Structure-Activity Relationship Study to Develop Peptide Amphiphiles as Species-Specific Antimicrobials

Insight into the Antibacterial Activities of Pyridinium-Based Cationic Pillar[5]arene with Controllable Hydrophobic Chain Lengths against Staphylococcus aureus

The increasing number of infections caused by pathogenic bacteria has severely affected human society. More and more deaths were originated from Gram-positive methicillin-resistant Staphylococcus aureus (MRSA) infection each year. The potential and excellent bacteriostatic activity and resistance to biofilm formation of pillar[5]arene with different functional groups attract important attention to further study the relationship between antimicrobial activity and cytotoxicity by varying the length of the hydrophobic chain, the number of positive charges, and the hydrophobic/hydrophilic balance of the molecule. In this work, four pyridinium-based cationic pillar[5]arene (PPs) with linear aliphatic chains of different lengths were synthesized. After systematic characterization, their inhibition activities against S. aureus were investigated. It revealed that PP6 (six methylenes in each linker) exhibited excellent inhibition activity against S. aureus (ATCC 6538) with a minimum inhibitory concentration (MIC) of 3.91 μg/mL and a minimum bactericidal concentration (MBC) of 62.50 μg/mL. As expected, PP6 exhibited the strongest antibiofilm ability and negligible antimicrobial resistance even after the 20th passage. A study of the action mechanism of selected PPs on the bacterial membrane depolarization and permeability by transmission electron microscopy (TEM) disclosed that the cationic pyridine groups of PPs inserted into the negatively charged bacterial membranes, thereby leading to membranolysis, cytoplasmic content leakage, and cell death. Importantly, PPs all showed very low toxicity to mammalian cells (L929 and HBZY-1), which provided a significant reference for the construction of hypotoxic antibacterial biomaterials for multiple drug-resistant bacteria based on pyridinium-grafted cationic macrocycles with controllable hydrophobic chain lengths.

Current scenario of indole hybrids with antibacterial potential against Acinetobacter baumannii pathogens: A mini-review

Acinetobacter baumannii with the capability to "escape" almost all currently available antibacterials is eroding the safety of basic medical interventions and is an increasing cause of mortality globally, prompting a substantial requirement for new classes of antibacterial agents. Indoles participate in the regulation of persistent bacterial formation, biofilm formation, plasmid stability, and drug resistance. In particular, indole hybrids demonstrated promising antibacterial activity against both drug-sensitive and drug-resistant A. baumannii pathogens, representing a fertile source for the discovery of novel therapeutic agents for clinical deployment in controlling A. baumannii infections. This mini-review outlines the current innovations of indole hybrids with antibacterial activity against A. baumannii pathogens, covering articles published from 2020 to the present, to open new avenues for exploring novel anti-A. baumannii candidates.

Enhanced Hydrogen Bonding by Urea Functionalization Tunes the Stability and Biological Properties of Peptide Amphiphiles

Self-assembled nanostructures such as those formed by peptide amphiphiles (PAs) are of great interest in biological and pharmacological applications. Herein, a simple and widely applicable chemical modification, a urea motif, was included in the PA's molecular structure to stabilize the nanostructures by virtue of intermolecular hydrogen bonds. Since the amino acid residue nearest to the lipid tail is the most relevant for stability, we decided to include the urea modification at that position. We prepared four groups of molecules (13 PAs in all), with varying levels of intermolecular cohesion, using amino acids with distinct β-sheet promoting potential and/or containing hydrophobic tails of distinct lengths. Each subset contained one urea-modified PA and nonmodified PAs, all with the same peptide sequence. The varied responses of these PAs to variations in pH, temperature, counterions, and biologically related proteins were examined using microscopic, X-ray, spectrometric techniques, and molecular simulations. We found that the urea group contributes to the stabilization of the morphology and internal arrangement of the assemblies against environmental stimuli for all peptide sequences. In addition, microbiological and biological studies were performed with the cationic PAs. These assays reveal that the addition of urea linkages affects the PA-cell membrane interaction, showing the potential to increase the selectivity toward bacteria. Our data indicate that the urea motif can be used to tune the stability of a wide range of PA nanostructures, allowing flexibility on the biomaterial's design and opening a myriad of options for clinical therapies.