Genetically diverse drug-resistant Pseudomonas aeruginosa found in Ukraine combat wounds

Researchers who sequenced thirteen Pseudomonas aeruginosa isolates recovered from combat-related wounds in Ukraine between December 2024 and June 2025 report a strikingly mixed picture: no single dominant clone, but a collection of ten different sequence types spanning internationally disseminated high-risk lineages, with four isolates classified as extensively drug resistant and the remaining nine as multidrug resistant. The study, published in New Microbes and New Infections, combined whole-genome sequencing, antimicrobial susceptibility testing and quantitative virulence phenotyping to characterise bacteria taken from thirteen distinct patients.

The phenotypic results were uniformly concerning. Every isolate showed reduced susceptibility, meaning intermediate or resistant categories under EUCAST criteria, to ciprofloxacin, ceftazidime, piperacillin–tazobactam and aztreonam. Reduced susceptibility to ceftolozane–tazobactam appeared in five of the thirteen isolates, and to meropenem in four. All four meropenem-non-susceptible isolates tested positive for carbapenemase production in a phenotypic assay. Because meropenem and ceftolozane–tazobactam are key components of contemporary antipseudomonal therapy, the authors note that non-susceptibility to these drugs is clinically concerning where treatment options are already constrained.

Genomic analysis traced meropenem non-susceptibility to acquired metallo-β-lactamase genes. The gene blaIMP-1 was identified in three isolates belonging to ST1047 and ST235 backgrounds, while blaNDM-1 was detected in a single ST773 isolate. Beyond carbapenemases, the collection carried a wide repertoire of additional β-lactamase genes with isolate-specific patterns, including blaCTX-M-15, multiple blaOXA variants and blaTEM alleles, alongside chromosomal blaPDC variants. Genes associated with aminoglycoside resistance were found in all isolates, and the 16S rRNA methyltransferase gene rmtB, which can confer high-level aminoglycoside resistance, was identified in two ST773 isolates.

Chromosomal variation added another layer. Multiple nonsynonymous variants were detected in the carbapenem porin OprD across the collection, although the authors emphasise that the functional consequences of individual variants were not experimentally determined. Nonsynonymous variants in ampD, a regulator of AmpC β-lactamase expression, appeared across the isolates, and additional mutations were observed in dacB, a gene previously implicated in β-lactam resistance through AmpC derepression. High-level ciprofloxacin resistance was strongly associated with the canonical gyrA Thr83Ile substitution, present in all fully ciprofloxacin-resistant isolates, with additional substitutions in parC supporting stepwise accumulation of quinolone resistance determinants. A recurrent missense mutation in the MexAB–OprM efflux repressor MexR, Val126Glu, was identified in multiple resistant isolates, and variants were also detected in other efflux regulatory genes including nalC, nalD, nfxB and mexT.

Notably, several isolates carried substitutions in colistin-associated regulatory genes such as phoQ Y85F, pmrA L71R and several pmrB substitutions, yet all isolates remained colistin-susceptible, with minimum inhibitory concentrations between 0.5 and 1 mg/L. The authors interpret this as evidence that these variants were not sufficient to confer a resistant phenotype in these strains. Colistin remains one of the last-line antibiotics for resistant Gram-negative infections, so the absence of phenotypic resistance is a small but meaningful reassurance in this collection.

Multi-locus sequence typing revealed ten distinct sequence types among the thirteen isolates. Internationally disseminated lineages detected included ST235, ST244, ST253, ST773 and ST1047, with ST1047, ST244 and ST773 each represented by two isolates and the remaining sequence types by single isolates. ST235 is among the most prevalent high-risk clones worldwide and is frequently associated with carbapenemase acquisition and severe disease. Comparison with publicly available genomes showed that several of the sequence types identified have been reported across multiple geographic regions, and a comparison with previously published P. aeruginosa genomes from Ukraine, numbering 117, showed that several of the sequence types found here had already been reported in the country.

