Antibacterial mechanisms of GN-2 derived peptides and peptoids against Escherichia coli

Optimization of sequence and chiral content enhances therapeutic potential of tilapia piscidin peptides

Because antimicrobial peptides (AMPs) often exhibit broad-spectrum bactericidal potency, we sought to develop peptide-based antimicrobials for potential clinical use against drug-resistant pathogens. To accomplish this goal, we first optimized the amino acid sequence of a broad-spectrum AMP known as Tilapia Piscidin 4 (TP4). Then, we used the optimized sequence to create a pair of heterochiral variants (TP4-α and TP4-β) with different percentages of D-enantiomers, as poly-L peptides often exhibit poor pharmacokinetic profiles. The conformations of the peptide pair exhibited inverted chirality according to CD and NMR spectroscopic analyses. Both heterochiral peptides displayed enhanced stability and low hemolysis activities. Irrespective of their different d-enantiomer contents, both heterochiral peptides exhibited bactericidal activities in the presence of human serum or physiological enzymes. However, the peptide with higher d-amino acid content (TP4-β) caused better bacterial clearance when tested in mice infected with NDM-1 K. pneumoniae. In addition, we observed a relatively higher hydrogen bonding affinity in a simulation of the interaction between TP4-β and a model bacterial membrane. In sum, our results demonstrate that the current design strategy may be applicable for development of new molecules with enhanced stability and in vivo antimicrobial activity.

Nano-topography and functionalization with the synthetic peptoid GN2-Npm9 as a strategy for antibacterial and biocompatible titanium implants

In recent years, antimicrobial peptides (AMPs) have attracted great interest in scientific research, especially for biomedical applications such as drug delivery and orthopedic applications. Since they are readily degradable in the physiological environment, scientific research has recently been trying to make AMPs more stable. Peptoids are synthetic N-substituted glycine oligomers that mimic the structure of peptides. They have a structure that does not allow proteolytic degradation, which makes them more stable while maintaining microbial activity. This structure also brings many advantages to the molecule, such as greater diversity and specificity, making it more suitable for biological applications. For the first time, a synthesized peptoid (GN2-Npm9) was used to functionalize a nanometric chemically pre-treated (CT) titanium surface for bone-contact implant applications. A preliminary characterization of the functionalized surfaces was performed using the contact angle measurements and zeta potential titration curves. These preliminary analyses confirmed the presence of the peptoid and its adsorption on CT. The functionalized surface had a hydrophilic behaviour (contact angle = 30°) but the hydrophobic tryptophan-like residues were also exposed. An electrostatic interaction between the lysine residue of GN2-Npm9 and the surface allowed a chemisorption mechanism. The biological characterization of the CT_GN2-Nmp9 surfaces demonstrated the ability to prevent surface colonization and biofilm formation by the pathogens Escherichia coli and Staphylococcus epidermidis thus showing a broad-range activity. The cytocompatibility was confirmed by human mesenchymal stem cells. Finally, a bacteria-cells co-culture model was applied to demonstrate the selective bioactivity of the CT_GN2-Nmp9 surface that was able to preserve colonizing cells adhered to the device surface from bacterial infection.

Sequential rearrangement and stereochemical reorganization to design an antimicrobial peptide with enhanced stability

Antimicrobial peptides (AMPs) are natural molecules that function within the innate immune system to counteract pathogenic invasion and minimize the detrimental consequences of infection. However, utilizing these molecules for medical applications has been challenging. In this study, we selected a model AMP with poor stability, Tilapia Piscidin 4 (TP4), and modified its sequence and chirality (TP4-γ) to improve its potential for clinical application. The strategy of chirality inversion was inspired by the cereulide peptide, which has a DDLL enantiomer pattern and exhibits exceptional stability. Sequential substitution of key residues and selective chirality inversion yielded a less toxic peptide with enhanced stability and notable antimicrobial activity. In addition to its superior stability profile and antimicrobial activity, TP4-γ treatment reduced the level of LPS-induced nitric oxide (NO) release in a macrophage cell line. This reduction in NO release may reflect anti-inflammatory properties, as NO is widely known to promote inflammatory processes. Hence, our heterochiral peptide construct shows a more suitable pharmacokinetic profile than its parental compound, and further studies are warranted to develop the molecule for potential clinical application.

