A high prevalence of tylosin resistance among Staphylococcus aureus strains isolated from bovine mastitis.

The macrolides appear to have considerable effects for treatment of bovine mastitis because of excellent diffusion into the mammary gland, long half-life, low protein binding, high intracellular concentration and lipid solubility. Acquired resistance to macrolides in Staphylococcus aureus is primarily related to target-site modification through acquisition of an erm gene. In the present study the prevalence of both phenotypic and genotypic tylosin resistance in S. aureus isolates (n = 103) from subclinical mastitis in nine dairy farms belonging to three different province of Iran were investigated. Overall, ermA, ermB and ermC was found in 7.80%, 32.00%, and 20.40% of S.aureus isolates, respectively. A very high percent of isolates (56.90%) were resistant to tylosin. MIC90 and MIC50 values were 64 and 32 µg mL-1, respectively. Most of tylosin resistant isolates did not harbour any erm gene but ermB was dominant gene among 58 tylosin resistant isolates of S. aureus. In overall, tylosin resistance was prevalent in S. aureus isolates obtained from bovine mastitis in Iran.