Resistance to tetracycline and distribution of tetracycline resistance determinants in commensal Escherichia coli isolated from clinically healthy chickens and pigs.

【Objective】 Tetracycline is a broad-spectrum antibiotic used wildly in humans and animals,the resistance to tetracycline has been one of the major global concerns.Since commensal E.coli from clinically healthy animals is considered as an indicator organism of antimicrobial susceptibility and a major reservoir for tetracycline resistance genes,it is important to investigate the antimicrobial susceptibility of these isolates to tetracycline and their resistance mechanisms involved in order to prevent and treat bacterial diseases in humans and animals.【Method】 Commensal E.coli strains were isolated from clinically healthy chickens and pigs and antimicrobial susceptibility to tetracycline was performed using the broth microdilution method,and PCR was used to detect the presence of 8 tetracycline resistance genes in tetracycline-resistant strains.Statistical analysis was performed in order to contrast the differences in both resistance to tetracycline and distribution of tetracycline resistance genes between the isolates from chickens and pigs.【Result】 A total of 269 E.coli isolates were obtained and 86.2% of them were resistant to tetracycline.There was a significantly higher proportion of tetracycline-resistant E.coli isolated from healthy pigs than from healthy chickens(P0.05).Tetracycline resistance genes tet(A),tet(B) and tet(M) were widespread in commensal E.coli from clinically healthy chickens and pigs,and the carrier rates were 87.9%,28.4% and 15.5%,respectively.All the tetracycline-resistant strains harbored tet(A) or tet(B) genes,and 25.0% and 2.6% of the isolates had two and three tetracycline resistance genes,respectively.Tet(M) was coexist with tet(A) or tet(A) + tet(B) genes.There was a statistically significant difference between the carrier rates of tet(A) + tet(M) of isolates from healthy chickens and pigs(P0.01).【Conclusion】 The resistance to tetracycline was common in commensal E.coli isolated from clinically healthy chickens and pigs,and tetracycline resistance genes were widespread.The efflux of the drugs was the major resistance mechanism of commensal E.coli to tetracycline,whereas further studies should be performed to assess the influence of ribosomal protection genes on tetracycline resistance mechanism in E.coli.