Human serum albumin and biological fluids reduce cefiderocol activity in carbapenem-resistant Gram-negative bacteria

• HSA attenuates cefiderocol bactericidal activity • FDC killing is reduced across multiple carbapenem-resistant species • HSA modulates FDC-induced transcription of iron uptake and β-lactamase genes • Host factors like albumin impact in vitro FDC susceptibility testing Human serum albumin (HSA), the predominant plasma protein, influences bacterial physiology and antibiotic susceptibility. We previously showed that HSA exposure alters Acinetobacter baumannii transcriptional profiles, reducing biofilm formation and modulating the expression of β-lactam resistance genes, while decreasing cefiderocol (FDC) activity through downregulation of TonB-dependent receptors (TBDRs). Since FDC uptake relies on siderophore-mediated transport, we hypothesized that albumin’s iron content could affect bacterial susceptibility to FDC. However, its effect on cefiderocol (FDC) activity across carbapenem-resistant Gram-negative pathogens remains underexplored. Here, we evaluated FDC bactericidal efficacy in the presence of physiological HSA concentrations and various human biological fluids against five different species of carbapenem-resistant strains. Time-kill assays revealed that HSA markedly attenuates FDC activity, permitting bacterial regrowth in species including A. baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli , and Achromobacter xylosoxidans . Human fluids with high protein content similarly reduced FDC efficacy. Transcriptional analyses indicated downregulation of iron uptake genes, suggesting that albumin-mediated iron availability modulates siderophore-dependent drug uptake. These findings highlight the significant role of host proteins and fluid environments in modulating FDC activity, underscoring the need to consider such factors when interpreting susceptibility information and optimizing treatment strategies for infections caused by carbapenem-resistant pathogens.