Encapsulation of Staphylococcus aureus isolated from bovine milk

Evidence is presented that nearly all Staphylococcus aureus infections of the bovine mammary gland are by encapsulated organisms (94% of isolates examined). This observation is based on the demonstration of diffuse growth in serum-soft agar, of cultures taken directly from mastitic milk without subculture on artificial media. Only milks containing pure cultures of S. aureus were examined. It was previously reported that only 6% had capsules. Evidence is presented that as few as 3 or 4 subcultures and/or a short storage on artificial media results in loss of encapsulation of most S. aureus of bovine origin. Antisera raised against encapsulate strains inhibit the expression of capsule formation on bacteria that are encapsulated in the presence of normal serum.