Expression of a pseudocapsule by Staphylococcus aureus: influence of cultural conditions and relevance to mastitis.

Strains of Staphylococcus aureus from cases of mastitis in ruminants were cultured in various media and the cells examined electron microscopically for a pseudocapsule. Organisms grown inside the udder, or in nutrient broth supplemented with ovine, bovine or caprine milk whey produced a large, well-defined pseudocapsule outside the cell wall, but such cells had no true capsule when tested by the India ink technique. Modified staphylococcus 110 medium induced secretion by the organisms of copious extracellular slime but did not induce the pseudocapsule. Organisms grown in the presence of milk whey showed a strong propensity to autoagglutinate and produced pseudocapsular material which contained antigens in common with staphylococci grown in vivo. Supplementing media with lactose, dextrose or casein failed to modify the cell surface of S aureus in a manner analogous to that induced by ruminant whey.