Survival of S. suis serotype 2 in porcine blood is restricted by active complement. (a) Survival of S. suis strain 10 (wt), 10∆ideSsuis (∆ideSsuis) and the two complemented strains 10∆ideSsuis∇ideSsuis_EcoRI (∇ideSsuis_EcoRI) and 10∆ideSsuis∇ideSsuis_C195S (∇ideSsuis_C195S) was analyzed in a whole blood survival assay with the addition of the complement inhibitor vaccinia virus complement control protein (VCP). Porcine whole blood from growing piglets (n = 6) with high anti-S. suis IgM titers was pre-incubated with 0.4, 4, 20 µg VCP/ml for five minutes at 37°C. 4 × 106 CFU/ml of the indicated S. suis strains were then added and the survival factor determined by plate counting after two hours at 37°C. (b) Soluble C5b-9 was determined in porcine plasma after the blood survival assay by sC5b-9 ELISA. A concentration dependent reduction in sC5b-9 levels was detectable after addition of VCP to porcine whole blood (Pearson r values: wt: −0,9487; 10∆ideSsuis: −0,8485; 10∆ideSsuis∇ideSsuis_EcoRI: −0,9999; 10∆ideSsuis∇ideSsuis_C195S: 0,9821). Bars and error bars indicate mean and standard deviation. Significant differences are indicated by asterisks. Probabilities were considered as follows p < 0.05 *, p < 0.01 **, p < 0.001 ***.