Antiphagocytic effect of the capsule of Streptococcus uberis.

Non-opsonized encapsulated and non-encapsulated strains of Streptococcus uberis were incubated with bovine mammary macrophages and the percentage of phagocytosis and intracellular killing were determined. Seventy-seven percent of macrophages ingested non-encapsulated organisms with a killing rate of 75%. In contrast, 48% of macrophages ingested encapsulated bacteria with a killing rate of 35%. When strains were opsonized with homologous antiserum, differences were detected in the percentage of phagocytosis (84% vs. 48%) and intracellular killing (52% vs. 35%) of the encapsulated strain only. Effects were partially abolished when antiserum was absorbed with purified capsule, or when macrophages were pre-treated with purified capsular material. Electron microscopy of mammary macrophages incubated with the encapsulated strain of S. uberis showed the microorganism in contact with the macrophage cell membrane without signs of membrane activation. In contrast, the non-encapsulated strain induced formation of pseudopods and membrane ruffling. These results suggest that capsule may protect S. uberis from phagocytosis which may be due to a direct interaction of capsular material with macrophages.