A zoonotic Streptococcus equi subsp. zooepidemicus strain survives in activated blood neutrophils of pigs through SzM expression.

Streptococcus equi subsp. zooepidemicus (SEZ) causes diseases in various animals and humans. In pigs, it is an emerging pathogen that causes septicemia and other severe pathologies. SEZ expresses the M-like protein SzM, which recruits host proteins such as fibrinogen to the bacterial surface. The objective of this study was to investigate the survival mechanism of a zoonotic SEZ strain in porcine blood. The zoonotic SEZ C33 strain proliferated in porcine blood of 10-week-old piglets in vitro despite comparably high levels of IgM binding to the bacterial surface. Comparison with the isogenic szm deletion mutant showed that SzM_C33 expression is crucial for porcine blood survival. Flow cytometry analysis revealed that SEZ C33 was associated with porcine blood granulocytes, exhibiting a prominent generation of reactive oxygen species. Cytochalasin D treatment significantly inhibited this association. Furthermore, we found viable intracellular SEZ C33 in a penicillin/streptomycin protection assay in porcine blood and purified porcine granulocytes, in agreement with the microscopic detection of streptococci in neutrophils. Experimental infection with SEZ C33 confirmed the virulence of this strain in pigs, including one case of fibrino-suppurative meningoencephalitis and tarsitis. In conclusion, SEZ C33, originally isolated from a diseased human, demonstrated SzM-mediated intracellular survival in porcine blood granulocytes in vitro and is most likely involved in the pathogenesis of diseases such as meningitis and arthritis in vivo.