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.