A dual beta-lactam strategy suppressed resistance and outperformed single-drug treatment against a high-risk Pseudomonas aeruginosa strain in laboratory models, supporting the development of future genome-guided antibiotic therapies.
Study: Dual β-lactam therapy against high-risk Pseudomonas aeruginosa isolates: a dynamic in-vitro infection model study integrating population genomics with quantitative systems pharmacology modelling and simulations. Image credit: Jun MT/Shutterstock.com
A recent study published in The Lancet Microbe examined whether pairing two beta-lactam antibiotics, meropenem and ceftolozane-tazobactam, could help suppress sequence type 235 (ST235), one of the most pathogenic strains of P. aeruginosa.
Dual Beta-Lactam Therapy Targets Drug-Resistant P. aeruginosa
The World Health Organization ranks P. aeruginosa as one of the most pathogenic drug-resistant bacteria that can infect humans. It carries an unusually large set of resistance mechanisms and often activates several at once, which makes it difficult to predict its response to antibiotics.
The ST235 lineage spreads across hospitals worldwide and causes more severe outcomes than other strains of the species. The resistome, or total collection of antibiotic resistance genes, in this strain also encodes more than 60 variants of β-lactamase, which degrade β-lactam antibiotics.
Meropenem, a broad-spectrum carbapenem antibiotic, has been a dependable treatment against this pathogen, although the bacterium is rapidly developing resistance against it. Ceftolozane-tazobactam is a newer combination drug that currently serves as a preferred treatment against carbapenem-resistant infections in the United States and Europe. However, resistance to it is also increasing.
Laboratory Model Mimics Antibiotic Exposure In Patients
In the present study, researchers from Spain combined laboratory infection modeling with hollow-fiber infection models (HFIMs) and genetic analysis and computer simulations to test whether a dual approach with both meropenem and ceftolozane-tazobactam could kill P. aeruginosa more effectively and prevent the development of antibiotic resistance.
The team examined three clinical isolates of P. aeruginosa, collected from different patients admitted to different Spanish tertiary care hospitals and preserved in the national collection at Hospital Son Espases in Palma de Mallorca, Spain, between 2017 and 2022. Each isolate already carried several resistance mutations before any antibiotic exposure, and the three responded differently to meropenem, though all remained susceptible to ceftolozane-tazobactam at the outset.
To recreate how these bacteria behave inside a patient, the scientists placed them in an HFIM, a laboratory system that continuously exposes bacteria to antibiotic concentrations rising and falling the way they would in a hospitalized patient's bloodstream.
One set of chambers received meropenem alone, delivered as a continuous infusion matching a 6 g daily dose. Another received ceftolozane-tazobactam alone, dosed every eight hours to mirror standard clinical use. A third set combined both drugs. Each experiment ran for ten days, with an antibiotic-free control run in parallel for every treatment arm.
Throughout this period, the team repeatedly measured how many bacteria survived and how many had turned resistant. Once the experiments ended, the researchers sequenced the genomes of surviving colonies and whole bacterial populations, and compared them against each isolate's original genetic makeup to pinpoint which mutations allowed bacteria to survive treatment.
Finally, the team built a mathematical model linking antibiotic concentrations, drug action, and the genetic changes identified through sequencing to the bacterial counts observed in the experiments. Using this model, they simulated one thousand virtual infected patients receiving each regimen to estimate how often meropenem, ceftolozane-tazobactam, or the combination would successfully achieve sustained bacterial killing under realistic dosing.
Combination Therapy Suppresses Resistance Across Isolates
The study found that combining meropenem with ceftolozane-tazobactam killed P. aeruginosa more effectively and blocked resistance far better than either drug alone. However, when both antibiotics were given separately, the resistant bacterial populations in nearly every monotherapy arm grew larger than those in untreated control samples within two days.
Only ceftolozane-tazobactam avoided producing detectable resistant colonies against one of the three isolates. For the isolate with the highest meropenem resistance, ceftolozane-tazobactam alone took eight days to substantially reduce bacterial counts, while the combination achieved a similar reduction within a single day.
Genetic sequencing traced these treatment failures to specific mutations. Bacteria that survived ceftolozane-tazobactam in the sequenced isolates developed changes in the ampC gene, which encodes AmpC β-lactamase that increases inactivation of ceftolozane. Bacteria that survived meropenem developed a mutation in ftsI, a gene that alters the bacterial protein targeted by the drug.
Computer simulations built from these results predicted that meropenem alone would fail to control infection in the vast majority of patients within days, and ceftolozane-tazobactam alone would fail similarly against two of the three isolates of the ST235 strain. The combination, by contrast, was projected to achieve the predefined bacterial killing target in approximately 89% to 99% of simulated patients, depending on the isolate.
The authors also noted some limitations. The bacteria studied lacked certain acquired resistance enzymes found in other strains. Additionally, the immune system's role in fighting infection could not be captured in this laboratory model. Lastly, the study examined only one high-risk lineage, which makes it difficult to conclude how well the results extend to other resistant strains.
Results Support Further Testing Of Dual Beta-Lactam Therapy
Overall, the findings showed that pairing meropenem with ceftolozane-tazobactam suppressed resistance and improved efficacy against a highly pathogenic, antibiotic-resistant strain of P. aeruginosa far more reliably than either drug alone.
The genetics-based model the researchers built explained why single-drug treatment often failed and why a combination of the two drugs was significantly more successful. The findings support future animal infection studies and, ultimately, clinical trials, and strengthen the case for genome-guided, personalized antibiotic regimens.
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Jornal Reference
Breen, S. K. J., Cortés-Lara, S., Tait, J. R., Rogers, K. E., Lee, W. L., Faith, M., Terrill, A. E., Fuhs, D. T., Harper, M., López-Causapé, C., Nation, R. L., Boyce, J. D, Oliver, A., & Landersdorfer, C. B. (2026). Dual β-lactam therapy against high-risk Pseudomonas aeruginosa isolates: A dynamic in-vitro infection model study integrating population genomics with quantitative systems pharmacology modelling and simulations. The Lancet Microbe, 101364. DOI:10.1016/j.lanmic.2026.101364, https://www.thelancet.com/journals/lanmic/article/PIIS2666-5247(26)00019-4/fulltext
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