Coexistence of tet(X4), mcr-1, and bla(NDM-5) in ST6775 Escherichia coli Isolates of Animal Origin in China

Emergence of pathogens harboring multiple resistance genes incurs great concerns. Cooccurrence of mobile resistance genes conferring resistance to tigecycline, colistin, and carbapenems in Escherichia coli has not been investigated. This study aimed to characterize three E. coli isolates coharboring tet(X4), mcr-1, and bla(NDM-5). Isolates coharboring tet(X4), mcr-1, and bla(NDM-5) were identified and characterized by PCR, Sanger sequencing, antimicrobial susceptibility testing, conjugation assays, Illumina sequencing, nanopore sequencing, and bioinformatic analysis. Three E. coli isolates carrying tet(X4), mcr-1, and bla(NDM-5) were identified from pigeons in China. They were resistant to almost all antimicrobials except enrofloxacin. tet(X4) and bla(NDM-5) could be conjugated into E. coli C600, but mcr-1 was nontransferable in three isolates. Three isolates belonged to sequence type 6775 (ST6775), and clonal dissemination of isolates carrying tet(X4), mcr-1, and bla(NDM-5) existed in the pigeon farm. Genetic analysis revealed that mcr-1 mediated by the Tn6330 was located on the chromosome, tet(X4) was located on the IncFII plasmid, and bla(NDM-5) was located on the IncX3 plasmid. We first characterized the E. coli isolates carrying tet(X4), mcr-1, and bla(NDM-5) simultaneously. Relevant measures should be taken to decrease the prevalence of pathogens carrying tet(X4), mcr-1, and bla(NDM-5). IMPORTANCE Tigecycline and colistin are regarded as vital antimicrobials to treat multidrug-resistant (MDR) bacterial infections, such as that caused by carbapenemase-producing Enterobacteriaceae (CPE). Cooccurrence of mobile resistance genes conferring resistance to last-resort antimicrobials in E. coli remains unknown. Here, we characterized E. coli strains coharboring tet(X4), mcr-1, and bla(NDM-5) phenotypically and genetically. Resistance genes tet(X4), mcr-1, and bla(NDM-5) were located on transposons or plasmids that were mobile genetic elements related to the capture, accumulation, and dissemination of such important resistance genes. The emergence of E. coli isolates carrying tet(X4), mcr-1, and bla(NDM-5) highlights the importance of monitoring the coexistence of novel mobile resistance genes in different settings with a One Health approach. Risk of transmission of such MDR pathogens from animals to humans should be evaluated comprehensively.