Prevalence of (X4) in From Duck Farms in Southeast China.

OBJECTIVES

Carbapenems, colistin, and tigecycline are critically important antibiotics in clinics. After the global appearance of and mediating the resistance to carbapenems and colistin, respectively, tigecycline becomes the last-resort drug against severe human infections caused by multidrug-resistant bacteria. Recently, a mobile tigecycline resistance gene (X4) has been identified in , , and that causes high resistance to tigecycline and other tetracyclines. In this study, the prevalence of (X4) in isolates from duck and goose farms in Southeast China was identified and characterized.

METHODS

Feces, soil, sewage, and dust samples were collected from duck and goose farms along with the southeast coast provinces of China. Antimicrobial susceptibility testing and polymerase chain reaction screening were performed to investigate the phenotype and genotype of tigecycline resistance. Conjugation, S1 pulsed-field gel electrophoresis (PFGE), and whole-genome sequencing were used to determine the transferability, genetic location, and the genomic characteristics of (X4).

RESULTS

In total, 1,716 samples were collected, and 16 isolates (0.9%) recovered from Guangdong, Shandong, and Jiangsu were positive for (X4) gene with tigecycline minimum inhibitory concentrations ≥16 mg/L. Notably, among these (X4)-positive isolates, seven of them were from the environment samples (soil and sewage). PFGE and multilocus sequence typing demonstrated that ST3997 was the most prevalent sequence type (eight isolates, 50%) in Jiangsu province. By conjugation assays, 11 isolates were able to transfer (X4) plasmid to C600 recipient, and these plasmids belonged to IncHI1 and IncX1 detected by sequence analysis. (X4) was found adjacent to an insertion sequence IS downstream and a D gene upstream for all isolates. In addition, multiple-drug resistance to tigecycline, chlortetracycline, ampicillin, florfenicol, ciprofloxacin, gentamicin, trimethoprim/sulfamethoxazole, and fosfomycin was profiled in most of the (X4)-positive isolates.

CONCLUSION

The identification of (X4) harboring strains in duck farms and their surrounding environment enlarges our knowledge of the variety and prevalence of tigecycline resistance. The prevalence of (X4) raises concern for the use of tetracyclines in animal farming, and the (X4) gene should be listed as primary gene for resistance surveillance.