Construction and phenotypic analysis of a low molecular weight tyrosine phosphatase mutant in Streptococcus suis serotype 2

Streptococcus suis ( S. suis ) is a major zoonotic pathogen that causes severe infections in both pigs and humans. Low-molecular-weight protein tyrosine phosphatases (LMWPTPs) are a conserved family of enzymes involved in capsule synthesis and the pathogenicity of various Gram-positive pathogens, but their functional role in S. suis 2 remains unclear. We constructed a homologous Lmwptp deletion mutant (Δ Lmwptp ) in S. suis 2. This mutant exhibited moderate growth retardation during the logarithmic growth phase; transmission electron microscopy revealed a reduction in capsule thickness, and its capsular polysaccharide content was reduced by approximately 12% ( p < 0.05). However, no significant differences were detected in colony morphology, adhesion to Hep-2 cells, anti-phagocytic activity, or virulence in a mouse infection model. In vitro dephosphorylation assays indicated that LMWPTP cannot directly dephosphorylate the bacterial tyrosine kinase (BY-kinase) Cps2BctC. Although LMWPTP possesses tyrosine phosphatase activity and partially affects capsule production, it is not essential for the pathogenicity of S. suis 2. These findings highlight the functional diversity of LMWPTP homologs across different Streptococcus species and underscore the need for caution when extrapolating mechanisms among related pathogens.