Emergence of Ceftazidime/Avibactam and Tigecycline Resistance in Carbapenem-Resistant Due to In-Host Microevolution.

can cause both hospital- and community-acquired clinical infections. Last-line antibiotics against carbapenem-resistant (CRKP), such as ceftazidime/avibactam (CZA) and tigecycline (TGC), remain limited as treatment choices. This study aimed to investigate the mechanisms by which CRKP acquires CZA and TGC resistance under β-lactam antibiotic and TGC exposure. Three CRKP strains (XDX16, XDX31 and XDX51) were consecutively isolated from an inpatient with a urinary tract infection in two months. PFGE and MLST showed that these strains were closely related and belonged to sequence type (ST) 4496, which is a novel ST closely related to ST11. Compared to XDX16 and XDX31, XDX51 developed CZA and TGC resistance. Sequencing showed that double copies of were located on a 108 kb IncFII plasmid, increasing expression in XDX51. In addition, was interrupted by Insertion sequence (IS) Kpn14 in XDX51, with this strain exhibiting upregulation of , and expression compared with XDX16 and XDX31. Furthermore, LPS analysis suggested that the O-antigen in XDX51 was defective due to IS insertion in the rhamnosyl transferase gene , which slightly reduced TGC susceptibility. In brief, CZA resistance was caused mainly by duplication, and TGC resistance was caused by inactivation with additional LPS changes due to IS element insertion in . Notably, CRKP developed TGC and CZA resistance within one month under TGC and β-lactam treatment without exposure to CZA. The CRKP clone ST4496 has the ability to evolve CZA and TGC resistance rapidly, posing a potential threat to inpatients during antibiotic treatment.