The purpose of this study was to detect the antibiotic resistance of forty-one Escherichia coli isolates from the intestinal contents of slaughtered broiler chickens using the disk diffusion method according to Kirby-Bauer. Mueller-Hinton agar plates were inoculated with 0.1 ml overnight broth cultures of individual E. coli isolates and the disks with the following concentrations of antibiotics were applied onto them: ampicillin (10 μg), cefotaxime (30 μg), gentamicin (10 μg), streptomycin (10 μg), azithromycin (15 μg), tetracycline (30 μg), ciprofloxacin (30 μg) and levofloxacin (3 μg). After the incubation at 37 °C for 16—18 hours, the inhibition zones were measured and interpreted in accordance with the Clinical and Laboratory Standard Institute (CLSI) zone diameter breakpoints. Almost all E. coli isolates showed resistance to tetracycline (92.68 %), most of them were resistant to gentamicin (75.61 %) and levofloxacine (70.73 %). Phenotypic resistance to tetracycline was further confirmed with the help of the Polymerase Chain Reaction (PCR) procedure focused on the presence of specific tet(A) and tet(B) genes. These genes were detected in all 41 E. coli isolates. On the contrary, E. coli isolates were highly susceptible to both azithromycin and streptomycin. In conclusion, the study highlighted the role of commensal E. coli bacteria isolated from the intestines of broiler chickens as an important reservoir of tetracycline resistance genes.
