Identification of isolates of Streptococcus canis infecting humans

During a survey of Group G and C streptococcal infections of humans two epidemiologically unrelated Group G streptococcal isolates were identified, one from a case of bacteremia and one from a wound infection. These isolates were atypical among this sample in that the emm gene could not be amplified from them by PCR. Biochemical characterization identified the isolates as Streptococcus canis, an organism normally associated with animal hosts. The biochemical identification was confirmed by sequencing of the 16S rRNA gene from both isolates and comparison with sequences of the S. canis type strain and other related streptococci of animals and humans. Comparative sequencing of fragments of two other housekeeping genes, sodA and mutS, confirmed that the isolates are most closely related to S. canis. The identification of two isolates of S. canis from a relatively small sample set suggests that the practice of identifying streptococci only by the Lancefield serological group may result in underestimation of the presence of S. canis in the human population.