Pathogenic characteristics of an unencapsulated Streptococcus suis serotype 31 strain isolated from a patient in Thailand

Streptococcus suis is a zoonotic pathogen that causes invasive infections in individuals who are in close contact with infected pigs or contaminated pork-derived products. An unencapsulated S. suis serotype 31 strain 43640 (sequence type 221) was first isolated from a human in Thailand in 2015; however, the mechanisms underlying host-cell interactions and virulence in animal models of this serotype are still limited. This study aimed to determine the virulence of the human unencapsulated S. suis serotype 31 strain 43640 using in vitro (A549 epithelial cells) and in vivo (C57BL/6 mouse) models. The adhesion rate of the unencapsulated strain 43640 to A549 cells was significantly lower than that of the highly pathogenic S. suis serotype 2 strain P1/7 after infection at multiplicity of infection (MOI) 1 (p < 0.01) and MOI 10 (p < 0.05). The invasion of epithelial cells by S. suis serotype 31 strain 43640 was significantly higher than that of serotype 2 strain P1/7 and the reference strain 92-4172 4 h after infection. S. suis serotypes 31 and 2 significantly induced cytotoxicity in epithelial cells by more than 50% and 90%, respectively, after 4 h and 18 h of infection. S. suis serotype 31 strain 43640 induced apoptosis in A549 cells via a caspase 9-dependent pathway. Using an in vivo mouse model, we demonstrated that S. suis serotype 31 strain 43640 induced 13.33% and 46.67% mortality due to septicemia after 24 h and 48 h, respectively. Bacterial counts in the blood of mice infected with strain 43640 were significantly lower than those in mice infected with the reference strain 12 h and 24 h after infection (p < 0.05). This study provides information on serotype 31-ST221 virulence in in vitro and in vivo models and proves that ST221 (CC221/234), an emerging human S. suis clone in Thailand, is potentially virulent.