Antibiotic resistance and enterotoxin gene profiles of Staphylococcus aureus isolated from raw milk in Iran

Staphylococcus aureus is a food-borne pathogen frequently isolated from raw milk all over the world. The emergence of antibiotic resistant strains of S. aureus has become an important global public health issue. In spite of this problem, the contamination rate of raw milk with enterotoxigenic and antibiotic resistant S. aureus strains has been rarely evaluated in Iran. This study aimed to evaluatethe prevalence and antibiotic resistance ofS. Aureus strains isolat-ed from bulk tank milk samples in Mashhad, and evaluate the carriage frequency of genes encoding the staphylococcal enterotoxins (SEs). For this, a total of 250 milk samples were tested for S. aureus by the conventional plating method and PCR. The disc diffusion method was used to assess the antibiotic susceptibility of the isolated S. aureus. Finally, the PCR method was used for the detection of staphylococcal enterotoxin genes.S. aureus was detected in 46 (18.4%) of the collected bulk tank milk samples with mean concentrations of 3.5 x 105 CFU/ml. Among the tested isolates, 86.96% (40/46) were resistant to at least one antibiotic, and 15% (6/40) were detected as multidrug resistant strains. The highest resistance rate was observed against penicillin (65.22%), ampicillin (41.31%), and methicillin (41.31%). Overall,71.74% of the isolates harbored at least one of the SEs encoding genes.The sec and sea genes were the preva-lent ones.In conclusion, the prevalence of enterotoxigenic and antibiotic-resistant S. aureusin the milk samples collect-ed from Mashhad dairy farms shows the need of implementing strict hygiene practices and rational antibiotic usage.