Because the success of treating Staphylococcus aureus (S. aureus) mastitis depends on numerous factors, different cure rates have been reported. Since biofilm production is one of the most critical factors for pathogenicity and antibiotic resistance, this study aimed to assess the ability of S. aureus to produce biofilm and compare antibiotic resistance in biofilm-positive and negative S. aureus strains. Milk samples (n=110) were collected from two herds of cows and subjected to bacteriological analysis. PCR (nucA gene) was used to detect accurately the cause of mastitis. Colorimetric microtiter plate assay was used to evaluate the biofilm formation capacity of the strains. The agar disk diffusion technique was utilised to specify the susceptibility to common antibiotics, including ampicillin, enrofloxacin, tylosin, penicillin, tetracycline, lincomycin, erythromycin, trimethoprim/sulfamethoxazole, oxacillin, ceftriaxone, methicillin, vancomycin, and cefazolin. From isolated S. aureus strains in this study, 31.8% were able to produce biofilm. In general, compared to biofilm-negative isolates, a higher percentage of biofilm-positive strains showed antibiotic resistance to antibiotics used, except for tylosin, cefazolin, and enrofloxacin. Resistance or susceptibility to penicillin, lincomycin, trimethoprim/sulfamethoxazole, oxacillin, methicillin and vancomycin was significantly distinct (P<0.05) between biofilm-positive and negative strains. Positive and negative biofilm groups demonstrated the highest sensitivity against enrofloxacin and cefazolin - most of biofilm-positive (85.7% and 77.1%, respectively) and biofilm-negative isolates (75.4% and 63.1%, respectively) were susceptible to these antibiotics. The present investigation revealed that a high percentage of S. aureus isolates causing bovine mastitis in Iran can form biofilms, so a practical therapeutic approach should be considered

Keywords