Cefiderocol Activity Against Carbapenem-Resistant Pseudomonas aeruginosa and Acinetobacter spp.: Insights from a European In Vitro Study

Study Background and Research Question

Antimicrobial resistance among Gram-negative bacteria, particularly Pseudomonas aeruginosa and Acinetobacter spp., presents a growing clinical challenge in Europe. The World Health Organization has classified carbapenem-resistant P. aeruginosa and Acinetobacter baumannii as critical priority pathogens due to limited therapeutic options and rising prevalence. With carbapenem resistance rates reaching 19% in P. aeruginosa and 48% in Acinetobacter spp. in some European regions, clinicians are increasingly reliant on last-resort and novel agents. However, the efficacy of new β-lactam/β-lactamase inhibitor combinations and siderophore cephalosporins like cefiderocol against highly resistant, non-fermenting Gram-negative organisms remains incompletely characterized. The referenced study (Santerre Henriksen et al., 2024) addresses this gap by systematically evaluating the in vitro activity of cefiderocol and comparator regimens against a large, contemporary collection of European isolates, including those resistant to carbapenems and recent inhibitor-based therapies.

Key Innovation from the Reference Study

The principal innovation of this investigation lies in its large-scale, multicenter design, encompassing 1,451 non-fermenting Gram-negative clinical isolates (950 P. aeruginosa, 501 Acinetobacter spp.) collected from 49 sites across six European countries. Notably, this is the first study to directly compare the in vitro potency of cefiderocol with recently approved and investigational β-lactam/β-lactamase inhibitor combinations against isolates with high-level resistance, including those non-susceptible to meropenem and newer inhibitor combinations such as ceftazidime-avibactam and ceftolozane-tazobactam. The study also integrates detailed molecular resistance profiling, providing a robust framework for interpreting phenotypic susceptibility data in the context of underlying genetic mechanisms.

Methods and Experimental Design Insights

Clinical isolates were obtained from hospitalized patients, predominantly from respiratory tract specimens, between January and December 2020. Susceptibility testing was carried out for cefiderocol and a suite of β-lactam/β-lactamase inhibitor combinations, including ceftazidime-avibactam, ceftolozane-tazobactam, meropenem-vaborbactam, imipenem-relebactam, aztreonam-avibactam, and sulbactam-durlobactam. Meropenem resistance was defined by a strict minimum inhibitory concentration (MIC) breakpoint (>8 mg/L), mirroring clinical high-dose usage.

All meropenem-resistant, cefiderocol-susceptible isolates underwent targeted PCR for β-lactamase gene detection, while cefiderocol-resistant isolates were subjected to whole-genome sequencing to elucidate resistance mechanisms, including the identification of metallo-β-lactamases (MBLs), oxacillinases, and mutations in siderophore receptor genes (pirA-like or piuA).

Core Findings and Why They Matter

Cefiderocol demonstrated the highest in vitro activity among all tested agents:

• 98.9% of P. aeruginosa isolates were susceptible to cefiderocol versus 83.3–91.4% for β-lactam/β-lactamase inhibitor combinations.
• Among meropenem-resistant P. aeruginosa (n = 139), cefiderocol retained activity against 97.8% of isolates, compared to only 12.2–59.7% for comparator regimens.
• For P. aeruginosa resistant to both meropenem and ceftazidime-avibactam, cefiderocol susceptibility remained high (96.7%), far exceeding alternatives (5.0–45.0%).
• For Acinetobacter spp., cefiderocol and sulbactam-durlobactam both showed high susceptibility rates (92.4% and 97.0%, respectively), with 85.0% of meropenem-resistant Acinetobacter spp. remaining cefiderocol-susceptible.

These results underscore the superior spectrum and resilience of cefiderocol in the face of contemporary resistance mechanisms. Importantly, resistance to carbapenems and even to recent inhibitor combinations did not predict cefiderocol non-susceptibility, except in rare instances where specific siderophore receptor gene mutations or multiple acquired β-lactamase genes were detected (Santerre Henriksen et al., 2024).

