Detection of Plasmid-Mediated Tigecycline Resistance Gene tet(X4) in a Salmonella enterica Serovar Llandoff Isolate

Abstract. The emergence and spread of plasmid-mediated tigecycline resistance genes have attracted extensive attention worldwide. We investigated the distribution of mobile tigecycline resistance genes in Salmonella genomes generated by both our laboratory and public bacterial genomes downloaded from the NCBI GenBank. The tet(X4)-positive strains were subjected to susceptibility testing and conjugation assays. The genetic features of the tet(X4)-bearing plasmid sequence were analyzed. Here, we report the identification of the plasmid-mediated tigecycline resistance gene tet(X4) in a conjugative plasmid of the Salmonella enterica serovar Llandoff strain SH16G3606, isolated from a man in China in 2016, the first reported serovar Llandoff in China as a novel sequence type ST8300. The tet(X4)-mediated resistance phenotype was successfully transferred from the Salmonella Llandoff strain into Escherichia coli J53, resulting in a 32-fold increase in the minimal inhibitory concentration of tigecycline. The tet(X4) gene was located between two copies of ISCR2 in the plasmid pSal21GXH-tetX4. To our knowledge, this is the first report of the plasmid-mediated tigecycline resistance gene tet(X4) in a Salmonella Llandoff strain isolated from a human stool sample in China. In addition, our findings demonstrated that a total of 171 isolates are carrying tet(X)-like genes distributed in 21 countries or areas across 6 continents, posing a serious threat to humans and public health. Overall, our timely discovery of the recent emergence of the tet(X4) gene in Salmonella isolates and other Enterobacteriaceae bacteria species supports the need for rapid surveillance to prevent the tet(X)-like gene from spreading.