Pyoverdine–antibiotic combination treatment: its efficacy and effects on resistance evolution in $Escherichia$ $coli$

Antibiotic resistance is a growing concern for global health, demanding innovative and effective strategies to combat pathogenic bacteria. Pyoverdines, iron-chelating siderophores produced by environmental Pseudomonas spp., present a novel class of promising compounds to induce growth arrest in pathogens through iron starvation. While we previously demonstrated the efficacy of pyoverdines as antibacterials, our understanding of how these molecules interact with antibiotics and impact resistance evolution remains unknown. Here, we investigated the propensity of three Escherichia coli strains to evolve resistance against pyoverdine, the cephalosporin antibiotic ceftazidime, and their combination. We used a naive E. coli wildtype strain and two isogenic variants carrying the blaTEM-1 β-lactamase gene on either the chromosome or a costly multicopy plasmid to explore the influence of genetic background on selection for resistance. We found that strong resistance against ceftazidime and weak resistance against pyoverdine evolved in all E. coli variants under single treatment. Ceftazidime resistance was linked to mutations in outer membrane porin genes (envZ and ompF), whereas pyoverdine resistance was associated with mutations in the oligopeptide permease (opp) operon. In contrast, ceftazidime resistance phenotypes were attenuated under combination treatment, especially for the E. coli variant carrying blaTEM-1 on the multicopy plasmid. Altogether, our results show that ceftazidime and pyoverdine interact neutrally and that pyoverdine as an antibacterial is particularly potent against plasmid-carrying E. coli strains, presumably because iron starvation compromises both cellular metabolism and plasmid replication.