Phagocytosis of Streptococcus uberis by Bovine Mammary Macrophages: Opsonizing Effect of Bovine Antiserum

Five encapsulated and five non-encapsulated strains of Streptococcus uberis were tested for their ability to resist phagocytosis and intracellular killing by bovine mammary macrophages obtained during the non-lactating period. The percentage of phagocytosis and intracellular killing of encapsulated strains ranged from 43 to 51 and from 25 to 40, respectively. By contrast, the percentage of phagocytosis and intracellular killing of non-encapsulated strains ranged from 66 to 74 and from 65 to 75, respectively. Two encapsulated and two non-encapsulated strains were opsonized with homologous and heterologous pre-immune and immune bovine sera. Phagocytosis and intracellular killing of encapsulated strains increased significantly when opsonized with serum of cows vaccinated with a homologous and heterologous encapsulated strain. By contrast, no significant differences were observed when serum from cows vaccinated with a non-encapsulated strain was used. These results suggest that resistance to phagocytosis is a common feature among encapsulated strains of S. uberis, and that cross-reactivity exists between encapsulated strains.