Streptococcus suis serovar 9 responses to elevated temperature and co-culture with Glaesserella parasuis.

Streptococcus suis and Glaesserella parasuis are commensal organisms that can shift from a benign to pathogenic state and cause severe disease in swine. We hypothesized that a change in host temperature and/or interactions with G. parasuis could impact S. suis growth dynamics. We compared phenotypic properties of a clinical S. suisserovar 9 strain (SS9C) with clinical serovar 2 and healthy serovar 9 isolates grown at 37°C and 41°C. We further investigated how co-culturing with G. parasuis affected biofilm formation of SS9C. Crystal violet staining indicated that SS9C produced significantly more biofilm than the other strains when grown at 37°C; this difference was amplified at 41°C. However, cell counts did not increase at the higher temperature. Biofilms of SS9C at 37°C and 41°C were unaffected by DNase I digestion, while other strains were both susceptible at 41°C. All biofilms were susceptible to proteinase K and α-amylase digestion at both temperatures. We showed that growth at 41°C increased biofilm formation and shifted the phenotype of SS9C; however, neither increased temperature nor co-culture with G. parasuis increased planktonic or sessile cell counts. Our study suggests that increased temperature in the host may be an important factor in understanding S. suis disease development.