Detection of a new tet(X6)-encoding plasmid in Acinetobacter towneri.

OBJECTIVES:The aim of this study was to characterise a novel tet(X6)-carrying plasmid detected in a livestock-associated Acinetobacter towneri isolate. METHODS:PCR screening was performed to detect tet(X) variants in livestock-associated Acinetobacter spp. isolates. The tet(X6)-positive isolate was analysed by whole-genome sequencing. Functional cloning was performed to detect the activity of Tet(X6). Antibiotic susceptibility was determined by broth dilution and microbiological degradation assays. Site-directed mutagenesis was performed to identify the role of 23-Ala residue of Tet(X6) in tigecycline resistance. RESULTS:The tet(X6) gene was detected on a 159-kb plasmid (pAT205) carried by a tigecycline-susceptible A. towneri isolate recovered from a swine faecal sample. The genetic context of tet(X6) [ΔISVsa3-tet(X6)-abh-guaA-ISVsa3] is highly similar to that of the reported plasmid-borne tet(X) variants, suggesting that it may represent a common structure mediating the dissemination of plasmid-borne tet(X) genes. Additional resistance genes detected on pAT205 were carried by a Tn6205-like region and a disrupted class 2 integron. Gene expression and microbiological degradation assays consistently suggested that the activity of tet(X6) is weaker than that of tet(X3) and tet(X4). The 23-Ala residue of the first FAD-binding site conferred higher activity to Tet(X6) than the Gly reside conserved in the other plasmid-borne tet(X) variants, indicating that the site might be under selection. CONCLUSION:This study alerts to the silent dissemination possibility of tigecycline resistance mediated by a novel plasmid. Continuous monitoring of plasmid-borne tet(X) is imperative for tackling its dissemination.