Abstract
The interactions of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPEs) and oligo-phenylene ethynylenes (OPEs) with E. coli cells are investigated to gain insights into the differences in the dark killing mechanisms between CPEs and OPEs. A laboratory strain of E. coli with antibiotic resistance is included in this work to study the influence of antibiotic resistance on the antimicrobial activity of the CPEs and OPEs. In agreement with our previous findings, these compounds can efficiently perturb the bacterial cell wall and cytoplasmic membrane, resulting in bacterial cell death. Electron microscopy imaging and cytoplasmic membrane permeability assays reveal that the oligomeric OPEs penetrate the bacterial outer membrane and interact efficiently with the bacterial cytoplasmic membrane. In contrast, the polymeric CPEs cause serious damage to the cell surface. In addition, the minimum inhibitory concentration (MIC) and hemolytic concentration (HC) of the CPEs and OPEs are also measured to compare their antimicrobial activities against two different strains of E. coli with the compounds’ toxicity levels against human red blood cells (RBC). MIC and HC measurements are in good agreement with our previous model membrane perturbation study, which reveals that the different membrane perturbation abilities of the CPEs and OPEs are in part responsible for their selectivity towards bacteria compared to mammalian cells. Our study gives insight to several structural features of the PPE-based CPEs and OPEs that modulate their antimicrobial properties and that these features can serve as a basis for further tuning their structures to optimize antimicrobial properties.
Highlights
Due in part to the misuse of antibiotics, pathogenic bacteria have evolved a number of resistance strategies to attenuate the effect of antibiotics, such as generating novel proteolytic enzymes, altering membrane protein and lipid components, and changing the role of transporters [1]
We have demonstrated that most of the conjugated polyelectrolytes (CPEs) and oligo-phenylene ethynylenes (OPEs) can significantly perturb model bacterial membranes made of total lipids extracted from E. coli cells (ATCC 11303); the same strain of E. coli cells is used in the current study
We selected several cationic CPEs and OPEs with the same backbones but different numbers of repeat units or side chains to investigate the relationships among antimicrobial activity, hemolytic activity, and molecular structure
Summary
Due in part to the misuse of antibiotics, pathogenic bacteria have evolved a number of resistance strategies to attenuate the effect of antibiotics, such as generating novel proteolytic enzymes, altering membrane protein and lipid components, and changing the role of transporters [1]. Since the discovery of the antimicrobial properties of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPEs) [10], a wide range of structurally similar polymeric CPEs and oligomeric. OPEs (oligo-phenylene ethynylenes) have been synthesized [11,12,13,14,15,16]. These PPE-based CPE and OPE compounds are efficient broad-spectrum bactericides against both Gram-positive and Gram-negative bacteria. We have demonstrated that the CPEs and OPEs exhibit remarkable light-activated biocidal activities and moderate killing efficiencies in the dark. The light-induced biocidal activity of these compounds has been attributed to the ability of their excited states to generate corrosive reactive oxygen species after activation with UV-visible light, which can strongly damage biomacromolecules [17,18]
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