Serious Risk of Tigecycline Resistance in Escherichia coli Isolated from Swine Manure

The emergence of the plasmid-mediated tigecycline resistance gene tet X family in pig farms has attracted worldwide attention. The use of tetracycline antibiotics in pig farms has a facilitating effect on the prevalence of the tet X family, but the relationship among its presence, expression, and resistance phenotype in resistant bacteria is unknown. In this study, the presence and expression characteristics of tetracycline resistance genes (TRGs) in 89 strains of doxycycline-resistant E. coli (DRE) isolated from pig manure samples from 20 pig farms under low concentrations of doxycycline stress (2 μg/mL) were analyzed. The detection rate of tet O was 96.63%, which is higher than those of other TRGs, such as tet A (94.38%), tet X (76.40%), tet B (73.03%), and te t(X4) (69.66%). At least three TRG types were present in DRE strains, which thus showed extensive resistance to tetracycline antibiotics, and 37% of these strains were resistant to tigecycline. In the presence of a low concentration of doxycycline, tet A played an important role, and the expression and existence ratio of TRGs indicated low expression of TRGs. Furthermore, the doxycycline resistance of DRE was jointly determined by the total absolute abundance of TRGs, and the absolute abundance of tet X and tet (X4) was significantly positively associated with tigecycline resistance in DRE ( P < 0.05). Overall, DRE isolated from swine manure is an important reservoir of the tet X family, which suggests that DRE in swine manure has a high risk of tigecycline resistance, poses a potential threat to human health, and should be of public concern.