Opsonization of Staphylococcus aureus with a fibronectin-binding protein antiserum induces protection in mice

The virulence of Staphylococcus aureus opsonized with an antiserum raised against a recombinant fibronectin-binding protein (FnBP) was compared with homologous, non-opsonized bacteria (treated with pre-immune serum) in a mouse mastitis model. Virulence was evaluated comparing the number of bacteria removed from the infected mammary glands and according to the type of lesions produced. The average number of bacteria recovered from the mammary glands inoculated with S. aureus opsonized with FnBP-antiserum was significantly lower (up to 10(7) cfu/ml) than the average number of bacteria recovered after inoculation with non-opsonized bacteria (up to 10(10) cfu/ml). Gross examination of infected mammary glands showed that 65% of glands infected with opsonized bacteria developed low grade/or had no pathological changes, and 35% developed severe mastitis whereas, 75% of glands inoculated with non-opsonized bacteria developed severe mastitis and 25% low grade mastitis or had no pathological changes. According to the histopathological examination eight out of 10 glands inoculated with opsonized bacteria produced disseminated focal necrosis or had no pathological changes and two glands produced non reactive necrotic lesions. In contrast, only three out of 10 glands inoculated with non-opsonized homologous bacteria developed disseminated focal necrosis and had no pathological changes while seven glands developed total necrosis.