Ultrashort antibacterial and antifungal lipopeptides

Antimicrobial Peptide Mimics for Clinical Use: Does Size Matter?

The search for efficient antimicrobial therapies that can alleviate suffering caused by infections from resistant bacteria is more urgent than ever before. Infections caused by multi-resistant pathogens represent a significant and increasing burden to healthcare and society and researcher are investigating new classes of bioactive compounds to slow down this development. Antimicrobial peptides from the innate immune system represent one promising class that offers a potential solution to the antibiotic resistance problem due to their mode of action on the microbial membranes. However, challenges associated with pharmacokinetics, bioavailability and off-target toxicity are slowing down the advancement and use of innate defensive peptides. Improving the therapeutic properties of these peptides is a strategy for reducing the clinical limitations and synthetic mimics of antimicrobial peptides are emerging as a promising class of molecules for a variety of antimicrobial applications. These compounds can be made significantly shorter while maintaining, or even improving antimicrobial properties, and several downsized synthetic mimics are now in clinical development for a range of infectious diseases. A variety of strategies can be employed to prepare these small compounds and this review describes the different compounds developed to date by adhering to a minimum pharmacophore based on an amphiphilic balance between cationic charge and hydrophobicity. These compounds can be made as small as dipeptides, circumventing the need for large compounds with elaborate three-dimensional structures to generate simplified and potent antimicrobial mimics for a range of medical applications. This review highlight key and recent development in the field of small antimicrobial peptide mimics as a promising class of antimicrobials, illustrating just how small you can go.

R. V, Debnath S, Satpati P, Chatterjee S. Delineating the Mechanism of Action of a Protease Resistant and Salt Tolerant Synthetic Antimicrobial Peptide against Pseudomonas aeruginosa

Rapidly growing antimicrobial resistance (AMR) against antibiotics has propelled the development of synthetic antimicrobial peptides (AMPs) as potential antimicrobial agents. An antimicrobial peptide Nle-Dab-Trp-Nle-Dab-Dab-Nle-CONH₂ (P36; Nle = norleucine, Dab = diaminobutyric acid, Trp = tryptophan) potent against Pseudomonas aeruginosa (P. aeruginosa) has been developed in the present study. Rational design strategy adopted in this study led to the improvisation of the therapeutic qualities such as activity, salt tolerance, cytotoxicity, and protease resistance of the template peptide P4, which was earlier reported from our group. P36 exhibited salt tolerant antimicrobial potency against P. aeruginosa, along with very low cytotoxicity against mammalian cell lines. P36 was found to be nonhemolytic and resistant toward protease degradation which qualified it as a potent antimicrobial agent. We have investigated the mechanism of action of this molecule in detail using several experimental techniques (spectroscopic, biophysical, and microscopic) and molecular dynamics simulations. P36 was a membrane active AMP with membrane destabilization and deformation abilities, leading to leakage of the intracellular materials and causing eventual cell death. The interaction between P36 and the microbial membrane/membrane mimics was primarily driven by electrostatics. P36 was unstructured in water and upon binding to the microbial membrane mimic SDS, suggesting no influence of secondary structure on its antimicrobial potency. Positive charge, optimum hydrophobic-hydrophilic balance, and chain length remained the most important concerns to be addressed while designing small cationic antimicrobial peptides.

Microbial Metabolite Inspired β-Peptide Polymers Displaying Potent and Selective Antifungal Activity

Potent and selective antifungal agents are urgently needed due to the quick increase of serious invasive fungal infections and the limited antifungal drugs available. Microbial metabolites have been a rich source of antimicrobial agents and have inspired the authors to design and obtain potent and selective antifungal agents, poly(DL-diaminopropionic acid) (PDAP) from the ring-opening polymerization of β-amino acid N-thiocarboxyanhydrides, by mimicking ε-poly-lysine. PDAP kills fungal cells by penetrating the fungal cytoplasm, generating reactive oxygen, and inducing fungal apoptosis. The optimal PDAP displays potent antifungal activity with minimum inhibitory concentration as low as 0.4 µg mL-1 against Candida albicans, negligible hemolysis and cytotoxicity, and no susceptibility to antifungal resistance. In addition, PDAP effectively inhibits the formation of fungal biofilms and eradicates the mature biofilms. In vivo studies show that PDAP is safe and effective in treating fungal keratitis, which suggests PDAPs as promising new antifungal agents.

