Tigecycline-resistant ST761 carrying (X4) in a pig farm, China.

This study aimed to investigate the prevalence and characterization of (X4) in isolates from a pig farm in Shanghai, China, and to elucidate (X4) dissemination mechanism in this swine farm. Forty-nine (80.33%) strains were isolated from 61 samples from a pig farm and were screened for the presence of (X). Among them, six (12.24%) strains were positive for (X4) and exhibited resistance to tigecycline (MIC ≥ 16 mg/L). They were further sequenced by Illumina Hiseq. Six (X4)-positive strains belonged to ST761 with identical resistance genes, resistance profiles, plasmid replicons, and cgMLST type except that additional ColE10 plasmid was present in isolate SH21PTE35. Isolate SH21PTE31, as a representative ST761 strain, was further sequenced using Nanopore MinION. The (X4) in SH21PTE31 was located on IncFIA18/IncFIB(K)/IncX1 hybrid plasmid pYUSHP31-1, highly similar to other (X4)-carrying IncFIA18/IncFIB(K)/IncX1 plasmids from ST761 and other lineages in China. These IncFIA18/IncFIB(K)/IncX1 plasmids shared closely related multidrug resistance regions, and could reorganize, acquire or lose resistance modules mediated by mobile elements such as IS and IS. Phylogenetic analysis were performed including all (X4)-positive isolates obtained in this pig farm combined with 43 (X4)-positive from pigs, cow, pork, wastewater, and patients with the same ST from NCBI. The 50 (X4)-carrying ST761 isolates from different areas in China shared a close phylogenetic relationship (0-49 SNPs). In conclusion, clonal transmission of (X4)-positive ST761 has occurred in this swine farm. ST761 has the potential to become a high-risk clone for (X4) dissemination in China.