A conjugate of chlorin e6 and cationic amphipathic peptoid: a dual antimicrobial and anticancer photodynamic therapy agent

Cationic amphipathic structures are often utilized in natural membrane-active host-defense peptides. Negatively charged surface membranes of rapidly proliferating bacterial and cancer cells have been targeted by various synthetic peptides and peptidomimetics adopting the structural motif. Herein, we synthesized a set of conjugates composed of cationic amphipathic peptoids (i.e., oligo-N-substituted glycines) and a chlorin photosensitizer, named chlorin e6 (Ce6)-peptoid conjugates (CPCs). Among the nine CPCs, CPC 7, composed of Ce6, a PEG linker, and guanidine-rich helical amphipathic peptoids, exhibited a distinct photoresponsive inactivation of Gram-positive and Gram-negative bacteria. Subsequent studies showed that CPC 7 effectively killed various cancer cells after irradiation with red light (655 nm), suggesting the potential of CPC 7 as a dual antimicrobial and anticancer agent. Confocal laser scanning microscopy and flow cytometry data suggested that CPC 7 could induce apoptotic cell death. Our results show the potential of peptoid-based photosensitizer conjugates as a versatile platform for antimicrobial and anticancer photodynamic therapy agents and peptoid therapeutics.

Efficacy of natural antimicrobial peptides versus peptidomimetic analogues: a systematic review

Aims: This systematic review was carried out to determine whether synthetic peptidomimetics exhibit significant advantages over antimicrobial peptides in terms of in vitro potency. Structural features - molecular weight, charge and length - were examined for correlations with activity. Methods: Original research articles reporting minimum inhibitory concentration  values against Escherichia coli, indexed until 31 December 2020, were searched in PubMed/ScienceDirect/Google Scholar and evaluated using mixed-effects models. Results: In vitro antimicrobial activity of peptidomimetics resembled that of antimicrobial peptides. Net charge significantly affected minimum inhibitory concentration values (p < 0.001) with a trend of 4.6% decrease for increments in charge by +1. Conclusion: AMPs and antibacterial peptidomimetics exhibit similar potencies, providing an opportunity to exploit the advantageous stability and bioavailability typically associated with peptidomimetics.

Development of an Anti-Biofilm Screening Technique Leads to the Discovery of a Peptoid with Efficacy against Candida albicans

Bacteria and fungi can secrete and reside within a complex polysaccharide matrix, forming a biofilm that protects these pathogens from the immune response and conventional antibiotics. Because many microbial pathogens grow within biofilms in clinical settings, there is a need for antimicrobial agents effective against biofilm-protected infections. We report the adaptation of a phenotypic high-throughput assay for discovering antimicrobial peptoids toward the screening of combinatorial libraries against established biofilms. This method, termed the Inverted Peptoid Library Agar Diffusion (iPLAD) assay, required optimization of growth media, reducing reagent, and fungal viability reporter. Once optimized, iPLAD was used to screen a combinatorial peptoid library against Candida albicans, a biofilm-forming fungal pathogen responsible for most hospital-acquired infections. This screening resulted in a lipopeptoid termed RMG9-11 with excellent activity against several species of Candida, including drug-resistant strains of C. albicans and the emerging and dangerous C. auris. Additionally, the cytotoxicity of RMG9-11 against several mammalian cell lines was minimal. This work provides a new method for the identification of compounds effective against biofilm-protected pathogens and demonstrates its utility by identifying a promising anti-Candida peptoid.

Selective Antifungal Activity and Fungal Biofilm Inhibition of Tryptophan Center Symmetrical Short Peptide

Candida albicans, an opportunistic fungus, causes dental caries and contributes to mucosal bacterial dysbiosis leading to a second infection. Furthermore, C.albicans forms biofilms that are resistant to medicinal treatment. To make matters worse, antifungal resistance has spread (albeit slowly) in this species. Thus, it has been imperative to develop novel, antifungal drug compounds. Herein, a peptide was engineered with the sequence of RRFSFWFSFRR-NH2; this was named P19. This novel peptide has been observed to exert disruptive effects on fungal cell membrane physiology. Our results showed that P19 displayed high binding affinity to lipopolysaccharides (LPS), lipoteichoic acids (LTA) and the plasma membrane phosphatidylinositol (PI), phosphatidylserine (PS), cardiolipin, and phosphatidylglycerol (PG), further indicating that the molecular mechanism of P19 was not associated with the receptor recognition, but rather related to competitive interaction with the plasma membrane. In addition, compared with fluconazole and amphotericin B, P19 has been shown to have a lower potential for resistance selection than established antifungal agents.

Approaches for the inhibition and elimination of microbial biofilms using macromolecular agents

Biofilms are complex three-dimensional structures formed at interfaces by the vast majority of bacteria and fungi. These robust communities have an important detrimental impact on a wide range of industries and other facets of our daily lives, yet their removal is challenging owing to the high tolerance of biofilms towards conventional antimicrobial agents. This key issue has driven an urgent search for new innovative antibiofilm materials. Amongst these emerging approaches are highly promising materials that employ aqueous-soluble macromolecules, including peptides, proteins, synthetic polymers, and nanomaterials thereof, which exhibit a range of functionalities that can inhibit biofilm formation or detach and destroy organisms residing within established biofilms. In this Review, we outline the progress made in inhibiting and removing biofilms using macromolecular approaches, including a spotlight on cutting-edge materials that respond to environmental stimuli for "on-demand" antibiofilm activity, as well as synergistic multi-action antibiofilm materials. We also highlight materials that imitate and harness naturally derived species to achieve new and improved biomimetic and biohybrid antibiofilm materials. Finally, we share some speculative insights into possible future directions for this exciting and highly significant field of research.

