BACKGROUND: Pseudomonas aeruginosa readily evolves antimicrobial resistance through regulatory plasticity and stress-adaptive pathways. Clinically, antibiotics lacking intrinsic antipseudomonal activity are often favoured with the assumption that they avoid selective pressure on P. aeruginosa. Whether subinhibitory exposure to such 'non-antipseudomonal antibiotics' (NAPA) can nevertheless select for canonical resistance pathways remains incompletely defined. METHODS: Three P. aeruginosa strains (ATCC 27853 and two bloodstream isolates) were serially passaged over 14 days in the presence of ertapenem, ceftriaxone or moxifloxacin at one-third the baseline MIC. MICs for antipseudomonal antibiotics (meropenem, ceftazidime, ciprofloxacin) were measured at serial time points and after a 3-day antibiotic-free recovery (Day 14). Whole-genome sequencing was performed longitudinally to identify mutations. RESULTS: NAPA exposure led to reproducible elevations in antipseudomonal MICs: ertapenem triggered up to a 29-fold increase in meropenem MIC, ceftriaxone up to a 31-fold rise in ceftazidime MIC and moxifloxacin up to a 12-fold increase in ciprofloxacin MIC. Elevated MICs persisted on Day 14 despite absence of further antibiotic pressure. Genomic analysis revealed convergent evolution of mutations in efflux regulator genes (nfxB, nalC, nalD, amrR) and the β-lactamase-regulating gene dacB, emerging during periods of MIC escalation and mapping to regulatory pathways governing efflux and AmpC expression. CONCLUSIONS: Subinhibitory exposure to antibiotics without intrinsic antipseudomonal activity reproducibly selected for heritable multidrug-resistant phenotypes in P. aeruginosa. Convergent mutations arose in regulatory genes classically associated with direct antipseudomonal antibiotic pressure, demonstrating that resistance architectures can be selected independent of target engagement and underscoring the potential for collateral resistance under antibiotic stress.
Lasry et al. (2026) studied this question.