One finding stood out against the global pattern. ST1047 was the most frequently represented sequence type among the publicly available Ukrainian genomes included in the comparison, whereas globally ST235, ST253 and ST244 were more frequently represented. Prior genomic studies have linked ST1047 to conflict- and displacement-associated healthcare networks and carbapenemase-associated subclonal structure. The authors are careful, however, to note that the small size of the present collection precludes conclusions about local expansion or clonal dominance. In the global phylogeny built from 1500 publicly available genomes using forty universal single-copy marker genes, the study isolates were distributed across the tree rather than forming a single clonal cluster, indicating that combat-related wound infections in Ukraine are caused by genetically diverse sequence types rather than one expanding clone.

Virulence gene content was equally heterogeneous. The cytotoxic type III secretion effector gene exoU was detected in seven isolates belonging to multiple sequence types, while exoS was present in the other six, a mutually exclusive distribution consistent with the broader population structure of P. aeruginosa, in which exoS is strongly associated with the PAO1-like major lineage and exoU with the PA14-like lineage. Iron acquisition systems were broadly distributed, with pyochelin biosynthesis genes detected in all but one isolate and pyoverdine-associated genes present in six to twelve isolates depending on the gene. Type IV pili genes were present in one to ten isolates depending on the specific gene, and algP, a transcriptional regulator of alginate biosynthesis, was detected in eleven isolates. When isolates were stratified by type III secretion effector genotype, exoU-positive isolates carried more acquired resistance genes on average, a mean of sixteen against eight, but fewer virulence-associated genes, a mean of twenty against twenty-eight. The authors stress these differences are descriptive only and should not be interpreted as evidence that exoU or exoS carriage determines overall resistance or pathogenic potential.

Phenotypic assays of quorum sensing, the chemical communication system that coordinates expression of multiple secreted virulence factors, revealed substantial variability. Compared with the reference strain PAO1, the reporter signal for the quorum sensing signal molecule 3-oxo-C12-HSL was significantly reduced in nine of thirteen isolates, while rhamnolipid production was significantly reduced in six, and pyocyanin levels varied without statistically significant differences. In three isolates, reduced 3-oxo-C12-HSL production coincided with high-impact loss-of-function mutations in lasR, and in one isolate it coincided with a missense substitution in lasI affecting a conserved residue reported to be essential for AHL synthase activity. These isolates belonged to different clonal backgrounds, suggesting quorum sensing attenuation can arise through distinct genetic routes. The authors caution that variation in lasR is common among P. aeruginosa isolates generally and should not necessarily be read as a specific adaptation to combat-associated wound infection.

The study has clear limitations. Thirteen isolates from thirteen patients limit statistical power for genotype–phenotype comparisons and preclude definitive conclusions about clonal expansion or population-level epidemiology. Clinical metadata including patient outcomes, infection severity, antibiotic treatment history and transmission events were not available, restricting infection-control conclusions. The authors also note that the mixed population structure is consistent with multiple potential sources of acquisition, including field contamination, initial hospital care and subsequent referral centres, but the present data cannot distinguish among these possibilities. Despite these constraints, the work argues for integrating rapid antimicrobial susceptibility testing with lineage-resolved genomic surveillance to inform antimicrobial management, infection prevention and control, and cross-border surveillance in conflict-associated healthcare pathways, where highly resistant organisms among evacuated patients have raised concerns about both treatment and cross-border transmission.

Subject of Research: Molecular epidemiology and virulence profiles of multidrug-resistant Pseudomonas aeruginosa from combat-related wounds in Ukraine

Article Title: Molecular epidemiology and virulence profiles of MDR and XDR Pseudomonas aeruginosa from combat-related wounds in Ukraine

Article References: Palasca, O., Schwartz, C. H., Kristensen, R., Koshova, I., Murashevych, B., Thomsen, T. R., Hertz, F. B., Bjarnsholt, T., & Jakobsen, T. H. (2026). Molecular epidemiology and virulence profiles of MDR and XDR Pseudomonas aeruginosa from combat-related wounds in Ukraine. New Microbes and New Infections, 74, Article 101880. https://doi.org/10.1016/j.nmni.2026.101880

Image Credits: AI Generated

DOI: 10.1016/j.nmni.2026.101880

Keywords: Pseudomonas aeruginosa, antimicrobial resistance, Ukraine, combat wounds, whole-genome sequencing, carbapenemase, ST235, ST1047, XDR, quorum sensing, virulence genes, MLST