Short Guanidinium-Functionalized Poly(2-oxazoline)s Displaying Potent Therapeutic Efficacy on Drug-Resistant Fungal Infections

New antifungals are urgently needed to combat invasive fungal infections, due to limited types of available antifungal drugs and frequently encountered side effects, as well as the quick emergence of drug-resistance. We previously developed amine-pendent poly(2-oxazoline)s (POXs) as synthetic mimics of host defense peptides (HDPs) to have antibacterial properties, but with poor antifungal activity. Hereby, we report the finding of short guanidinium-pendent POXs, inspired by cell-penetrating peptides, as synthetic mimics of HDPs to display potent antifungal activity, superior mammalian cells versus fungi selectivity, and strong therapeutic efficacy in treating local and systemic fungal infections. Moreover, the unique antifungal mechanism of fungal cell membrane penetration and organelle disruption explains the insusceptibility of POXs to antifungal resistance. The easy synthesis and structural diversity of POXs imply their potential as a class of promising antifungal agents.

Spermine-Conjugated Short Proline-Rich Lipopeptides as Broad-Spectrum Intracellular Targeting Antibacterial Agents

Toward the design of new proline-rich peptidomimetics, a short peptide segment, present in several proline-rich antimicrobial peptides (AMPs), was selected. Fatty acids of varying lengths and spermine were conjugated at the N- and C-terminals of the peptide, respectively. Spermine-conjugated lipopeptides, C₁₀-PR-Spn and C₁₂-PR-Spn, exhibited minimum inhibitory concentrations within 1.5-6.2 μM against the tested pathogens including resistant bacteria and insignificant hemolytic activity against human red blood cells up to 100 μM concentrations and demonstrated resistance against trypsin digestion. C₁₀-PR-Spn and C₁₂-PR-Spn showed synergistic antimicrobial activity against multidrug-resistant methicillin-resistant Staphylococcus aureus with several tested antibiotics. These lipopeptides did not permeabilize bacterial membrane-mimetic lipid vesicles or damage the Escherichia coli membrane like the nonmembrane-lytic AMP, buforin-II. The results suggested that C₁₀-PR-Spn and C₁₂-PR-Spn could interact with the 70S ribosome of E. coli and inhibit its protein synthesis. C₁₀-PR-Spn and C₁₂-PR-Spn demonstrated superior clearance of bacteria from the spleen, liver, and kidneys of mice, infected with S. aureus ATCC 25923 compared to levofloxacin.

Design and Characterization of Myristoylated and Non-Myristoylated Peptides Effective against Candida spp. Clinical Isolates

The increasing resistance of fungi to antibiotics is a severe challenge in public health, and newly effective drugs are required. Promising potential medications are lipopeptides, linear antimicrobial peptides (AMPs) conjugated to a lipid tail, usually at the N-terminus. In this paper, we investigated the in vitro and in vivo antifungal activity of three short myristoylated and non-myristoylated peptides derived from a mutant of the AMP Chionodracine. We determined their interaction with anionic and zwitterionic membrane-mimicking vesicles and their structure during this interaction. We then investigated their cytotoxic and hemolytic activity against mammalian cells. Lipidated peptides showed a broad spectrum of activity against a relevant panel of pathogen fungi belonging to Candida spp., including the multidrug-resistant C. auris. The antifungal activity was also observed vs. biofilms of C. albicans, C. tropicalis, and C. auris. Finally, a pilot efficacy study was conducted on the in vivo model consisting of Galleria mellonella larvae. Treatment with the most-promising myristoylated peptide was effective in counteracting the infection from C. auris and C. albicans and the death of the larvae. Therefore, this myristoylated peptide is a potential candidate to develop antifungal agents against human fungal pathogens.

