The recent rise of multidrug resistant microbial strains requires development of new and novel therapeutic alternatives. In this study, we present a novel antibacterial system that comprises of modified naturally abundant antimicrobial peptides in conjugation with silver nanoparticles. Further, we propose a simple route to incorporate a cysteine residue either at the N- or C-terminal of the parent peptide. Tagging a cysteine residue at the terminals not only enhances the binding propensity of the resultant peptide with the silver nanoparticle, but also increases its antimicrobial property against several pathogenic bacterial strains including K. pneumoniae. The minimum inhibitory concentration (MIC) values of the cysteine tagged nanoconjugates were obtained in the range of 5–15 μM compared to 50 μM for peptides devoid of the cysteines. The origin and mechanism of such improved activity of the conjugates were investigated using NMR spectroscopy and molecular dynamics (MD) simulations. The application of ¹³C-isotope labelled media to track the metabolic lifecycle of E. coli cells provided further insights into the system. MD simulations showed that pore formation in membrane bilayer is mediated through a hydrophobic collapse mechanism. The design strategy described herein opens up new-avenues for using biocompatible nanomedicines as a potential alternative to conventional antibiotics.

Antimicrobial peptides are important candidates for developing new classes of antibiotics because of their potency against antibiotic-resistant pathogens. Current research focuses on topical applications and it is unclear how to design peptides with systemic efficacy. To address this problem, we designed two potent peptides by combining database-guided discovery with structure-based design. When bound to membranes, these two short peptides with an identical amino acid composition can adopt two distinct amphipathic structures: A classic horizontal helix (horine) and a novel vertical spiral structure (verine). Their horizontal and vertical orientations on membranes were determined by solid-state ¹⁵N NMR data. While horine was potent primarily against gram-positive pathogens, verine showed broad-spectrum antimicrobial activity. Both peptides protected greater than 80% mice from infection-caused deaths. Moreover, horine and verine also displayed significant systemic efficacy in different murine models comparable to conventional antibiotics. In addition, they could eliminate resistant pathogens and preformed biofilms. Significantly, the peptides showed no nephrotoxicity to mice after intraperitoneal or intravenous administration for 1 wk. Our study underscores the significance of horine and verine in fighting drug-resistant pathogens. Significance It is the dream of many scientists to develop antimicrobial peptides (AMPs) into potent antibiotics to combat resistant pathogens and their biofilms. However, the success is low due to limited knowledge on peptide design. Natural AMPs are diverse in sequence, activity, and source. We found a linear relationship between the averaged contents of arginine (R) and hydrophobic amino acids (Pho) of over 3,000 AMPs in the current Antimicrobial Peptide Database (http://aps.unmc.edu/AP). Based on this R–Pho relationship, we identified a peptide template and designed two representative amphipathic peptides with distinct structures and activities. Remarkably, horine and verine showed systemic efficacy in mice, but no nephrotoxicity to rodents. Our study advances peptide design and provides two antibiotics to combat drug-resistant pathogens.

This review describes the discovery, structure, activity, engineered constructs, and applications of KR-12, the smallest antibacterial peptide of human cathelicidin LL-37, the production of which can be induced under sunlight or by vitamin D. It is a moonlighting peptide that shows both antimicrobial and immune-regulatory effects. Compared to LL-37, KR-12 is extremely appealing due to its small size, lack of toxicity, and narrow-spectrum antimicrobial activity. Consequently, various KR-12 peptides have been engineered to tune peptide activity and stability via amino acid substitution, end capping, hybridization, conjugation, sidechain stapling, and backbone macrocyclization. We also mention recently discovered peptides KR-8 and RIK-10 that are shorter than KR-12. Nano-formulation provides an avenue to targeted delivery, controlled release, and increased bioavailability. In addition, KR-12 has been covalently immobilized on biomaterials/medical implants to prevent biofilm formation. These constructs with enhanced potency and stability are demonstrated to eradicate drug-resistant pathogens, disrupt preformed biofilms, neutralize endotoxins, and regulate host immune responses. Also highlighted are the safety and efficacy of these peptides in various topical and systemic animal models. Finaly, we summarize the achievements and discuss future developments of KR-12 peptides as cosmetic preservatives, novel antibiotics, anti-inflammatory peptides, and microbiota-restoring agents.