Genotypic variation among Staphylococcus aureus isolates from cases of clinical mastitis in Swedish dairy cows

The main aim of this study was to examine genotypic diversity of 82 Staphylococcus aureus isolates from cases of acute clinical mastitis in cattle using pulsed-field gel electrophoresis, and to investigate pulsotype differences between geographical regions of Sweden. The work also investigated haemolysis type (alpha, alphabeta), beta-lactamase production, and the association between S. aureus pulsotypes and breed, parity and presence of teat lesions. All isolates came from different farms. In total, 25 pulsotypes were found, but most only once or twice. The most common pulsotype accounted for 26% of the isolates and was found in all regions. The distribution of pulsotypes in the southern region of the country differed from the northern and middle regions. Alphabeta- or alpha-haemolysis was found in 66% and 34% of the isolates, respectively, and beta-lactamase production was found in 9% of the isolates. Variation in haemolysis type or beta-lactamase production was found within some pulsotypes. The results indicate that some pulsotypes have spread widely between herds within the country.