WGS analysis and virulence of Streptococcus suis serotype 4 ST1689 isolated from an asymptomatic pig, Thailand

Streptococcus suis causes severe disease in pigs and is implicated increasingly in human infections, posing a substantial zoonotic threat to global public health, with many human and pig infections being caused by S. suis serotype 2, especially strains belonging to clonal complex (CC) 1. However, infections due to non-serotype 2 and non CC1 have been increasing worldwide, especially CC94. This study investigated the genome characteristics and virulence capacity of S. suis serotype 4 ST1689. Whole genome sequencing, cytotoxicity, adhesion, invasion, apoptosis, hemolytic assay, and a mouse experiment were conducted. The genomic analysis revealed that serotype 4 ST1689 strain 3453 was classified into minimum core-genome group 3 and that the serotype 4 isolate was closely related with the porcine strains from Thailand. Furthermore, this strain carried genes conferring resistance to macrolide (ermB), tetracycline tet(O), aminoglycoside (spw), and lincosamide (lsa(E), lnu(B)). This strain was multidrug resistant with resistance to penicillin, cefepime, cefotaxime, azithromycin, erythromycin, and tetracycline. β-lactam resistance in this strain was associated with the mutations in the PBP1A, PBP2B, and PBP2X proteins. In addition, the adhesion and invasion capacity were significantly lower than S. suis serotype 2 (P1/7). There was a strong cytotoxic, apoptotic effect on the epithelial cell line, similar to strain P1/7. In addition, this strain produced 80 % and 100 % mortality rates in a mouse model within 12 h and 24 h post-infection, respectively. Mice infected with S. suis serotype 4-ST1689 strain exhibited a higher bacteria burden, particular in the blood (p < 0.05), brain (p < 0.001) and kidney (p < 0.01) compared with serotype 2 (P1/7)-infected mice. S. suis serotype 4-ST1689 strain infection induced an acute and extremely high inflammatory cytokine response, including significantly (p < 0.001) increased IL-6, IL-1β, and TNF-α levels. These results suggested that the highly virulent S. suis serotype 4-ST1689 strain induced high levels of pro-inflammatory mediators and invasion of multiple organs, subsequence leading to high mortality. These results should provide important insights into the development of control strategies for transmission of S. suis serotype 4 ST1689 in public and pig farms.