Incorporation of N-acetyl neuraminic acid (NANA), or sialic acid, onto lipooligosaccharide (LOS) enhances the virulence of several bacterial species. In the present study, we assessed the effect of sialylation of Histophilus somni LOS on complement-mediated killing, binding of complement factor H (which converts C3b to inactive C3b (iC3b) and inhibit the alternative complement pathway) to the bacteria, complement activation by the LOS, and phagocytosis and killing of the bacteria by bovine polymorphonuclear leukocytes (PMN). Killing of H. somni by alternative complement pathway was measured by incubation of sialylated or non-sialylated H. somni with antibody-free precolostral calf serum (PCS) followed by viable plate count. A complement dose-dependent response to killing of non-sialylated H. somni by PCS was observed. However, sialylated H. somni were significantly (P = 0.001) more resistant to killing at any of the concentrations of PCS used. Sialylated H. somni LOS activated (P = 0.025) and consumed (P = 0.001) less complement than non-sialylated LOS, as determined by reduction in hemolysis of opsonized sheep red blood cells or rabbit red blood cells, and by western blotting of C3 activation products. Sialylated H. somni bound more factor H than non-sialylated bacteria (determined by enzyme-linked immunosorbent assay) (P = 0.004), supporting the deficiencies observed in complement activation and consumption by sialylated LOS. Sialylation of H. somni inhibited both PMN phagocytosis of 3H-thymidine-labelled bacteria (P = 0.004) and intracellular killing of the bacteria (P = 0.0001), compared to non-sialylated bacteria. Therefore, sialylation of the LOS results in enhanced binding of complement factor H to the bacteria, resulting in diminished complement activation, resistance to complement-mediated lysis, and PMN phagocytosis and killing. / Master of Science