Background and Objective: Nowadays, the spread of pathogenic Escherichia coli strains containing tetracycline resistance genes is considered as one of the major health concerns worldwide. The main reason for their spread is the excessive and inappropriate use of antibiotics. The aim of the present study was to determine the antibiotic resistance patterns and the frequency of tetracycline resistance genes (tetA, tetB, tetC) in total isolates of poultry bacillus in Ilam city. Materials and Methods: This descriptive-cross-sectional study was conducted in 2019 on one hundred and fifty APEC isolates collected from total cases of poultry bacillus in 20 broiler flocks in Ilam city. Standard bacteriological and biochemical tests were used to determine the identity of the isolates. The isolates were stored at -20°C in nutrient broth in the presence of 15% glycerol for subsequent steps. To determine the antibiotic resistance pattern of the isolated strains, disc diffusion was used according to the Kirby-Boer method based on the CLSI. In the PCR test, specific primers were used to prove the presence of tetA, tetB and tetC genes in the studied isolates.
Findings: The results of the study of the phenotypic antibiotic resistance of Escherichia coli strains producing colibacillosis showed that the highest level of antibiotic resistance was against tetracycline antibiotics. The results of the PCR test using specific primers to detect and identify the gene encoding the efflux pump tetA, tetB and tetC showed that 130 isolates (86.67%) had the tetA gene, 47 isolates (31.3%) had the tetB gene and only 9 isolates (6%) had the tetC gene.

Conclusion: The high frequency of microbial resistance genes to tetracycline in total isolates of poultry bacillus indicates the indiscriminate use of tetracycline antibiotics in poultry farms in the study area. It is necessary to perform an antibiogram before treating the disease to prevent the emergence and spread of resistance in the bacterial population at the flock level.