Isolation, Identification, and Molecular Genetic Characteristics of a Pathogenic Strain of Streptococcus suis Serotype 3

Streptococcus suis (S. suis) is considered as one of the most crucial bacterial pathogens that leads to serious economic losses to the swine industry. Different S. suis serotypes exhibit diverse characteristics in population structure and pathogenicity. Epidemiology data underscore the importance of S. suis serotype 3 (SS3). However, except for a few epidemiological information, limited study information is available on this serotype. Herein, a pathogenic SS3 (the S. suis strain YA) was isolated from infected piglets in clinical practice, and then whole genome sequencing and analysis, hemolytic activity, antimicrobial susceptibility, pathogenicity to mice and piglets were conducted. The results of the whole genome sequencing of the S. suis strain YA showed that the complete genome was 2,167,682 bp in length with a G + C content of 41.2% and exhibited a unique sequence type (ST1801). The result of phylogenetic tree showed that it was most closely related to strain DNC15 and 6407 (ST54) from Denmark. The tet(W) and erm(B) resistant genes were identified in the S. suis strain YA by inserting into rum locus, in accordance with the result of resistance to tetracyclines and macrolide-lincosamide-streptogramin antibiotics. Twenty-seven key virulence factors were detected in the S. suis strain YA, including sly, ef and mrp, which contribute to pathogenicity in mice and piglets, causing bleeding and congestion in multiple tissue organs especially in the brains. And the LD50 value for mice was 1.54 x 107 CFU. Therefore, our research emphasizes the importance of understanding SS3, and provides valuable information for the scientific prevention and control of S. suis.