Characterization of two multidrug-resistant harboring tigecycline-resistant gene (X4) in China.

Objectives: Tigecycline is recognized as one of the last-line antibiotics to treat serious bacterial infection caused by carbapenem-resistant (CRKP). The plasmid-borne gene (X4) mediates high resistance to tigecycline. However, the prevalence and genetic context of (X4) in from various sources are not fully understood. Here, we investigated the prevalence of (X4)-positive and characterized the genetic context of (X4)-bearing plasmids in isolates.

Methods: Polymerase chain reaction (PCR) was used to detect the (X4) gene. The transferability of the (X4)-carrying plasmids was tested by conjugation assays. The infection model was used to test virulence of (X4)-positive strains. Whole-genome sequencing and genome-wide analysis were performed to identify the antimicrobial resistance and the virulence genes, and to clarify the genetic characteristics of the (X4)-positive isolates.

Results: Among 921 samples, we identified two (X4)-positive strains collected from nasal swabs of two pigs (0.22%, 2/921). The two (X4)-positive isolates exhibited high minimum inhibitory concentrations to tigecycline (32-256 mg/L) and tetracycline (256 mg/L). The plasmids carrying the (X4) gene can transfer from the donor strain to the recipient strain J53. Genetic analysis of the complete sequence of two (X4)-carrying plasmids pTKPN_3-186k-tetX4 and pTKPN_8-216k-tetX4 disclosed that the (X4) gene was flanked by delta IS and IS, which may mediate the transmission of the (X4) gene.

Conclusion: The prevalence of (X4)-positive among different sources was low. IS and IS may contribute to the horizontal transfer of (X4) gene. Effective measures should be taken to prevent the transmission of (X4)-producing in humans or animals.