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

Streptococcus equi subsp. zooepidemicus (SEZ) causes disease in various animals and humans. In pigs, it is an emerging pathogen causing 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 mechanisms of survival 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 comparable high levels of IgM binding to the bacterial surface. Comparison with the isogenic szm deletion mutant showed that SzM_C33 expression is crucial for survival in porcine blood. Flow cytometry analysis revealed that SEZ C33 is associated with porcine blood granulocytes exhibiting prominent generation of reactive oxygen species. Cytochalasin D inhibited this association significantly. Furthermore, we found viable intracellular SEZ C33 in a penicillin/streptomycin protection assay in porcine blood and in purified porcine granulocytes in agreement with microscopic detection of streptococci in neutrophils. Experimental infection with SEZ C33 confirmed 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, demonstrates SzM-mediated, intracellular survival in porcine blood granulocytes in vitro which is most likely involved in the pathogenesis of disease such as meningitis and arthritis in vivo.