The contribution of a capsule to survival of staphylococci within bovine neutrophils.

The encapsulated strain M of Staphylococcus aureus and the encapsulated strain 76 of S. simulans survived incubation with bovine neutrophils in media containing heated bovine serum 0.5% v/v or milk whey 10% v/v. With two unencapsulated strains of S. aureus, M12 and M60, less than 5% of the original inoculum survived. When the concentrations of bovine serum and milk whey were increased to 2% and 50% respectively, survival of the encapsulated strains was similar to that of the unencapsulated strains. The ability of skimmed milk 50% v/v to opsonise strain M was found in milk from 95% of 62 cows. The individual values for percentage intracellular survival of strains of S. aureus phagocytosed by bovine neutrophils in the presence of bovine serum 2% v/v or milk whey 50% v/v were 0.48, 0.30 and 0.24 for strains M60, M and M12 respectively. Intracellular survival of strain M60 was significantly greater than that of strains M12 and M. The encapsulated coagulase-negative strain 76 (S. simulans) was not susceptible to lysis by lysostaphin, and it was not possible to destroy extracellular staphylococci to measure intracellular survival. It is concluded that the capsule of S. aureus strain M does not contribute to intracellular survival.