Hetero-antagonism of avibactam and sulbactam with cefiderocol in carbapenem-resistant spp.

Cefiderocol, a siderophore-cephalosporine conjugate antibiotic, shows promise as a therapeutic option for carbapenem-resistant (CR) infections. While resistance has already been reported in , combination therapies with avibactam or sulbactam reduce MICs of cefiderocol, extending its efficacy. However, careful consideration is necessary when using these combinations. In our experiments, exposure of and to cefiderocol and sulbactam or avibactam led to the selection of cefiderocol-resistant strains. Three of those were subjected to whole genome sequencing and transcriptomic analysis. The strains all possessed synonymous and non-synonymous substitutions and short deletions. The most significant mutations affected efflux pumps, transcriptional regulators, and iron homeostasis genes. Transcriptomics showed significant alterations in expression levels of outer membrane proteins, iron homeostasis, and β-lactamases, suggesting adaptive responses to selective pressure. This study underscores the importance of carefully assessing drug synergies, as they may inadvertently foster the selection of resistant variants and complicate the management of CR infections.IMPORTANCEThe emergence of carbapenem-resistant strains as a serious global health threat underscores the urgent need for effective treatment options. Although few drugs show promise against CR infections, resistance to both drugs has been reported. In this study, the molecular characterization of spontaneous cefiderocol-resistant variants, a CR strain with antagonism to sulbactam, and an strain with antagonism to avibactam, provides valuable insights into the mechanisms of resistance to cefiderocol. Some mechanisms observed are associated with mutations affecting efflux pumps, regulators, and iron homeostasis genes. These findings highlight the importance of understanding resistance mechanisms to optimize treatment options. They also emphasize the importance of early evaluation of drug synergies to address the challenges of antimicrobial resistance in infections.