Self-Assembled Polyurethane Capsules with Selective Antimicrobial Activity against Gram-Negative E. coli

This article reports the antimicrobial activity of two segmented amphiphilic polyurethanes, PU-1 and PU-2, containing a primary or secondary amine group, respectively. In acidic water, intrachain H-bonding among the urethanes followed by hierarchical assembly resulted in the formation of capsules (D_h = 120 ± 20 and 100 ± 17 nm for PU-1 and PU-2, respectively) with a highly positive surface charge. They showed selective interactions with bacterial cell mimicking liposomes over mammalian cell mimicking liposomes with favorable enthalpy and entropy contributions, which was attributed to the electrostatic interaction and hydrophobic effect. Antimicrobial studies with Escherichia coli revealed very low minimum inhibitory concentration (MIC) values of 7.8 and 15.6 μg/mL for PU-1 and PU-2, respectively, indicating their ability to efficiently kill Gram-negative bacteria. Killing of Gram-positive Staphylococcus aureus was noticed only at C = 500 μg/mL, indicating unprecedented selectivity for E. coli, which was further confirmed by scanning electron microscopy (SEM) studies. Hemolysis assay revealed HC₅₀ values of 453 and 847 μg/mL for PU-1 and PU-2, respectively, which were >50 times higher than their respective MIC values, thus making them attractive antimicrobial materials. Ortho-nitrophenyl-β-galactoside (ONPG) assay and live-dead fluorescence assay confirmed that for both the polymers, a membrane disruption pathway was operative for wrapping of the bacterial membrane, similar to what was proposed for antimicrobial peptides. SEM images of polymer-treated E. coli bacteria helped in visualization of the pore formation and the disrupted membrane structure.