Spreading Ability of <i>Tet(X)</i> -Harboring Plasmid and Effect of Tetracyclines as a Selective Pressure

Tigecycline is a last-resort antimicrobial in humans. Tetracyclines are the most widely used antimicrobials in livestock. Mobile tigecycline resistance genes [<i>tet(X)</i>] are disseminated worldwide, and tetracycline use may have promoted the selection of <i>tet(X)</i> genes. Thus, the selective pressure on <i>tet(X)</i> genes and their plasmids in livestock must be elucidated. We performed a retrospective study to clarify the prevalence of tigecycline-resistant <i>Escherichia coli</i> from pigs in Thailand. Screening for tigecycline resistance was performed on 107 <i>E. coli</i> strains from 25 samples, and <i>tet(X)</i>-carrying plasmids were characterized. <i>tet(X)</i> genes were cloned and expressed in <i>E. coli</i>. Bacterial growth rate in the presence of tetracycline as a result of the presence of <i>tet(X)</i> genes was also evaluated. Thirty-two <i>tet(X4)</i>-harboring tigecycline-resistant <i>E. coli</i> strains were detected in 10/25 samples (40%). The <i>tet(X4)</i> genes were carried on various Inc-type plasmids and flanked by IS<i>CR2</i>. The <i>tet(X)</i>-carrying plasmids were transferred to <i>E. coli</i> and <i>Klebsiella pneumoniae</i>. Acquisition of <i>tet(X)</i> genes and their plasmids improved bacterial growth in the presence of tetracycline. In summary, tetracycline use exerts selective pressure on <i>tet(X)</i> genes and their various backbone plasmids; therefore, a reduced amount of tetracycline use is important to limit the spreading of <i>tet(X)</i> genes.