How do terminal modifications of short designed IIKK peptide amphiphiles affect their antifungal activity and biocompatibility?

HYPOTHESIS

The widespread and prolonged use of antifungal antibiotics has led to the rapid emergence of multidrug resistant Candida species that compromise current treatments. Natural and synthetic antimicrobial peptides (AMPs) offer potential alternatives but require further development to overcome some of their current drawbacks. AMPs kill pathogenic fungi by permeabilising their membranes but it remains unclear how AMPs can be designed to maximise their antifungal potency whilst minimising their toxicity to host cells.

EXPERIMENTS

We have designed a group of short (IIKK)₃ AMPs via selective terminal modifications ending up with different amphiphilicities. Their antifungal performance was assessed by minimum inhibition concentration (MICs) and dynamic killing to 4 Candida strains and Cryptococcus neoformans, and the minimum biofilm-eradicating concentrations to kill 95% of the C. albicans biofilms (BEC₉₅). Different antifungal actions were interpreted on the basis of structural disruptions of the AMPs to small unilamellar vesicles from fluorescence leakage, Zeta potential, small angle neutron scattering (SANS) and molecular dynamics simulations (MD).

FINDING

AMPs possess high antifungal activities against the Candida species and Cryptococcus neoformans; some of them displayed faster dynamic killing than antibiotics like amphotericin B. G(IIKK)₃I-NH₂ and (IIKK)₃II-NH₂ were particularly potent against not only planktonic microbes but also fungal biofilms with low cytotoxicity to host cells. It was found that their high selectivity and fast action were well correlated to their fast membrane lysis, evident from data measured from Zeta potential measurements, SANS and MD, and also consistent with the previously observed antibacterial and anticancer performance. These studies demonstrate the important role of colloid and interface science in further developing short, potent and biocompatible AMPs towards clinical treatments via structure design and optimization.

Investigation of putative antimicrobial peptides in Carassius gibel, revealing a practical approach to screening antimicrobials

Antimicrobial peptides (AMPs) and their mimics are rapidly gaining attention as a new class of antimicrobials due to their clinical potential. AMPs are widely distributed throughout nature and participate in the innate host defense. In this study, 18 AMPs, including 3 β-defensins, 3 hepcidins, 4 liver-expressed antimicrobial peptide 2 (LEAP-2) compounds, 4 g-type lysozymes, 2 c-type lysozymes, and 2 NK-lysins, were identified from the genome of Carassius auratus by a homologous search and were further classified based on their fundamental structural features and molecular phylogeny. C. auratus AMPs were found to be ubiquitously distributed in all tested tissues and showed similar expression profiles, with the exception of β-defensins, when RT-qPCR was used to investigate the tissue distribution of AMPs in healthy Carassius gibel. In addition, the expression levels of NK-lysin genes in the tested tissues tended to be upregulated upon bacterial and viral infection when representative NK-lysins were chosen to examine their relative expression levels in various tissues. Importantly, the synthetic peptide caNKL2_102-119, which targets the functional domain of saposin B in caNK-lysins, could effectively counter Aeromonas hydrophila, Staphylococcus aureus, and Escherichia coli with minimum inhibitory concentration (MIC) values of 3-6 μg/mL, as well as inhibit the proliferation of spring viraemia of carp virus (SVCV). These results provide potential targets for antibiotic-free breeding in the aquaculture industry.

A Heterochiral Diphenylalanine Auxin Derivative empowers Remarkable Mechanical Integrity with promising Antiinflammatory and Antimicrobial Performances

The emergence of drug-resistant microbes in conjunction with antiinflammatory responses are one of the universal challenges in health care avenues. Although scientists have dedicated enormous efforts to discover potential constructs,...

