Isolation and characterization of a Streptococcus suis serotype 9 from a wild cat

Abstract:

[Objective] Streptococcus suis (S. suis) is an emerging zoonotic pathogenic bacterium capable of infecting piglets and human and with sporadic infections in a variety of mammalian species. The aim of this study is to investigate the prevalence of S. suis in wild cats. [Methods] We isolated an S. suis strain from a wild cat. We tested the serotype of the isolated strain by anti-serum agglutination and PCR. We determined the sequence type (ST) of the isolated strain by multilocus sequence typing tests (MLST). We constructed the 16S rRNA phylogenetic tree of the isolation and S. suis strains in NCBI database to demonstrated genetic relationship of different strains. We measured the antibiotic resistance of the isolated strain by triple disk diffusion method. We detected the virulence of the isolated strain by mice infection experiments. [Results] We isolated an S. suis strain m70 from a wild cat, which belongs to serotype 9. MLST showed that m70 fell into a new ST. The 16S rRNA phylogenetic tree of m70 and S. suis strains in NCBI database demonstrated that m70 was in a separate cluster. m70 was resistant to tetracycline, intermediate to erythromycin, and sensitive to ampicillin, corresponding to clinical S. suis isolates in China. The mortality of mice infected with 108 CFU of m70 was achieved 60%-80% (3/5-4/5). The mean LD₅₀ of mice infected with m70 was 5.1×10⁷ CFU, while the mean LD₅₀ of virulent S. suis strain HA9801 was 3.9×10⁷ CFU. There is no significant difference between the LD₅₀ of the two strains(P<0.05). [Conclusion] We isolated an S. suis strain from a wild cat, which belongs to the prevalent serotype and was a virulent strain, indicating the potential of transmission of S. suis from wild cats to humans, especially some prevalent serotype strains.