Effects of Conventional Surfactants on the Activity of Designed Antimicrobial Peptide.

In this article, the interaction between a designed antimicrobial peptide (AMP) G(IIKK)3I-NH2 (G3) and four typical conventional surfactants (sodium dodecyl sulfonate (SDS), hexadecyl trimethyl ammonium bromide (C16TAB), polyoxyethylene (23) lauryl ether (C12EO23), and tetradecyldimethylamine oxide (C14DMAO)) has been studied through surface tension measurement and circular dichroism (CD) spectroscopy. The antimicrobial activities of AMP/surfactant mixtures have also been studied with Gram-negative Escherichia coli, Gram-positive Staphylococcus aureus, and the fungus Candida albicans. The cytotoxicity of the AMP/surfactant mixtures has also been assessed with NIH 3T3 and human skin fibroblast (HSF) cells. The surface tension data showed that the AMP/SDS mixture was much more surface-active than SDS alone. CD results showed that G3 conformation changed from random coil, to ß-sheet, and then to α-helix with increasing SDS concentration, showing a range of structural transformation driven by the different interactions with SDS. The antimicrobial activity of G3 to Gram-negative and Gram-positive bacteria decreased in the presence of SDS due to the strong interaction of electrostatic attraction between the peptide and the surfactant. The interactions between G3 and C16TAB, C12EO23, and C14DMAO were much weaker than SDS. As a result, the surface tension of surfactants with G3 did not change much, neither did the secondary structures of G3. The antimicrobial activities of G3 were little affected in the presence of C12EO23, slightly improved by C14DMAO, and clearly enhanced by cationic surfactant C16TAB due to its strong cationic and antimicrobial nature, consistent with their surface physical activities as binary mixtures. Although AMP G3 did not show activity to fungus, the mixtures of AMP/C16TAB and AMP/C14DMAO could kill C. albicans at high surfactant concentrations. The mixtures had rather high cytotoxicity to NIH 3T3 and HSF cells although G3 is nontoxic to cells. Cationic AMPs can be formulated with nonionic, cationic, and zwitterionic surfactants during product development, but care must be taken when AMPs are formulated with anionic surfactants, as the strong electrostatic interaction may undermine their antimicrobial activity.

Reversible Thermoresponsive Peptide-PNIPAM Hydrogels for Controlled Drug Delivery.

Mixed thermoreversible gels were successfully fabricated by the addition of a thermosensitive polymer, poly(N-isopropylacrylamide) (PNIPAM), to fibrillar nanostructures self-assembled from a short peptide I3K. When the temperature was increased above the lower critical solution temperature of the PNIPAM, the molecules collapsed to form condensed globular particles, which acted as cross-links to connect different peptide nanofibrils and freeze their movements, resulting in the formation of a hydrogel. Since these processes were physically driven, such hydrogels could be reversibly switched between the sol and gel states as a function of temperature. As a model peptide, I3K was formulated with PNIPAM to produce a thermoreversible sol-gel system with a transition temperature of ∼33 °C, which is just below the body temperature. The antibacterial peptide of G(IIKK)3I-NH2 could be conveniently encapsulated in the hydrogel by the addition of the solution at lower temperatures in the sol phase and then increasing the temperature to be above 33 °C for gelation. The hydrogel gave a sustained and controlled linear release of G(IIKK)3I-NH2 over time. Using the peptide nanofibrils as three-dimensional scaffolds, such thermoresponsive hydrogels mimic the extracellular matrix and could potentially be used as injectable hydrogels for minimally invasive drug delivery or tissue engineering.

Aggregated Amphiphilic Antimicrobial Peptides Embedded in Bacterial Membranes.

Molecular dynamics (MD) simulations, stochastic optical reconstruction microscopy (STORM), and neutron reflection (NR) were combined to explore how antimicrobial peptides (AMPs) can be designed to promote the formation of nanoaggregates in bacterial membranes and impose effective bactericidal actions. Changes in the hydrophobicity of the designed AMPs were found to have a strong influence on their bactericidal potency and cytotoxicity. G(IIKK)3I-NH2 (G3) achieved low minimum inhibition concentrations (MICs) and effective dynamic kills against both antibiotic-resistant and -susceptible bacteria. However, a G3 derivative with weaker hydrophobicity, KI(KKII)2I-NH2 (KI), exhibited considerably lower membrane-lytic activity. In contrast, the more hydrophobic G(ILKK)3L-NH2 (GL) peptide achieved MICs similar to those observed for G3 but with worsened hemolysis. Both the model membranes studied by Brewster angle microscopy, zeta potential measurements, and NR and the real bacterial membranes examined with direct STORM contained membrane-inserted peptide aggregates upon AMP exposure. These structural features were well supported by MD simulations. By revealing how AMPs self-assemble in microbial membranes, this work provides important insights into AMP mechanistic actions and allows further fine-tuning of antimicrobial potency and cytotoxicity.

How do Self-Assembling Antimicrobial Lipopeptides Kill Bacteria?

Antimicrobial peptides are promising alternatives to traditional antibiotics. A group of self-assembling lipopeptides was formed by attaching an acyl chain to the N-terminus of α-helix-forming peptides with the sequence Cx-G(IIKK)yI-NH2 (CxGy, x = 4-12 and y = 2). CxGy self-assemble into nanofibers above their critical aggregation concentrations (CACs). With increasing x, the CACs decrease and the hydrophobic interactions increase, promoting secondary structure transitions within the nanofibers. Antimicrobial activity, determined by the minimum inhibition concentration (MIC), also decreases with increasing x, but the MICs are significantly smaller than the CACs, suggesting effective bacterial membrane-disrupting power. Unlike conventional antibiotics, both C8G2 and C12G2 can kill Staphylococcus aureus and Escherichia coli after only minutes of exposure under the concentrations studied. C12G2 nanofibers have considerably faster killing dynamics and lower cytotoxicity than their nonaggregated monomers. Antimicrobial activity of peptide aggregates has, to date, been underexploited, and it is found to be a very promising mechanism for peptide design. Detailed evidence for the molecular mechanisms involved is provided, based on superresolution fluorescence microscopy, solid-state nuclear magnetic resonance, atomic force microscopy, neutron scattering/reflectivity, circular dichroism, and Brewster angle microscopy.