Bactericidal assays. Bacterial killing by murine neutrophils under opsonizing conditions. The data represent OPA antibody titers against SS2, SS7, SS9, and SS1/2 in sera from ten mice orally immunized with the indicated S.Choleraesuis strains 2 weeks after boosting. Serum from individual mouse was serially diluted with twofold to obtain titers. Error bars represent variation between mice. SS2, SS7, SS9, and SS1/2 were incubated at 37 ℃ with 5% serum and porcine neutrophils/polymorphonuclear leukocytes (PMNs) at a 1: 1 (CFU: PMN) ratio for 1 h, and bacterial survival was determined as described in the Materials and Methods. *P < 0.05; ***P < 0.001.