Protonation State-dependent Interaction of Polycationic Polymyxins with the <i>Pseudomonas aeruginosa</i> Outer Membrane Using Colistin A as a Model

The rapid spread of antimicrobial resistance (AMR) has relaunched Colistin A, a member of the polycationic polymyxin (PMBs) family, as a last resort option against infection by Gram-negative pathogens such as <i>Pseudomonas aeruginosa</i>. However, the presence of positively charged α,γ-Diamino Butyric acid (Dab) moieties in PMBs is associated with clinical toxicity, so efforts have been made toward the development of derivatives with a reduced total charge. In this study, we used all-atom molecular dynamics (MD) simulations to elucidate the interaction of PMBs with an evolved model of the <i>P. aeruginosa</i> outer membrane (OM) that includes Lipid A and the entire oligosaccharide core. To investigate the potential effects of pH and total charge on OM perturbation by Colistin A, used as a model scaffold, four different protonation states were studied. Results reveal that the Colistin A equipped with an overall +2 formal charge deeply penetrates the OM, displaces Ca²⁺ ions within the simulated time scale, and destabilizes OM integrity with a higher efficacy than other protonation states. These results highlight the importance of the total charge of small molecules directed toward the OM of Gram-negative pathogens such as <i>P. aeruginosa</i>, providing additional insights into the mechanism of action of Colistin A, essential for guiding the rational design of effective PMB derivatives.