Streptococcus suis is a porcine commensal and pathogen with zoonotic potential. We recently identified a novel Type I restriction–modification (R–M) system in a zoonotic S. suis clone which has emerged in the Netherlands. Here, we describe the DNA inversions in the specificity subunit of this system in S. suis serotype 2, clonal complex 20 and explain the absence of domain movement by the absence of repeats. In addition, we identified a core Type I R–M system present in 95% of the isolates and found an association of the distribution of Type I R–M systems in the S. suis genome with population structure. We speculate on the potential role of Type I R–M systems in S. suis given the recently described associations of Type I R–M systems with virulence and propose future research directions.
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