Characterization of tetracycline resistance genes in Escherichia coli isolated from feedlot cattle administered therapeutic or subtherapeutic levels of tetracycline

The effect of administering feedlot cattle subtherapeutic levels of chlortetracycline (CT) or CT and therapeutic levels of oxytetracycline (CT-OX) on resistance genotypes in Escherichia coli was investigated. Detection of genes tet(A), tet(B), and tet(C) encoded by tetracycline-resistant isolates (CT, N = 77; CT-OX, N = 99) was performed by multiplex polymerase chain reaction (PCR). Prevalence of tet(A) was similar in isolates across treatment regimes; however, prevalence of tet(B) was lower (18% versus 34%; P < 0.05) and tet(C) was higher (46% versus 28%; P < 0.05) in CT isolates compared with CT-OX isolates. To further characterize selection of resistance genotypes in E. coli, a group of intermediately tetracycline-resistant E. coli (N = 48) was analyzed. The tet(C) gene was present in 92% of these isolates. Copies of tet(C) transcripts, analyzed by real-time PCR, indicated that upregulation did not occur in tetracycline-resistant isolates when compared with intermediately resistant isolates. The minimum inhibitory concentrations of tetracycline, chlortetracycline, and oxytetracycline were also tested on isolates with different resistance genes. The minimum inhibitory concentration was dependent on the tetracycline analogue and the nature of encoded resistance. These data indicate that tetracycline analogues should not be used interchangeably to evaluate resistance and that prevalence of resistance genes in E. coli can vary according to the tetracycline analogue administered to cattle.