Interaction of quaternary ammonium ionic liquids with bacterial membranes – Studies with Escherichia coli R1–R4-type lipopolysaccharides

Abstract

Here we investigated the interaction between quaternary ammonium ionic liquids and the bacterial membrane of five different Escherichia coli strains. The strains were K-12 and R1–R4-type lipopolysaccharide strains. Additionally, Bacillus cereus was used as a typical Gram-positive bacterium for comparison. The ionic liquids used in the studies were theophylline- based quaternary ammonium salts with C8–C18 alkane chains. The studied compounds belong to third-generation ionic liquids comprising hydrophobic anions of natural origin, and they potentially exhibit biological activity. The interactions were studied using zeta potential measurements, a membrane lateral diffusion estimate based on the excimerization of pyrene, and cell surface hydrophobicity measurements. The minimum inhibitory and bactericidal concentrations were also studied to compare the responses of the bacterial strains to the ionic liquids. We found that the toxicity of the ionic liquids was strain-dependent, and that they strongly affected the Gram-negative bacterial membrane, causing changes of the zeta potential and membrane lateral diffusion. The results indicate that the interaction of ionic liquids with the bacteria depended significantly on the rough or smooth type of E. coli strains. Such relationships were not explained easily, however, some interesting correlations were found, e.g., the correlation between membrane lateral diffusion and ionic liquid toxicity. The results are important for further studies searching for new antibiotics and other drugs that affect the bacterial membrane. The ionic-liquid-mediated changes of the membrane zeta potential are also important for assessing the interactions of membranes and biofilms with nanoparticles.