Isolation of encapsulated strains of Staphylococcus aureus from bovine mastitis milk.

Among 1598 strains of Staphylococcus aureus isolated from bovine mastitis milk samples, 19 exhibited a diffuse-type growth using the serum-soft agar (SSA) technique. By the application of a capsular typing method using the SSA technique, they could be separated into five different serological types. Mouse virulence of individual strains varied significantly. Two strains showed high mouse virulence but the majority showed relatively low virulence or were avirulent. The avirulent strains and those of low virulence were capable of absorbing protective antibodies from rabbit antisera to type strains of human encapsulated S aureus. These results suggested that encapsulated strains of S aureus may play a part in the pathogenesis of bovine mastitis.