Antibiotic class shapes O-antigen-dependent phage outcomes in a clinical Pseudomonas aeruginosa isolate

ABSTRACT Pseudomonas aeruginosa is a high-priority opportunistic pathogen characterized by significant genetic plasticity and increasing multidrug resistance. Although phage–antibiotic combinations are increasingly explored to combat antimicrobial resistance, whether these interactions are synergistic or antagonistic and how different antibiotic classes influence phage replication and the subsequent development of resistance remain unclear. In this study, we examined combinations of the O-antigen-dependent phage phipa10 with amikacin (AMK), levofloxacin (LEV), or meropenem (MEM) against P. aeruginosa ZS-PA-35. At 24 h, the combinations showed antibiotic-dependent patterns of bacterial growth suppression. High concentrations of amikacin and meropenem reduced the extracellular yield of infectious phipa10. The highest tested levofloxacin concentration also reduced the recovered phage yield. Whole-genome sequencing of phage-resistant mutants showed that meropenem treatment drove large chromosomal deletions encompassing galU and hmgA , resulting in a characteristic brown pigmentation phenotype. LEV-selected isolates carried smaller deletions involving galU , whereas AMK-selected isolates shared a galU substitution. In a Galleria mellonella infection model, phipa10–antibiotic combinations improved host survival relative to either treatment alone, although the magnitude of the benefit varied across antibiotic classes. The meropenem combination yielded the highest observed survival proportion despite reduced phage yield in vitro . These results show that bacterial suppression, infectious phage yield, and resistance outcomes can diverge among the three tested antibiotic partners. IMPORTANCE Phage–antibiotic combinations may affect bacterial suppression, phage yield, and resistance-associated phenotypes differently. We compared three antibiotic partners in a single Pseudomonas aeruginosa –phage system. The observed growth, genetic, and larval survival outcomes varied across the tested treatments. Reduced phage yield did not preclude improved survival in the infection model. These observations support evaluating multiple outcomes when assessing a phage–antibiotic combination. Their broader relevance requires validation with additional isolates, phages, and antibiotics.