Design, synthesis and biological evaluation of P2-modified proline analogues targeting the HtrA serine protease in Chlamydia

High temperature requirement A (HtrA) serine proteases have emerged as a novel class of antibacterial target, which are crucial in protein quality control and are involved in the pathogenesis of a wide array of bacterial infections. Previously, we demonstrated that HtrA in Chlamydia is essential for bacterial survival, replication and virulence. Here, we report a new series of proline (P2)-modified inhibitors of Chlamydia trachomatis HtrA (CtHtrA) developed by proline ring expansion and Cγ-substitutions. The structure-based drug optimization process was guided by molecular modelling and in vitro pharmacological evaluation of inhibitory potency, selectivity and cytotoxicity. Compound 25 from the first-generation 4-substituted proline analogues increased antiCtHtrA potency and selectivity over human neutrophil elastase (HNE) by approximately 6- and 12-fold, respectively, relative to the peptidic lead compound 1. Based on this compound, second-generation substituted proline residues containing 1,2,3-triazole moieties were synthesized by regioselective azide-alkyne click chemistry. Compound 49 demonstrated significantly improved antichlamydial activity in whole cell assays, diminishing the bacterial infectious progeny below the detection limit at the lowest dose tested. Compound 49 resulted in approximately 9- and 22-fold improvement in the inhibitory potency and selectivity relative to 1, respectively. To date, compound 49 is the most potent HtrA inhibitor developed against Chlamydia spp.

De novo design of short antimicrobial lipopeptides

The increase in bacterial resistance to antibiotics available leads to the search for new compounds with antimicrobial potential, such as peptides and lipopeptides. In this work, eight short lipopeptides with the structural pattern Cn-X1 X2 X3-NH2 were de novo designed, synthesized by Fmoc solid phase and characterized by instrumental techniques. The results of the in vitro tests indicated that two of them, LIP 4 and LIP 12 display antibacterial activity against 4 pathogenic bacteria with minimum inhibitory concentrations (MIC) between 9.50 and 100 μM and between 8.50 and 10.0 μM, respectively; they did not displayed toxicity to human erythrocytes at concentrations between 3.13 and 50.0 μM. The antibacterial mechanism of action observed by scanning electron microscopy indicate that the cell membrane was the target, causing the formation of blisters and vesicles, with size ranging from 100 to 120 nm. The lipopeptide LIP 12, with higher activity, was stable to proteases of human blood serum.

Characterization and Antimicrobial Studies of Iturin-Like and Bogorol-Like Lipopeptides From Brevibacillus spp. Strains GI9 and SKDU10

Accession numbers for whole-genome sequence of Brevibacillus sp. strain GI9 and SKDU10 are CAGD01000001 to CAGD01000061 and LSSO00000000, respectively. Members of the genus Brevibacillus have been demonstrated to produce a variety of bioactive compounds including polyketides, lipopeptides and bacteriocins. Lipopeptides are non-ribosomally synthesized surface-active compounds with antimicrobial, antitumor, and immune-stimulatory activities. They usually exhibit strong antifungal and antibacterial activities and are considered as promising compounds in controlling fungal diseases. In this study, we have characterized two lipopeptides from Brevibacillus sp. strains GI9 and SKDU10. The corresponding lipopeptides were purified by reverse-phase high-performance liquid chromatography. Mass analysis and characterization by MALDI-TOF-MS (Matrix-assisted laser desorption ionization time-of-flight mass spectrometry) analysis revealed production of an iturin-like lipopeptide by strain GI9 and bogorol-like lipopeptide by strain SKDU10. Both lipopeptides exhibited broad spectrum antibacterial activity and inhibited the growth of various fungi. They showed minimum inhibitory concentration (MIC) values between 90 and 300 μg/ml against indicator strains of bacteria and drug-resistant Candida indicator strains. The lipopeptides did not show phytotoxic effect in seed germination experiments but caused hemolysis. Further, both lipopeptides inhibited the growth of fungi on fruits and vegetables in in vitro experiments, thereby exhibited potential use in biotechnological industry as effective biocontrol agents.

