Genotypic Characterization of Streptococcus canis Isolated from Distinct Hosts with Special Emphasis on Multilocus Sequence Typing

Background: The animal pathogen Streptococcus canis is increasingly being noticed in human infections. Our aim was to develop a new multilocus sequence typing (MLST) scheme for Streptococcus canis and to compare isolates recovered from house pets and humans, in order to define the clonal structure of the S. canis population and explore the zoonotic potential of distinct S. canis genetic lineages. Methods: Eighty-five S. canis isolates recovered from infections in animals (n = 78, recovered from 2000 to 2010 in three European countries, mainly from house pets) and humans (n = 7, recovered from 2006 to 2010 in Portugal) were studied. Isolates were identified by API 20 Strep, 23S rRNA gene targeted PCR and 16S rRNA gene sequencing, and characterized by MLST, pulsed-field gel electrophoresis (PFGE) and emm typing. Results: All isolates were successfully typed with the proposed MLST scheme, indicating its applicability to S. canis from distinct sources. The MLST analysis showed a polyclonal structure of the S. canis population, where the same genetic lineages are found infecting house pets and humans and are disseminated in distinct geographic locations. PFGE confirmed the MLST findings, as it identified the same prevailing lineages and further strengthened the similarity between animal and human isolates. Phylogenetic analysis conducted with the 16S rRNA and MLST loci sequence data indicated that S. canis was a divergent taxon of the sister species Streptococcus pyogenes and Streptococcus dysgalactiae subsp. equisimilis, and found evidence of acquisition of genetic material by S. canis from the latter species. The presence of emm-like genes was restricted to a few isolates and correlated with MLST defined genetic lineages. Conclusion: Our data shows that S. canis isolated from house pets and humans are a single population, and demonstrates that isolates belonging to the main genetic lineages identified are able to infect the human host, providing strong evidence for the zoonotic nature of S. canis infection in humans. A MLST database for S. canis was established at http://pubmlst.org/scanis/ (hosted by the Department of Zoology, University of Oxford, United Kingdom), constituting a valuable tool for future studies on the molecular epidemiology of this pathogen.