A comparative study of Staphylococcus aureus strains isolated from bovine subclinical mastitis during 1952-1956 and 1992.

Fifty-two strains of S. aureus isolated from cases of bovine subclinical mastitis in 52 different dairy herds in Denmark, in the periods 1952 to 1956 and 1992, were compared with regard to their phage- and EcoRI ribotypes. Furthermore, susceptibility to penicillin and production of fibrinolysin were used as additional phenotypic markers. Forty-nine strains (94%) could be separated into 12 phage types. Ribotyping assigned the 52 strains to 21 different types. Both methods showed that 57% of the 1950's strains and between 38-45% of the 1992 strains belonged to 3 dominating types. The remaining strains were placed by ribotyping in 8 types occurring among the 1952-1956 strains and 10 types occurring among the 1992 strains. In 87% of the strains the results of the 2 typing methods were in accordance. However, 7 strains gave different results by the 2 methods including 2 strains with major differences. Penicillin resistance only occurred in a single genotype from the 1950's compared to 6 different genotypes among the 1992 strains.