Boosting Synergistic Effects of Short Antimicrobial Peptides With Conventional Antibiotics Against Resistant Bacteria

The global spread of antibiotic-resistant infections has meant that there is an urgent need to develop new antimicrobial alternatives. In this study, we developed a strategy to boost and/or synergize the activity of conventional antibiotics by combination with antimicrobial peptides tagged with the bulky non-natural amino acid β-naphthylalanine (Nal) to their N- or C-terminus. A checkerboard method was used to evaluate synergistic effects of the parent peptide and the Nal-tagged peptides. Moreover, boron-dipyrro-methene labeled vancomycin was used to characterize the synergistic mechanism of action between the peptides and vancomycin on the bacterial strains. These Nal-tagged antimicrobial peptides also reduced the antibiotic-induced release of lipopolysaccharide from Gram-negative bacteria by more than 99.95%. Our results demonstrate that Nal-tagged peptides could help in developing antimicrobial peptides that not only have enhanced antibacterial activities but also increase the synergistic effects with conventional antibiotics against antibiotic-resistant bacteria.

Virus-inspired surface-nanoengineered antimicrobial liposome: A potential system to simultaneously achieve high activity and selectivity

Enveloped viruses such as SARS-CoV-2 frequently have a highly infectious nature and are considered effective natural delivery systems exhibiting high efficiency and specificity. Since simultaneously enhancing the activity and selectivity of lipopeptides is a seemingly unsolvable problem for conventional chemistry and pharmaceutical approaches, we present a biomimetic strategy to construct lipopeptide-based mimics of viral architectures and infections to enhance their antimicrobial efficacy while avoiding side effects. Herein, a surface-nanoengineered antimicrobial liposome (SNAL) is developed with the morphological features of enveloped viruses, including a moderate size range, lipid-based membrane structure, and highly lipopeptide-enriched bilayer surface. The SNAL possesses virus-like infection to bacterial cells, which can mediate high-efficiency and high-selectivity bacteria binding, rapidly attack and invade bacteria via plasma membrane fusion pathway, and induce a local "burst" release of lipopeptide to produce irreversible damage of cell membrane. Remarkably, viral mimics are effective against multiple pathogens with low minimum inhibitory concentrations (1.6-6.3 μg mL-1), high bactericidal efficiency of >99% within 2 h, >10-fold enhanced selectivity over free lipopeptide, 99.8% reduction in skin MRSA load after a single treatment, and negligible toxicity. This bioinspired design has significant potential to enhance the therapeutic efficacy of lipopeptides and may create new opportunities for designing next-generation antimicrobials.

Antifungal Activity of Linear and Disulfide-Cyclized Ultrashort Cationic Lipopeptides Alone and in Combination with Fluconazole against Vulvovaginal Candida spp

Vulvovaginal candidiasis (VVC) occurs in over 75% of women at least once during their lifetime and is an infection that significantly affects their health. Candida strains resistant to standard azole antifungal therapy and relapses of VVC are more and more common. Hypothetically, biofilm is one of the main reasons of relapses and failure of the therapy. Ultrashort cationic lipopeptides (USCLs) exhibit high antimicrobial activities. Our previous study on USCLs revealed that disulfide cyclization can result in selective antifungal compounds. Therefore, four USCL were selected and their antifungal activity were studied on 62 clinical strains isolated from VVC. The results confirmed previous premises that cyclic analogs have increased selectivity between fungal cells and keratinocytes and improved anticandidal activity compared to their linear analogs against both planktonic and biofilm cultures. On the other hand, linear lipopeptides in combination with fluconazole showed a synergistic effect. It was found that the minimum inhibitory concentrations of the tested compounds in combination with fluconazole were at least four times lower than when used separately. Our results indicate that combination therapy of VVC with USCLs and fluconazole at low non-toxic concentrations can be beneficial owing to the synergistic effect. However, further in vivo studies are needed to confirm this hypothesis.

