Emergence and characterization of a novel plasmid co-harbouring tmexC3D2-toprJ2 and tet(X6) in Thiopseudomonas alkaliphila.

OBJECTIVE: The tmexCD-toprJ and tet(X) are key determinants mediating tigecycline resistance in bacteria, and investigating their epidemiological characteristics in different bacteria is crucial for controlling the spread of tigecycline-resistant bacteria. This study aimed to conduct a detailed investigation on the characterization of a Thiopseudomonas alkaliphila isolate SA1F2-1K carrying a novel plasmid co-harbouring tmexCD-toprJ and tet(X). METHODS: An isolate SA1F2-1K co-harbouring tmexCD-toprJ and tet(X) was identified by PCR and was further characterized using antimicrobial susceptibility testing, conjugation and electroporation experiments, S1-PFGE, whole-genome sequencing and bioinformatic analysis. RESULTS: A T. alkaliphila strain SA1F2-1K was isolated, which co-carried the tigecycline resistance gene cluster tmexC3D2-toprJ2 and the tet(X6) gene. This strain exhibited high-level resistance to tigecycline, kanamycin, florfenicol and tetracycline, and the tigecycline resistance genes of SA1F2-1K were non-transferable. S1-PFGE and whole-genome sequencing revealed that tmexC3D2-toprJ2 and tet(X6) were located on a novel untypable MDR plasmid pSA1F2-1K_200k. Genetic environment analysis showed that the acquisition of tmexC3D2-toprJ2 and tet(X6) might be associated with the △umuC gene and △ISCR2 element, respectively. Phylogenetic analysis indicated that T. alkaliphila strains isolated in recent years have acquired a greater number of resistance genes and insertion sequences compared with earlier isolates. CONCLUSIONS: This study first reported the characterization of a T. alkaliphila strain carrying a novel large plasmid co-harbouring tmexC3D2-toprJ2 and tet(X6) genes, revealing the potential reservoir and transmission risk of clinically important tigecycline resistance genes.