Delaying cefiderocol resistance development in NDM-producingEnterobacter cloacaecomplex by combining cefiderocol with aztreonamin vitro

Abstract Background The rapid development of cefiderocol resistance poses a significant concern, particularly in Enterobacterales that produce New Delhi metallo-β-lactamase (NDM). This study explores the potential of inhibiting the development of cefiderocol resistance by combining cefiderocol with aztreonam. Methods A resistance induction experiment using 20 clinical isolates was performed to assess the impact of cefiderocol-aztreonam on preventing cefiderocol resistance development at 4x and 10x cefiderocol MIC, with and without aztreonam (2, 4, 8 µg/ml). Additionally, serial passaging with doubling cefiderocol concentrations was performed with and without aztreonam. Whole genome sequencing (WGS) was performed to identify potential genetic factors associated with the phenotype. Results Among the 20 E. cloacae complex isolates, 40% (8/20) exhibited a significant reduction in cefiderocol MIC (≥4-fold MIC reduction) in the presence of 4 µg/ml aztreonam. Combining cefiderocol with a fixed concentration of 4 µg/ml aztreonam inhibited cefiderocol resistance development in these eight isolates at an inoculum of 10 7 cfu/ml. Additional resistance induction experiments through serial passaging indicated a delayed emergence of cefiderocol-resistant clones when cefiderocol was combined with aztreonam. WGS analysis revealed a significant positive association between bla CTX-M-15 , bla OXA-1 , and other co-localized genes with a substantial MIC reduction for cefiderocol-aztreonam compared to cefiderocol alone. Conclusion Our study suggested that cefiderocol resistance development in NDM-producing E. cloacae complex can be delayed or inhibited by combining cefiderocol with aztreonam, even in the presence of multiple β-lactamase genes. A MIC reduction of at least 4-fold emerges as the most reliable predictor for inhibiting resistance development with this dual β-lactam combination.