Gemini Lipopeptide Bearing an Ultrashort Peptide for Enhanced Transfection Efficiency and Cancer-Cell-Specific Cytotoxicity

Cationic gemini lipopeptides are a relatively new class of amphiphilic compounds to be used for gene delivery. Through the possibility of incorporating short peptides with cell-penetrating functionalities, these lipopeptides may be advantageous over traditional cationic lipids. Herein, we report the design, synthesis, and application of a novel cationic gemini lipopeptide for gene delivery. An ultrashort peptide, containing four amino acids, arginine-cysteine-cysteine-arginine, serves as a cationic head group, and two α-tocopherol moieties act as hydrophobic anchoring groups. The new lipopeptide (ATTA) is incorporated into the conventional liposomes, containing 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and 1,2-dioleoyl-sn-glycerol-3-phosphoethanolamine (DOPE), at different molar ratios. The formulated liposomes are characterized and screened for better transfection efficiency. Transfection activity in multiple human cell lines from cancerous and noncancerous origins indicates that the inclusion of an optimal ratio of ATTA in the liposomes substantially enhances the transfection efficiency, superior to that of a traditional liposome, DOTAP-DOPE. Cytotoxicity of ATTA-containing formulations against multiple cell lines indicates potentially distinct activity between cancer and noncancer cell lines. Furthermore, lipoplexes of the ATTA-containing formulations with anticancer therapeutic gene, plasmid encoding tumor necrosis factor-related apoptosis-inducing ligand (pTRAIL), induce obviously more cytotoxicity than conventional formulations. The results indicate that arginine-rich cationic lipopeptide appears to be a promising ingredient in gene delivery vector formulations to enhance transfection efficiency and cell-selective cytotoxicity.

Membrane-disruptive peptides/peptidomimetics-based therapeutics: Promising systems to combat bacteria and cancer in the drug-resistant era

Membrane-disruptive peptides/peptidomimetics (MDPs) are antimicrobials or anticarcinogens that present a general killing mechanism through the physical disruption of cell membranes, in contrast to conventional chemotherapeutic drugs, which act on precise targets such as DNA or specific enzymes. Owing to their rapid action, broad-spectrum activity, and mechanisms of action that potentially hinder the development of resistance, MDPs have been increasingly considered as future therapeutics in the drug-resistant era. Recently, growing experimental evidence has demonstrated that MDPs can also be utilized as adjuvants to enhance the therapeutic effects of other agents. In this review, we evaluate the literature around the broad-spectrum antimicrobial properties and anticancer activity of MDPs, and summarize the current development and mechanisms of MDPs alone or in combination with other agents. Notably, this review highlights recent advances in the design of various MDP-based drug delivery systems that can improve the therapeutic effect of MDPs, minimize side effects, and promote the co-delivery of multiple chemotherapeutics, for more efficient antimicrobial and anticancer therapy.

Novel Glycerophospholipid, Lipo- and N-acyl Amino Acids from Bacteroidetes: Isolation, Structure Elucidation and Bioactivity

