Genotyping of Staphylococcus aureus from clinical mastitis in a commercial dairy farm

The epidemiology of intramammary infections is specific of each farm as well as the distribution of pathogens causing clinical mastitis. Staphylococcus aureus is an important contagious pathogen that diffuses throughout a herd essentially during milking. However, new infections can occur depending on strains specificities and management. Milk samples from clinical mastitis were collected in one commercial dairy farm for twenty-two months and cultured according to NMC guidelines. The farm consisted of 150 lactating cows. S. aureus isolates were genotyped by 16-23S rRNA intergenic spacer region (RSPCR) and characterized by PCR for detection of a panel of virulence factors. A total of 96 isolates were tested. In spite of the different phenotypes at culture, one genotype (GTB) resulted prevalent (77/96), 17 isolates belonged to a subtype of GTB and two isolates to a novel genotype. All the isolates were positive for enterotoxins A, D and J, and negative for Panton Valentine Leukocidin, mecA gene and Toxic shock syndrome toxin. The results of RSPCR on this farm confirm the diffusion of a restricted number of strains, consistent with contagiousness, and a notable prevalence of GTB. The virulence patterns are typical of bovine strains.