Streptococcus canis Are a Single Population Infecting Multiple Animal Hosts Despite the Diversity of the Universally Present M-Like Protein SCM

Abstract

<i>Streptococcus canis</i> is an animal pathogen which occasionally causes infections in humans. The <i>S. canis</i> M-like protein (SCM) encoded by the <i>scm</i> gene, is its best characterized virulence factor but previous studies suggested it could be absent in a substantial fraction of isolates. We studied the distribution and variability of the <i>scm</i> gene in 188 <i>S. canis</i> isolates recovered from companion animals (<i>n</i> = 152), wild animal species (<i>n</i> = 20), and humans (<i>n</i> = 14). Multilocus sequence typing, including the first characterization of wildlife isolates, showed that the same lineages are present in all animal hosts, raising the possibility of extensive circulation between species. Whole-genome analysis revealed that <i>emm</i>-like genes found previously in <i>S. canis</i> correspond to divergent <i>scm</i> genes, indicating that what was previously believed to correspond to two genes is in fact the same <i>scm</i> locus. We designed primers allowing for the first time the successful amplification of the <i>scm</i> gene in all isolates. Analysis of the <i>scm</i> sequences identified 12 distinct types, which could be divided into two clusters: group I (76%, <i>n</i> = 142) and group II (24%, <i>n</i> = 46) sharing little sequence similarity. The predicted group I SCM showed extensive similarity with each other outside of the <i>N</i>-terminal hypervariable region and a conserved IgG binding domain. This domain was absent from group II SCM variants found in isolates previously thought to lack the <i>scm</i> gene, which also showed greater amino acid variability. Further studies are necessary to elucidate the possible host interacting partners of the group II SCM variants and their role in virulence.