Figure 4

Killing of S. suis strains in porcine blood (A) is associated with the deposition of C3 (B) on the bacterial surface. Survival of the different strains was determined in heparinized blood samples of 8-week-old piglets in vitro (A). The specific bacterial contents (CFU/mL) were determined through plating of serial dilutions after 0 min and 120 min of incubation at 37 ℃. The survival factor (SF) represents the ratio of the CFUs after 120 min to the CFUs at time zero. The FI of C3 deposition on the bacterial surface (B) of the indicated S. suis strains after incubation in plasma samples obtained from the blood used also in the bactericidal assay shown in A. Staining of C3 was conducted with a cross-reactive FITC-conjugated anti-human C3c antibody and measured by flow cytometry. HIPs (heat-inactivated plasma) was used as a negative control. Flow cytometric data and survival data was analyzed using Friedman test followed by Dunn’s multiple comparisons test and is presented as mean values with SDs. Error bars represent SD and are too small to be visible outside the symbol when not present. Significant differences are indicated with the star symbols (*p < 0.05, **p < 0.01, and ***p < 0.001). Spearman correlation was calculated between C3 deposition on the streptococcal surface with the respective SFs of the indicated S. suis strains in blood. The correlation between SFs and the FI of the samples (C) is shown.