In vitro antimicrobial activity and resistance mechanisms of cefiderocol against clinical carbapenem-resistant gram-negative bacteria

Background The rise of carbapenem-resistant gram-negative bacteria (CRGNB) necessitates new therapeutic options such as cefiderocol. Objective To evaluate the in vitro efficacy of cefiderocol against clinical CRGNB and investigate associated resistance mechanisms. Methods A total of 370 CRGNB isolates were analyzed. Minimum inhibitory concentration (MIC) values were determined, and whole genome sequencing, efflux pump inhibition assays, and RT-qPCR were conducted to assess resistance-related mutations, gene loss, and expression changes. Results Cefiderocol demonstrated potent in vitro activity, with high susceptibility rates in C. freundii (100%), K. pneumoniae (93.3%), and E. hormaechei (92.2%), and notable activity against P. aeruginosa (80.0%) and Escherichia coli (76.8%). Efflux pump inhibition by Carbonyl Cyanide m-Chlorophenyl Hydrazone (CCCP) significantly reduced MICs in resistant strains. Key resistance mechanisms included β -lactamase gene variants ( bla OXA-66 , bla OXA-23 , bla SHV-12 ), mutations in envZ , cirA , nuoC , ampC , and loss or altered expression of iron transporter genes ( piuA , pirA , fepA ). Conclusion Cefiderocol is highly effective against CRGNB; however, resistance may arise through diverse mechanisms, including efflux pump activity. Continued surveillance of emerging resistance is essential to guide its optimal clinical use.