Multiple Mechanisms of Tigecycline Resistance in from a Pig Farm, China.

We isolated eight tigecycline-resistant strains from a pig farm in Shanghai, China, including Escherichia coli ( = 1), Proteus cibarius ( = 1), and Enterobacter hormaechei ( = 6). Two of them (E. coli and P. cibarius) were positive for (X). E. coli SH19PTE6 contained an IncFIA18/IncFIB(K)/IncX1 hybrid plasmid pYUSHP6-tetX, highly similar to other (X)-bearing hybrid plasmids from E. coli in China. In P. cibarius SH19PTE4, (X) was located within a new chromosomal integrative and conjugative element (ICE), ICEChn2, belonging to the SXT/R391 ICE family. All tigecycline-resistant E. hormaechei isolates carried the (A) variant; cloning and transfer of this (A) variant into various hosts increased their MICs for tigecycline (4- to 8-fold). Tigecycline resistance observed on a pig farm is mediated by the (A) variant and (X) via a plasmid or ICE. The rational use of antibiotics such as doxycycline and surveillance of tigecycline resistance in livestock are warranted. As a last-resort antimicrobial agent to treat serious infections, the emergence and spread of tigecycline resistance in and Acinetobacter have raised global concerns. Multiple mechanisms mediate tigecycline resistance in , such as the monooxygenase Tet(X), mutations in Tet proteins, and overexpression of efflux pumps. Although tigecycline is not approved for animals, tigecycline resistance has been observed in Escherichia coli, Proteus cibarius, and Enterobacter hormaechei isolates on a pig farm, mediated by the (A) variant and (X) via a plasmid or ICE. The heavy use of tetracyclines such as doxycycline in food-producing animals in China may be the reason for the emergence and transmission of tigecycline resistance.