Extensive spread of tet(X4) in multidrug-resistant Escherichia coli of animal origin in western China.

The recent emergence of plasmid-mediated tigecycline resistance gene tet(X) variants represents a threat to public health. tet(X4) is highly endemic in northwestern China. Here, we extended the region to western China to further understand its epidemiology and distribution. During 2018-2020, 1497 faecal samples from pigs, sheep and goats in western China were screened for tet(X4)-positive strains. A total of 134 tigecycline-nonsusceptible Escherichia coli were isolated; among them, the largest number were isolated from Guangxi (66.42%, 89/134), Shaanxi (17.16%, 23/134) and Ningxia (8.21%, 11/134). Antibiotic susceptibility testing showed that all of these isolates were multidrug-resistant bacteria that were also resistant to florfenicol. Eighty-nine of 134 tet(X4)-positive Escherichia coli were analysed by whole-genome sequencing. They belonged to twenty-seven sequence types, and ST10, ST48, ST189 and ST2223 were the main types of Escherichia coli. Seventy-six of the 134 tet(X4)-positive Escherichia coli could transfer tet(X4) into the recipient Escherichia coli 26 R 793. Southern blot analysis showed that there were multiple plasmids carrying the tet(X4) gene in a single strain. The plasmids of the 89 tet(X4)-bearing isolates in this study were analysed. tet(X4)-bearing plasmids ranged from 20 to 400 kb, and the main plasmid types were categorised as IncX1, IncY, and ColRNAI. Guangxi, Ningxia, and Shanxi are likely to be the seminal and most affected areas. Additional efforts are needed to evaluate the impact of the spread of these tigecycline resistance genes and tigecycline-resistant isolates.