Abstract: The effect of administering feedlot cattle subtherapeutic levels of chlortetracycline (CT) or CT and therapeutic levels of oxytetracycline (CT-OX) on resistance genotype in Escherichia coli was investigated. Isolated E. coli from fecal samples. Isolates were tested for susceptibility to tetracycline, oxytetracycline, and chlortetracycline using disk diffusion or broth microdilution. Detection of tet(A), tet(B) and tet(C) genes encoded by tetracycline-resistant isolates (n＝176) were performed by multiplex PCR. All isolates encoded one or a combination of two resistance genes. Prevalence of tet(A) was similar between groups of E. coli, however prevalence of tet(B) was lower(P<0.05) and tet(C) was greater(P<0.05) in CT isolates. The nature of the tet determinants was further assessed in a group of intermediately tetracycline-resistant isolates (n＝52). Minimum inhibitory concentrations showed that susceptibility was dependent on tetracycline analogue and the type of resistance determinant. The tet(C) gene was present in 92% of these isolates. Copies of tet(C) transcripts, analyzed by real-time PCR, indicated that up-regulation did not occur in tetracycline-resistant isolates when compared to intermediately-resistant isolates. However, sequence analysis of the tet(C) gene revealed a T→G substitution at position 1063 in resistant isolates that may have affected phenotype. These data provide insight into the relationship between the type of tetracycline analogue administered to cattle and the prevalence of resistance genes in E. coli.

Key words: Escherichia coli; antimicrobial resistance; tetracycline; feedlot cattle