Resistance of Pasteurella multocida to rabbit neutrophil phagocytosis and killing

The susceptibilities of several Pasteurella multocida strains to serum bactericidins and to killing by rabbit polymorphonuclear neutrophils (PMN) were investigated, using in vitro assays. Strain Bunia II (serotype 5,12:E) was killed by both immune and normal rabbit sera containing active complement, and strains R1 (serotype 3,12:A) and 656 (serotype 12:B) were serum resistant. Strain R1 opsonized with immune or normal rabbit serum with complement or with immune antibody alone was phagocytized by neutrophils, and strain 656 was ingested only after opsonization with immune antibody in the presence or absence of complement. Complement alone was ineffective as an opsonin for the last 2 serotypes. Neither isolate was resistant to PMN killing. Growth of strain R1 in subcutaneously implanted chambers in rabbits did not increase the resistance of this organism to PMN. Comparison of phagocytosis and killing of this isolate with 2 virulent rabbit strains of P multocida--strains L-A (serotype 12:A) and 7228 (serotype 14:D)--demonstrated that strain L-A was resistant to destruction by neutrophils, whereas strains 7228 and R1 were killed. Resistance of strain L-A was not associated with the hyaluronic acid capsule.