Genetic characterization and antimicrobial resistance of Staphylococcus aureus isolated from bovine milk in Tunisia

Staphylococcus aureus is a major agent of bovine mastitis in dairy herds, causing economic losses in dairy industry worldwide. In addition, milk and milk-products contaminated by Staph.aureus can cause harmful human diseases. The aim of this study was to characterize Staph.aureus strains isolated from dairy farms in Tunisia. Bulk tank milk (n=32) and individual cow milk (n=130) samples were collected during the period of 2013-2014. Forty-three Staph.aureus isolates were recovered and typed by spa typing, 16S-23S rRNA intergenic spacer (RS-PCR) and multiplex PCRs for 22 virulence genes. Antimicrobial resistance was also investigated with a disc diffusion test. A selected subsample of 22 strains was additionally genotyped by multilocus sequence typing. Seventeen spa types were recovered, and t2421 (n=10), t521 (n=6) and t2112 (n=5) were the most common. Fourteen different RS-PCR genotypes grouped into 11 clusters were detected in our study, with predominance of the R-VI genotype (n=24). Eight sequence types were identified and Clonal Complex 97, corresponding to RS-PCR cluster R, was the most common (n=10), followed by CC1 (n=4), CC15 (n=3) and other four accounting for one or two strains. Different combinations of virulence genes were reported, and enterotoxin genes were present in few strains (seh, n=4; sea, n=2; sea and seh, n=2; sec and sel, n=2). The majority of strains were resistant only to penicillin; only one strain was found to be multiresistant and no methicillin-resistant Staph.aureus was demonstrated. Our study reported the isolation of CC97 from bovine milk in Tunisia for the first time and confirmed the relevance of this lineage in intramammary infection in cows.