Antimicrobial Resistance of Escherichia coli Isolates from Mastitic Milk and its Possible Relationship with Resistance and Virulence Genes

Mastitis caused by Escherichia coli is a notable threat to dairy industry due to its high genetic variability, enormous number of environmental sources and increased resistance to antibiotics. The present study was aimed to determine the antimicrobial resistance of E. coli isolates from bovine mastitis and identification of antimicrobial resistance and virulence genes associated with them. Antimicrobial susceptibility testing was performed by disk diffusion method. Resistance and virulence genes were detected by PCR. The results showed that 100% of isolates were resistant to penicillin, 54% to ampicillin, 44% to tetracycline and 30% to streptomycin, while none of them was resistant to chloramphenicol. These E. coli isolates carried one or more antimicrobial resistance genes. Genes present with highest frequency were tetA (42%), tetB (28%) and ampC (26%). Fewer E. coli isolates carried tetD (10%), tetE (8%) and tetG (8%) genes. None of the isolates was positive for bla2 resistance genes. PCR results of virulence genes confirmed that 66% of strains were carrying the traT gene, 26% the Shiga toxin gene and 16% the intimin (eae) gene, while all strains were negative for aerobactin gene. Conclusively, E. coli isolates were resistant to at least two or more antibiotics, irrespective of presence or absence of relevant resistance and virulence genes.