Characterisation of Staphylococcus aureus strains isolated from mastitis bovine milk in Argentina

The study reported in this Research Communication was conducted to characterise Staphylococcus aureus isolates recovered from mastitic bovine milk from dairy herds in Argentina. A total of 829 mastitic milk samples, both clinical and subclinical, were collected from 21 farms by veterinarians and submitted to the laboratory for testing from which 229 S. aureus isolates were recovered, an isolation rate of 281%. These isolates were tested for susceptibility to the antibiotics penicillin, erythromycin and clindamycin. Of the 229 isolates, 53 (231%) were resistant to penicillin, 31 (135%) to erythromycin and 28 (122%) to clindamycin. All isolates were negative for the mecA, mecC and pvl genes by PCR. Southernblot hybridisation revealed that the ermC gene was located on plasmid bands. Eighty isolates were randomly selected from the 229 for further characterisation. Restriction analysis of chromosomal DNA with Cf9I followed by PFGE of the 80 isolates revealed 23 distinct pulsotypes at 80% similarity. Seven major types (A, B, N, P, S, T, U and V) accounted for 687% of these isolates and 12 pulsotypes (A, B, F, G, J, K, M, N, P, S, T and U) occurred on more than one farm indicating genetic diversity within the farms. MLST of a representative isolate from dominant types identified the STs 97 705, 746, 2102 and 2187 with ST97 being the most predominant. Antibiotic susceptibility testing showed that 537% of the 80 randomly selected isolates were resistant to at least one of the three antibiotics tested. To our knowledge, this study represents the first large scale molecular studies on S. aureus isolates from dairy farms in Argentina.