Helical Antimicrobial Peptide Foldamers Containing Non-proteinogenic Amino Acids

Antimicrobial peptides (AMPs) are potential novel therapeutic drugs against microbial infections. Most AMPs function by disrupting microbial membranes because of their amphipathic properties and ordered secondary structures. In this minireview, we describe recent efforts to develop helical AMP foldamers containing non-proteinogenic amino acids, such as α,α-disubstituted α-amino acids, β-amino acids, γ-amino acids, side-chain stapling and N-alkyl glycines.

Isolation of Antimicrobial Genes from Oryza rufipogon Griff by Using a Bacillus subtilis Expression System with Potential Antimicrobial Activities

Antimicrobial genes are distributed in all forms of life and provide a primary defensive shield due to their unique broad-spectrum resistance activities. To better isolate these genes, we used the Bacillus subtilis expression system as the host cells to build Oryza rufipogon Griff cDNA libraries and screen potential candidate genes from the library at higher flux using built-in indicator bacteria. We observed that the antimicrobial peptides OrR214 and OrR935 have strong antimicrobial activity against a variety of Gram-positive and Gram-negative bacteria, as well as several fungal pathogens. Owing to their high thermal and enzymatic stabilities, these two peptides can also be used as field biocontrol agents. Furthermore, we also found that the peptide OrR214 (MIC 7.7-10.7 μM) can strongly inhibit bacterial growth compared to polymyxin B (MIC 5-25 μM) and OrR935 (MIC 33-44 μM). The cell flow analysis, reactive oxygen burst, and electron microscopy (scanning and transmission electron microscopy) observations showed that the cell membranes were targeted by peptides OrR214 and OrR935, which revealed the mode of action of bacteriostasis. Moreover, the hemolytic activity, toxicity, and salt sensitivity experiments demonstrated that these two peptides might have the potential to be used for clinical applications. Overall, OrR214 and OrR935 antimicrobial peptides have a high-throughput bacteriostatic activity that acts as a new form of antimicrobial agent and can be used as a raw material in the field of drug development.

Alternating Cationic-Hydrophobic Peptide/Peptoid Hybrids: Influence of Hydrophobicity on Antibacterial Activity and Cell Selectivity

The influence of hydrophobicity on antibacterial activity versus the effect on the viability of mammalian cells for peptide/peptoid hybrids was examined for oligomers based on the cationic Lys-like peptoid residue combined with each of 28 hydrophobic amino acids in an alternating sequence. Their relative hydrophobicity was correlated to activity against both Gram-negative and Gram-positive species, human red blood cells, and HepG2 cells. This identified hydrophobic side chains that confer potent antibacterial activity (e. g., MICs of 2-8 μg/mL against E. coli) and low toxicity toward mammalian cells (<10 % hemolysis at 400 μg/mL and IC₅₀ >800 μg/mL for HepG2 viability). Most peptidomimetics retained activity against drug-resistant strains. These findings corroborate the hypothesis that for related peptidomimetics two hydrophobicity thresholds may be identified: i) it should exceed a certain level in order to confer antibacterial activity, and ii) there is an upper limit, beyond which cell selectivity is lost. It is envisioned that once identified for a given subclass of peptide-like antibacterials such thresholds can guide further optimisation.

Synthesis of Peptoids Containing Multiple Nhtrp and Ntrp Residues: A Comparative Study of Resin, Cleavage Conditions and Submonomer Protection

Peptoids hold status as peptide-mimetics with versatile biological applications due to their proteolytic stability and structural diversity. Among those that have been studied in different biological systems, are peptoids with dominant balanced hydrophobic and charge distribution along the backbone. Tryptophan is an important amino acid found in many biologically active peptides. Tryptophan-like side chains in peptoids allow H-bonding, which is absent from the parent backbone, due to the unique indole ring. Furthermore, the rigid hydrophobic core and flat aromatic system influence the positioning in the hydrocarbon core and allows accommodating tryptophan-like side chains into the interfacial regions of bacterial membranes and causing bacterial membrane damage. Incorporating multiple tryptophan-like side chains in peptoids can be tricky and there is a lack of suitable, synthetic routes established. In this paper, we investigate the synthesis of peptoids rich in Nhtrp and Ntrp residues using different resins, cleavage conditions, and unprotected as well as tert-butyloxycarbonyl-protected amines suitable for automated solid-phase submonomer peptoid synthesis protocols.