is a major cause of zoonotic meningitis in humans. Here we investigate epigenetic gene regulation in S. suis by multiple phasevarions controlled by the phase-variable Type III DNA methyltransferase ModS. This is the first characterised example of a Type III R-M system regulating a phasevarion in a Gram-positive organism. We demonstrate that biphasic ON-OFF switching of ModS expression results in differences in bacterial growth and antibiotic resistance. Understanding the effects of ModS phase variation is required to determine the stably expressed antigenic repertoire of S. suis, which will direct and inform the development of antimicrobial treatments and vaccines against this important pathogen.