A Rapid Colorimetric Assay for Real-Time Detection of Imipenem/Relebactam Resistance in Pseudomonas aeruginosa

The accelerating spread of multidrug-resistant Gram-negative bacteria has introduced a troubling level of unpredictability in clinical care—especially in intensive care units, where rapid decision-making often determines patient outcomes. Among these resistant pathogens, the opportunistic organism Pseudomonas aeruginosa stands out due to its remarkable ability to both resist and adapt. Recently, carbapenem-resistant P. aeruginosa (CRPA) has become a familiar challenge across global healthcare systems. These strains are now frequently encountered in serious infections, including bacteremia, pneumonia, and complicated urinary tract infections. In light of this growing threat, the World Health Organization has designated CRPA as a critical priority pathogen, underlining the urgent demand for both novel treatments and faster diagnostic strategies. One of the most pressing issues in managing CRPA infections is not necessarily the absence of effective drugs, but rather the lag in identifying which drugs will work. Conventional antimicrobial susceptibility testing—whether by broth microdilution, gradient diffusion, or disk diffusion—can take 18 to 24 hours or more. In many cases, that delay forces clinicians to make therapeutic decisions without complete data which may lead either to inadequate coverage or unnecessary escalation to last-line agents. This approach carries risks for individual patients and contributes to the broader crisis of antimicrobial resistance. Imipenem is a carbapenem with broad activity, and relebactam is a β-lactamase inhibitor capable of targeting class A and C enzymes, including the chromosomal AmpC commonly overproduced in P. aeruginosa. Together, they can restore efficacy in a subset of CRPA strains, particularly those not harboring metallo-β-lactamases (MBLs).  The combination of imipenem and relebactam has shown recently promise against CRPA strains, however, for this combination to be used responsibly and effectively, clinicians need tools that can quickly and reliably determine whether a given strain is susceptible.

To this account, new research paper published in Diagnostic Microbiology and Infectious Disease and led by Professor Patrice Nordmann from the University of Fribourg in Switzerland and conducted by Maxime Bouvier, Mohamed Bachtarzi, and Laurent Poirel, researchers developed a rapid phenotypic test known as the Rapid IPR Pseudomonas NP assay. The test uses a straightforward colorimetric method based on glucose metabolism and pH shift to reveal resistance or susceptibility to IPR in just four hours.  To evaluate the diagnostic tool they had developed, the researchers drew on a set of 80 clinical Pseudomonas aeruginosa isolates, each previously characterized with respect to its resistance profile. Some isolates in the collection produced well-known carbapenemases, such as VIM, NDM, or IMP, while others relied on more insidious, non-enzymatic strategies: downregulation of porins, upregulation of efflux pumps, or overexpression of the chromosomal AmpC enzyme. Therefore, the clinical samples reflected the breadth of resistance mechanisms clinicians regularly confront

The researchers designed the Rapid IPR Pseudomonas NP test which is a phenotypic assay based on a smart and simple informative color change. The concept of the test rests on glucose metabolism. If the bacteria are resistant to imipenem/relebactam, they continue to grow and metabolize glucose, which causes the pH to rise and trigger a shift in the phenol red indicator from yellow to orange or red, however, if they’re susceptible, no growth occurs and the solution remains yellow. The research team tested each strain in parallel—one condition with the drug, one without. After incubation at 37°C, results were visually assessed at regular intervals over four hours. They found that the assay correctly identified all 42 isolates previously determined to be resistant using broth microdilution, producing a sensitivity of 100%. Of the 38 susceptible isolates, 34 were correctly categorized, while four showed a false-positive result. Notably, those four sat right at the edge of the susceptibility threshold, with MICs of 1–2 mg/L. All shared similar phenotypes—upregulated AmpC, reduced permeability, and efflux activity—making them particularly difficult to interpret, even with reference methods. These edge cases highlight the complexity of defining a sharp cutoff in a biologically fuzzy zone. According to the authors, the new assay delivered an impressive and strong overall performance: 95% accuracy, nearly 90% specificity, and over 91% precision.

In conclusion, what the new research of Professor Patrice Nordmann and his team offers is a pragmatic and timely solution: a diagnostic assay that takes just four hours to determine whether a P. aeruginosa isolate is susceptible to the imipenem/relebactam combination. That matters. The Rapid IPR Pseudomonas NP test doesn’t just save time—it changes the rhythm of clinical decision-making. In a critical care setting, those hours can make the difference between recovery and deterioration. And because the method is phenotypic, based on real-time bacterial growth and metabolic behavior, it sidesteps the need for expensive molecular tools or prior genetic knowledge of resistance mechanisms. This is especially important when dealing with a pathogen as diverse and adaptable as P. aeruginosa. But the implications stretch beyond bedside decisions. Rapid, low-cost diagnostics are foundational to any effective antimicrobial stewardship effort. When physicians know which patients truly need potent last-line therapies, they’re less likely to use them indiscriminately, helping preserve their efficacy. This is critical in resource-limited regions, where advanced lab infrastructure is scarce but resistant infections are widespread. The simplicity of this test—both in concept and execution—means it can be implemented in a wide range of clinical settings without heavy investment.