Effect of ramR loss-of-function insertion on tigecycline resistance in clinical isolates of carbapenem-resistant Klebsiella pneumoniae

Abstract Objectives Tigecycline is an antibacterial restricted for use against carbapenem-resistant Klebsiella pneumoniae (CRKP). This study aimed to identify the tigecycline-resistance mechanism in clinical CRKP isolates obtained from a 60-year-old female during tigecycline treatment. Methods Three K. pneumoniae isolates obtained during tigecycline treatment were subjected to antimicrobial susceptibility testing (AST), pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (MLST), whole-genome sequencing and analyzing. The function of ramR was confirmed by gene complementation. Results Three K. pneumoniae isolates named W814, W112, and W113 were collected on days 0, 10 and 13 respectively, from an ongoing tigecycline treatment. The AST results showed resistance to all antibiotics except tigecycline and ceftazidime/avibactam. The tigecycline minimum inhibitory concentration (MIC) for W814 and W112 was 4 mg/L, compared to W113 MIC of 16 mg/L. These three strains belonged to ST11 and their PGFE analysis showed a similar pattern. The ISKpn18 insertion sequence (IS) in ramR was identified in W113. A parent strain transformed with the plasmid pCR2.1-Hyg carrying ramR enhanced tigecycline susceptibility, thus confirming that loss-of-function insertion in ramR contributes to tigecycline resistance. Conclusion ISKpn18 insertion in the ramR gene contributes to the tigecycline-resistance mechanism in the isolated K. pneumoniae strains.