Comparative Genome Analysis of Streptococcus suis Serotype 9 Isolates from China, The Netherland, and the U.K.

Streptococcus suis (S. suis) is an important swine pathogen and an emerging zoonotic agent worldwide. Serotype 9 is the most prevalent serotype in several European countries but it is relatively rare in China. In this study, through the investigation of the serotypes of 279 S. suis strains isolated from China from 2015 to 2017, it was found that serotype 9 is the second most prevalent serotype (43 out of 279), behind serotype 2 (83 out of 279). Next, the 43 serotype 9 isolates were sequenced and compared with those from the Netherland (28) and the U.K. (eight). For the purpose of comparison, the strain D12 (GCA_000231905), which has completed genome sequences, was also incorporated. Phylogenetic tree analysis showed that the strains from China and the U.K. were heterogeneous. In contrast, all but one from the Netherland belonged to the same clade. The dominant clades of Chinese strains (33) and strains from the Netherland (27) were very similar. Both of them may have originated from the same strain about 70 years ago. Then, the distributions of virulence-associated genes and antibiotic resistance genes among different clades and sources were analyzed. By comparison, strains from the Netherland carried more virulence-associated genes and those from the U.K. had more antibiotic resistance genes. Additionally, some virulence-associated genes (salK and salR) and antibiotic resistance genes (lincomycin and spectinomycin) existed only in several Chinese strains. In conclusion, our data displayed the population characteristics and differences of S. suis serotype 9 between China and Europe, suggesting that they have taken different evolutionary paths.