Phylogenetic, comparative genomic and structural analyses of human Streptococcus agalactiae ST485 in China

Background: Streptococcus agalactiae (Group B Streptococcus, GBS) is a common bacteria species infecting both human and bovine. Previous studies have shown that the GBS isolated from human and bovine are mostly unrelated and belong to separate populations. However, recently, the bovine GBS CC103 has become the dominant epidemic strain and frequently isolated from human patients. In particular, the ST485 GBS, a member of CC103, has become the new dominant ST in China and exhibited very high pathogenicity. This phenomenon is not consistent with the established understanding about the relationship between bovine and human GBS, which needs to be reinvestigated. Results: The genome-based phylogenetic analysis showed that the human and bovine GBS CC103 strains had very close genetic relationship and they were alternately distributed on the evolutionary tree. CC103 strains evolved into several branches, including the ST485, which exhibited high pathogenicity and specifically infected human. Compared to other CC103 strains, the ST485 lacked Lac.2 gene structure and acquired the CadDX gene structure in their genomes. Conclusions: Our results indicate that GBS CC103 could propagate across human and bovine, and GBS ST485 might evolve from the ST103 that could infect both human and bovine. Moreover, the recombination of Lac.2 and CadDX gene structures might play an important role in the formation of highly pathogenic ST485 in China.