Efflux Pumps and Different Genetic Contexts of (X4) Contribute to High Tigecycline Resistance in from Pigs.

Tigecycline is a last-resort antibiotic for the treatment of infections caused by multidrug-resistant bacteria. The emergence of plasmid-mediated tigecycline resistance genes is posing a serious threat to food safety and human health and has attracted worldwide attention. In this study, we characterized six tigecycline-resistant strains from porcine nasal swab samples collected from 50 swine farms in China. All the isolates were highly resistant to tigecycline with minimal inhibitory concentration (MIC) values of 16-32 mg/L, and all contained the (X4) gene. In addition, 13-19 multiple resistance genes were identified in these isolates, revealed by whole-genome sequencing analysis. The (X4) gene was identified as being located in two different genetic structures, --(X4)-IS in five isolates and --(X4)-ΔIS-IS-IS in one isolate. The role of efflux pumps in tigecycline resistance was evaluated by using inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP). The MIC values of tigecycline showed a 2- to 4-fold reduction in the presence of CCCP, indicating the involvement of active efflux pumps in tigecycline resistance in . The (X4) gene was found to be transferable to J53 by conjugation and resulted in the acquisition of tigcycline resistances in the transconjugants. Whole-genome multilocus sequence typing (wgMLST) and phylogenetic analysis showed a close relationship of five isolates originating from different pig farms, suggesting the transmission of (X4)-positive between farms. In conclusion, our findings suggest that strains in pigs are reservoirs of a transferable (X4) gene and provide insights into the tigecycline resistance mechanism as well as the diversity and complexity of the genetic context of (X4) in .