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Antiphagocytic effect of the capsule of Streptococcus uberis
1Department of Animal Science, University of Tennessee, Knoxville 37901-1071.
Abstract:
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.
Insights
The capsule of Streptococcus uberis protects it from being engulfed and killed by bovine mammary macrophages. This bacterial capsule may directly interfere with macrophage immune responses.
Area of Science:
- Veterinary Immunology
- Bacteriology
- Cellular Microbiology
Background:
- Streptococcus uberis is a significant cause of bovine mastitis.
- Bovine mammary macrophages are crucial for controlling intramammary infections.
- The role of the S. uberis capsule in evading host immune responses is not fully understood.
Purpose of the Study:
- To investigate the effect of Streptococcus uberis capsule on phagocytosis and intracellular killing by bovine mammary macrophages.
- To elucidate the mechanism by which the S. uberis capsule may inhibit macrophage functions.
Main Methods:
- Incubation of encapsulated and non-encapsulated S. uberis strains with bovine mammary macrophages.
- Quantification of phagocytosis and intracellular killing percentages.
- Opsonization experiments using homologous antiserum.
- Absorption of antiserum and pre-treatment of macrophages with capsular material.
- Electron microscopy to visualize bacterial-macrophage interactions.
Main Results:
- Non-encapsulated S. uberis showed higher phagocytosis (77%) and killing rates (75%) compared to encapsulated strains (48% phagocytosis, 35% killing).
- Opsonization enhanced phagocytosis and killing of the encapsulated strain, but this effect was reduced by capsule absorption or pre-treatment.
- Electron microscopy revealed capsule-mediated inhibition of macrophage membrane activation (pseudopod formation, ruffling) during interaction with encapsulated S. uberis.
Conclusions:
- The capsule of Streptococcus uberis significantly inhibits phagocytosis and intracellular killing by bovine mammary macrophages.
- Capsular material may directly interact with macrophages, impairing their ability to engulf and destroy the bacteria.
- These findings highlight the capsule as a key virulence factor for S. uberis in bovine mastitis.