Cationic ∝-Conjugated Polyelectrolyte Shows Antimicrobial Activity by Causing Lipid Loss and Lowering Elastic Modulus of Bacteria

Cationic, ∝-conjugated oligo-/polyelectrolytes (CCOEs/CCPEs) have shown great potential as antimicrobial materials to fight against antibiotic resistance. In this work, we treated wild-type and ampicillin-resistant (amp-resistant) Escherichia coli (E. coli) with a promising cationic, ∝-conjugated polyelectrolyte (P1) with a phenylene-based backbone and investigated the resulting morphological, mechanical, and compositional changes of the outer membrane of bacteria in great detail. The cationic quaternary amine groups of P1 led to electrostatic interactions with negatively charged moieties within the outer membrane of bacteria. Using atomic force microscopy (AFM), high-resolution transmission electron microscopy (TEM), we showed that due to this treatment, the bacterial outer membrane became rougher, decreased in stiffness/elastic modulus (AFM nanoindentation), formed blebs, and released vesicles near the cells. These evidences, in addition to increased staining of the P1-treated cell membrane by lipophilic dye Nile Red (confocal laser scanning microscopy (CLSM)), suggested loosening/disruption of packing of the outer cell envelope and release and exposure of lipid-based components. Lipidomics and fatty acid analysis confirmed a significant loss of phosphate-based outer membrane lipids and fatty acids, some of which are critically needed to maintain cell wall integrity and mechanical strength. Lipidomics and UV-vis analysis also confirmed that the extracellular vesicles released upon treatment (AFM) are composed of lipids and cationic P1. Such surface alterations (vesicle/bleb formation) and release of lipids/fatty acids upon treatment were effective enough to inhibit further growth of E. coli cells without completely disintegrating the cells and have been known as a defense mechanism of the cells against cationic antimicrobial agents.