Genotypic diversity of Streptococcus suis and the S. suis-like bacterium Streptococcus ruminantium in ruminants

<jats:title>Abstract</jats:title><jats:p>Although <jats:italic>Streptococcus suis</jats:italic> has attracted public attention as a major swine and human pathogen, this bacterium has also been isolated from other animals, including ruminants. However, recent taxonomic studies revealed the existence of other species that were previously identified as <jats:italic>S. suis</jats:italic>, and some of these isolates were reclassified as the novel species <jats:italic>Streptococcus ruminantium</jats:italic>. In Japan, biochemically identified <jats:italic>S. suis</jats:italic> is frequently isolated from diseased ruminants; however, such isolates have not yet been identified accurately, and their aetiological importance in ruminants is unclear. Therefore, to understand the importance of <jats:italic>S. suis</jats:italic> and <jats:italic>S. suis</jats:italic>-like bacteria in ruminants, we reclassified <jats:italic>S. suis</jats:italic> isolates from ruminants according to the updated classification and investigated their genetic diversity. Although both <jats:italic>S. suis</jats:italic> and <jats:italic>S. ruminantium</jats:italic> were isolated from healthy and diseased ruminants, most of the isolates from diseased animals were <jats:italic>S. ruminantium</jats:italic>, implying that <jats:italic>S</jats:italic>. <jats:italic>ruminantium</jats:italic> is more likely to be associated with ruminant disease than <jats:italic>S. suis</jats:italic>. However, the ruminant <jats:italic>S. suis</jats:italic> and <jats:italic>S. ruminantium</jats:italic> isolates from diseased animals were classified into diverse genotypes rather than belonging to certain clonal groups. Genome sequence analysis of 20 <jats:italic>S</jats:italic>. <jats:italic>ruminantium</jats:italic> isolates provided information about the antibiotic resistance, potential virulence, and serological diversity of this species. We further developed an <jats:italic>S</jats:italic>. <jats:italic>ruminantium</jats:italic>-specific PCR assay to aid in the identification of this bacterium. The information obtained and the method established in this study will contribute to the accurate diagnosis of ruminant streptococcal infections.</jats:p>