Absence of encapsulation in strains of Staphylococcus aureus isolated from bovine mastitis

Thirty of 104 strains of Staphylococcus aureus isolated from clinical cases of bovine mastitis in England grew as diffuse colonies in serum soft agar (SSA), 45 grew as mixed diffuse and compact colonies and 29 yielded compact colonies only. The compact strains grew as diffuse colonies in SSA after one passage in the mammary gland of mice. However, none of the strains had an unstained halo when examined by the India ink technique and there was a 99.99 per cent reduction in the viable numbers of the bacteria in 30 representative strains 24 hours after inoculation into the peritoneal cavity of mice. By contrast the truly encapsulated strain M had an unstained halo by the India ink technique and resisted phagocytic killing in the peritoneal cavity. It is concluded that these strains from cases of mastitis are not encapsulated and that growth as diffuse colonies in SSA is not a reliable test of encapsulation.