Phenotypic convergence and collateral susceptibility development in Pseudomonas aeruginosa under antibiotic exposure in ICU patients.

Antibiotic combination therapy is often used to broaden the antimicrobial spectrum, limit resistance and improve treatment efficacy. Several antibiotics show collateral effects where resistance to one antibiotic increases susceptibility to another. In intensive care units (ICUs), antibiotic treatments are frequently adjusted based on patient outcomes, without considering collateral effects. This provides a setting to study these effects in Pseudomonas aeruginosa (PA), a highly adaptable, multidrug-resistant (MDR), nosocomial pathogen. We compared longitudinal PA isolates from twenty-five ventilated ICU patients receiving various antibiotics to laboratory strains undergoing in vitro adaptive evolution under four antipseudomonal monotherapies. Prolonged exposure to certain antibiotics produced resistance with collateral effects. In vitro, increasing antibiotic pressure drove distinct mutational trajectories. In patients, the number of antibiotics administered did not correlate with resistance changes to those antibiotics, suggesting that switching may reduce persistence of resistance. Notably, an inverse correlation between resistance to non-administered antibiotics and the number of different antibiotic classes administered, aligns with the principles of collateral susceptibility driven by multi-class exposure. This study provides s real-world evidence that empirical antibiotic mixing in ICU patients leverages evolutionary trade-offs. Consequently, diversifying antibiotic pressure via multi-class exposure may attenuate the fixation and persistence of MDR phenotypes in critical care.