Comparison of Virulence Levels in Streptococcus Strains

In our previous study, the virulence levels of S. parasuis clinical strains BS26 and BS27 were similar [7 (link)]. S. parasuis clinical strains BS26 and NN1 were selected in the present study. For comparison, the highly pathogenic S. suis strain P1/7 (serotype 2 and sequence type 1) isolated from a pig with fatal meningitis [16 (link)] was included as a control. The strains were grown overnight on Columbia blood agar base plates (Oxoid, London, UK) under 5% CO₂ at 37 °C, and five isolated colonies per strain were inoculated into 5 mL of Todd–Hewitt broth (THB; Oxoid) without shaking under 5% CO₂ at 37 °C until the OD600 value reached 0.6, which corresponded to 1 × 10⁹ CFU/mL, 8 × 10⁸ CFU/mL, and 5 × 10⁸ CFU/mL for P1/7, BS26, and NN1, respectively. The strains were washed twice in phosphate-buffered saline (PBS) (pH 7.4; Gibco) before infection. Serial dilutions of the bacterial suspension were plated onto THB agar plates to determine the final CFU/mL for each experiment.