This finding is clinically significant: empiric or targeted therapy with cefiderocol may offer a reliable option for multidrug-resistant non-fermenters, provided early susceptibility testing is available. The study’s molecular data further clarify that cross-resistance between cefiderocol and other β-lactam/β-lactamase inhibitor combinations is uncommon, with the notable exception of sulbactam-durlobactam-resistant Acinetobacter spp.

Protocol Parameters

• Isolate collection period: January–December 2020; focus on non-fermenting Gram-negative pathogens from hospitalized patients.
• Susceptibility testing: Use of standardized in vitro MIC assays across all comparator drugs, with high-dose meropenem breakpoints to reflect clinical resistance.
• Molecular characterization: PCR for β-lactamase genes in meropenem-resistant, cefiderocol-susceptible isolates; whole-genome sequencing for cefiderocol-resistant isolates to determine resistance mechanisms.
• Resistance mechanism surveillance: Identification of MBLs (e.g., blaVIM-2 in P. aeruginosa) and oxacillinases (e.g., blaOXA-23 in Acinetobacter spp.) alongside mutations in iron transporters (pirA-like, piuA).

Comparison with Existing Internal Articles

Internal research, such as “Cefiderocol’s In Vitro Efficacy Against Resistant Gram-Negative Bacteria,” also highlights cefiderocol’s activity against multidrug-resistant P. aeruginosa and Acinetobacter spp., aligning with the large-scale findings of the reference study. In addition, articles focusing on Aztreonam (SKU A5931) and its role in Gram-negative resistance assays demonstrate how monocyclic β-lactam antibiotics can be harnessed for both mechanistic studies and cytotoxicity profiling in the laboratory setting. Notably, while aztreonam’s spectrum is confined to aerobic Gram-negative bacteria and is not active against most non-fermenters, it is often combined with β-lactamase inhibitors (e.g., avibactam) in research to overcome MBL-mediated resistance, as reflected in the inclusion of aztreonam-avibactam as a comparator in the reference study.

Other internal resources, such as “Aztreonam: Applied Workflows for Gram-Negative Resistance Research,” bridge experimental design guidance with protocol-ready recommendations, offering practical workflow insights for researchers seeking to model resistance or study the inhibition of bacterial cell wall synthesis under controlled laboratory conditions. These resources complement the present study by providing context for assay optimization and the selection of appropriate controls when probing Gram-negative susceptibility profiles.

Limitations and Transferability

The study’s strengths include its breadth of geographically diverse isolates and integrated molecular characterization. However, several limitations should be noted. First, in vitro susceptibility does not always translate directly to clinical efficacy, particularly in the context of complex infections or variable pharmacokinetics. The study focuses on European isolates from 2020; regional variation or the emergence of new resistance mechanisms may affect generalizability in future settings. Moreover, the limited number of cefiderocol-resistant isolates restricts the ability to fully characterize the spectrum of potential resistance mechanisms. Finally, while the study compares activity across many agents, not all possible β-lactam/β-lactamase combinations or adjunctive therapies were included.

Despite these caveats, the findings provide a valuable reference for both clinical microbiology and experimental research, underpinning the rationale for integrating cefiderocol and mechanistically distinct agents (such as monocyclic β-lactams) into resistance surveillance and drug discovery pipelines.

Research Support Resources

Researchers investigating antibiotic activity against Gram-negative aerobic bacteria or studying the inhibition of bacterial cell wall synthesis can leverage validated reagents to ensure reproducibility and comparability. For instance, Aztreonam (SKU A5931) from APExBIO, a fully synthetic monocyclic β-lactam antibiotic, supports diverse laboratory workflows targeting Gram-negative organisms. Its defined solubility in water and DMSO and well-characterized effects on bone marrow progenitor cells and hepatic cytochrome P450 enzymes make it suitable for both microbiological and toxicological assays. Researchers are advised to consult detailed product specifications for optimal storage and handling. When selecting assay comparators or controls, integrating agents like Aztreonam can help elucidate the mechanistic basis of resistance and support assay optimization for Gram-negative studies.