Objectives: This study was conducted to describe antimicrobial resistance in fecal generic Escherichia coli isolated from newborn dairy calves and also to investigate relationship between antimicrobial resistance and possession of K99 gene by E .coli isolates, diarrhea and age of the calf. Materials & Methods: Seven dairy farms in Mashhad were visited on weekly basis and fecal samples were collected from a total of 95 diarrheic and 95 non-diarrheic newborn calves as assessed by a scoring system. Fecal samples were all cultured for Escherichia coli and the isolated bacteria were tested for the presence of K99 gene and antimicrobial susceptibility using PCR and disc diffusion respectively. Results & Conclusion: Out of 190 isolates, 22 (11.6%) isolates possessed K99 genes and 7.4 % of isolates were found multiresistant against antimicrobial agents. Higher resistance rates were detected for oxytetracycline (86.3%), amoxycillin (79.5%), spectinomycine (78.9%), trimethoprim-sulphamethoxazole (76.3%) and ampicillin (71.6) respectively. Comparisons between K99 positive and K99 negative isolates using binary logistic regression showed that isolates possessing K99 gene were significantly more resistant to neomycin and trimethoprim/sulfamethoxazole and significant differences were detected between different age groups with regard to the resistance to neomycin, trimethoprim/sulfamethoxazole, ampicillin, gentamycin and enrofloxacin and for all of these antimicrobial agents the least resistance was observed in bacteria isolated from 22-30 days old age group. No significant differences in terms of antimicrobial resistance were detected between diarrheic and non-diarrheic animals. In conclusion, antimicrobial susceptibilities against some antimicrobial agents in Escherichia coli differed with regard to possessing of K99 gene and age of the newborn dairy calves.

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