Development of a Serotyping Scheme for Streptococcus pasteurianus: An Underreported Zoonotic Pathogen.

Streptococcus pasteurianus is an opportunistic and underreported zoonotic pathogen capable of causing severe infections in both humans and animals, and it is also an etiological agent of swine streptococcosis. Despite its clinical and veterinary relevance, no systematic serotyping scheme has previously been established for this species. In this study, six representative strains harboring distinct cps gene clusters were selected for antiserum production, and agglutination assays identified five serotypes, designated serotypes 1a, 1b, 2, 3, and 4. Based on serotype-specific variation within the cps gene cluster, a multiplex PCR (mPCR) serotyping assay was subsequently developed and evaluated using 35 S. pasteurianus strains, including 14 pig isolates, 19 human isolates, and two cattle isolates. Serotype assignments obtained by mPCR were fully concordant with those determined by agglutination assays using rabbit antisera. In addition, transmission electron microscopy (TEM) revealed a visible capsular layer surrounding representative strains of each serotype. Collectively, these findings establish a robust serotyping framework for S. pasteurianus. The mPCR serotyping assay provides a valuable tool for future epidemiological studies, enhances diagnostic capability, and supports improved surveillance and control of S. pasteurianus infections.