Phenotypic and genotypic study on antimicrobial resistance patterns of E. coli isolates from bovine mastitis.

It is important to evaluate the antimicrobial resistance pattern by genotypic and phenotypic methods in epidemiological studies in order to control antimicrobial resistance and to improve the outcome of the treatments. Four-hundred and thirty clinical mastitis samples were collected from 14 dairy herds in five different cities. The antimicrobial susceptibility test was performed using agar disk diffusion for 70 identified Escherichia coli isolates. The antimicrobial resistance genes including strA, strB, aadA, sulI, sulII, sulIII, ampC were detected by PCR method. Phylogenic groups were determined by Clermont's multiplex PCR method, and RAPD typing was performed on all isolates. Most isolates were resistant to lincomicin and streptomycin, whereas sulfa-trimethoprim has the lowest resistance rate. Moreover, ampC, aadA and sul2 genes had the highest frequency (92.85%, 38.57%, and 32.85% respectively). 20% of all the isolates carried strA and strB genes, and 11.42% of the isolates had sul1 gene and 10% of the isolates had the less frequent sul3 gene. Of the total of 70 E. coli isolates, 26 (37.14%), 20 (28.5%), 17 (24.2%), 8 (11.4%) isolates belonged to B1, A, B2 and D phylogenic groups respectively. strA, strB, sul2and aadA resistance genes had the highest percentage in A phylogenic groups. Based on RAPD-PCR method, E. coli isolates were classified in four clusters. The result showed a high phenotypic and genotypic E. coli resistance to the current antimicrobials with a similar pattern in different cities; also the majority of E. coli isolates belonged to B1 group which mainly contains the commensal E. coli isolates.