Genotyping and antimicrobial resistance of Staphylococcus aureus isolates from dairy ruminants: differences in the distribution of clonal types between cattle and small ruminants

Staphylococcus aureus is part of the normal flora of animals, and represents one of the leading causes of contagious mastitis in dairy herds worldwide. Sixty-seven epidemiologically unrelated S. aureus isolates from nasal and mastitis milk samples of dairy-producing animals (32 cows, 25 sheep, and 10 goats) were characterized by antimicrobial susceptibility testing and spa typing followed by multilocus sequence typing (MLST) on representative isolates and SCCmec-typing on methicillin-resistant S. aureus (MRSA) isolates. The highest resistance was observed to penicillin (64.2%, 43/67), followed by tetracycline (23.9%, 16/67), erythromycin (22.4%, 15/67), and streptomycin (17.9%, 12/67). In general, 18 spa types (including newly identified t16958) and 13 sequence types (STs) belonging to 8 clonal complexes (CCs) were detected. The cow-associated isolates were mainly assigned to CC5 (n = 18, related to t267-ST97, t521-ST352, t527-ST97, t304-ST6, and t084-ST15), followed by CC398 (n = 6, t937-ST291), CC45 (n = 3, t230-ST45), CC88 (n = 2, t2526-ST88), CC22 (n = 2, t3680-ST22), and CC522 (n = 1, t3576-ST522). Small ruminant isolates were mostly clustered into CC522 (n = 29, related to t3576, t1534, t16958, t7308, t7311, t7305 [ST522], t1534-ST2057, and t5428-ST2079). Two isolates from cows with mastitis were found to be MRSA, exhibited a composite profile of t937-ST291-SCCmecIV. No isolates carried the PVL and mecC genes. A significant difference in clonal types of S. aureus isolates from cows in comparison with those from small ruminants was found. This study demonstrated the circulation of diverse clones of S. aureus among dairy animals in Iran, with a different clonal composition between cows and small ruminants. The current study also reports MRSA-related mastitis in dairy cows, emphasizing the need for comprehensive surveillance.