A total of 21 (4.3%) enterohemorrhagic E. coli strains were isolated by biochemical tests and identification of the eae+stx1+stx2+ genotype from 490 stool samples obtained from calves with diarrhea during 1-year period from a major farm in Tehran, Iran. All of the strains showed resistance to ampicillin, ciprofloxacin, trimethoprim, streptomycin, chloramphenicol and tetracycline, while 19% showed resistance to gentamicin. Out of 21 EHEC strains, 11 (53%) harbored class 1 integron. Two different amplification products, which were approximately 750 and 1,700 bp in size, were obtained from amplified variable regions (in-F/in-R primers) in 3 (14.3%) and 4 (19%) of the EHEC isolates, which corresponded to dfrA7(dihydrofolate reductase type I) and dfrA1/aadA1(dihydrofolate reductase/aminoglycoside adenyltransferase) resistance gene cassettes, respectively, and this was confirmed by sequencing. Genotyping analysis revealed a total of 16 pulsotypes that corresponded to 16 isolates with the similarity indices of 62% and 30% for the most and least similar isolates, respectively, 9 of which harbored class 1 integron. Analysis of pulsotypes showed an extensive diversity among the isolates harboring integron, which is indicative of a lack of any significant genetic relatedness among the isolates. No obvious relation could be deduced between integron content and special pulsotypes. The little data available on the genotyping patterns of EHEC isolates from cattle and their resistance gene contents emphasize the need to establish genotyping databases in order to monitor and source track the source of emergence and spread of new resistant and integron-carrying genotypes.

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