Huang, Wen-ChunWen-ChunHuangYU-TSUNG HUANGKo, Wen-ChienWen-ChienKoShih, Wei-AnWei-AnShihTeng, Ching-HaoChing-HaoTengWang, Jiun-LingJiun-LingWang2026-04-232026-04-232026-02-1022137165https://scholars.lib.ntu.edu.tw/handle/123456789/737451ObjectivesPlasmid-mediaed tet(X4), linked to high-level tigecycline resistance, was first identified in China with Escherichia coli (E. coli) as a major reservoir. No confirmed cases had been reported in Taiwan.MethodsWe examined 81 tigecycline-resistant E. coli isolates (MIC ≥ 4 mg/L) collected in Taiwan from 2015 to 2022, including 71.6% carbapenem-resistant and 28.4% carbapenem-susceptible strains. Thirty-six underwent whole-genome sequencing to investigate resistance mechanisms.ResultsTwo isolates (2.5%) carried tet(X4) on novel plasmids (pEC1360–1 and pEC1638–1). Both plasmids contained the ISVsa3-estT-tet(X4)-ISVsa3 (IETI) element, a mobile unit capable of transposon-mediated transfer without a fixed integration hotspot. The tet(X4)-positive strains showed distinct evolutionary divergence from the first reported Chinese strain (LHM10–1). Tet(X4) was located on different Inc-type plasmids, including a 66.8 kb IncR and a 159.3 kb IncR/IncFIB(K)/IncFIA(HI1) plasmid, across various sequence types. No tet(X4) was detected in carbapenem-resistant isolates. Other resistance genes, such as cmlA1 and floR, were more prevalent in carbapenem-susceptible isolates (66.7% vs. 25.9%, P = 0.077).ConclusionThis study reports the first tet(X4)-positive E. coli isolates in Taiwan, both from carbapenem-susceptible strains. The presence of novel mobile plasmids underscores the potential for horizontal gene transfer. Continuous surveillance of tet(X) and other last-line antibiotic resistance mechanisms is essential to mitigate the risk of further spread.entruePlasmidTet(X4)TigecyclineTigecycline-resistant E. colijournal article10.1016/j.jgar.2026.01.014416795142-s2.0-105034592373