The 'core' metabolome of the Bacteroidetes genus Chitinophaga was recently discovered to consist of only seven metabolites. A structural relationship in terms of shared lipid moieties among four of them was postulated. Here, structure elucidation and characterization via ultra-high resolution mass spectrometry (UHR-MS) and nuclear magnetic resonance (NMR) spectroscopy of those four lipids (two lipoamino acids (LAAs), two lysophosphatidylethanolamines (LPEs)), as well as several other undescribed LAAs and N-acyl amino acids (NAAAs), identified during isolation were carried out. The LAAs represent closely related analogs of the literature-known LAAs, such as the glycine-serine dipeptide lipids 430 (2) and 654. Most of the here characterized LAAs (1, 5-11) are members of a so far undescribed glycine-serine-ornithine tripeptide lipid family. Moreover, this study reports three novel NAAAs (N-(5-methyl)hexanoyl tyrosine (14) and N-(7-methyl)octanoyl tyrosine (15) or phenylalanine (16)) from Olivibacter sp. FHG000416, another Bacteroidetes strain initially selected as best in-house producer for isolation of lipid 430. Antimicrobial profiling revealed most isolated LAAs (1-3) and the two LPE 'core' metabolites (12, 13) active against the Gram-negative pathogen M. catarrhalis ATCC 25238 and the Gram-positive bacterium M. luteus DSM 20030. For LAA 1, additional growth inhibition activity against B. subtilis DSM 10 was observed.

New antimicrobial self-assembling short lipopeptides

Lipopeptides are an exceptional example of amphiphilic molecules that self-assemble into functional structures with applications in the areas of nanotechnology, catalysis or medicinal chemistry. Herein, we report a library of 21 short lipopeptides, together with their supramolecular characterization and antimicrobial activity against both Gram-negative (E. coli) and Gram-positive (S. aureus) strains. This study shows that simple lipoamino acids self-assemble into micellar or vesicular structures, while incorporating dipeptides capable of stablishing hydrogen bonds results in the adoption of advanced fibrilar structures. The self-assembly effect has proven to be key to achieve antimicrobial activity.

Chemically modified and conjugated antimicrobial peptides against superbugs

Antimicrobial resistance (AMR) is one of the greatest threats to human health that, by 2050, will lead to more deaths from bacterial infections than cancer. New antimicrobial agents, both broad-spectrum and selective, that do not induce AMR are urgently required. Antimicrobial peptides (AMPs) are a novel class of alternatives that possess potent activity against a wide range of Gram-negative and positive bacteria with little or no capacity to induce AMR. This has stimulated substantial chemical development of novel peptide-based antibiotics possessing improved therapeutic index. This review summarises recent synthetic efforts and their impact on analogue design as well as their various applications in AMP development. It includes modifications that have been reported to enhance antimicrobial activity including lipidation, glycosylation and multimerization through to the broad application of novel bio-orthogonal chemistry, as well as perspectives on the direction of future research. The subject area is primarily the development of next-generation antimicrobial agents through selective, rational chemical modification of AMPs. The review further serves as a guide toward the most promising directions in this field to stimulate broad scientific attention, and will lead to new, effective and selective solutions for the several biomedical challenges to which antimicrobial peptidomimetics are being applied.

The multifaceted nature of antimicrobial peptides: current synthetic chemistry approaches and future directions

Bacterial infections caused by 'superbugs' are increasing globally, and conventional antibiotics are becoming less effective against these bacteria, such that we risk entering a post-antibiotic era. In recent years, antimicrobial peptides (AMPs) have gained significant attention for their clinical potential as a new class of antibiotics to combat antimicrobial resistance. In this review, we discuss several facets of AMPs including their diversity, physicochemical properties, mechanisms of action, and effects of environmental factors on these features. This review outlines various chemical synthetic strategies that have been applied to develop novel AMPs, including chemical modifications of existing peptides, semi-synthesis, and computer-aided design. We will also highlight novel AMP structures, including hybrids, antimicrobial dendrimers and polypeptides, peptidomimetics, and AMP-drug conjugates and consider recent developments in their chemical synthesis.

D-Amino Acid-Containing Lipopeptides Derived from the Lead Peptide BP100 with Activity against Plant Pathogens

From a previous collection of lipopeptides derived from BP100, we selected 18 sequences in order to improve their biological profile. In particular, analogues containing a D-amino acid at position 4 were designed, prepared, and tested against plant pathogenic bacteria and fungi. The biological activity of these sequences was compared with that of the corresponding parent lipopeptides with all L-amino acids. In addition, the influence of the length of the hydrophobic chain on the biological activity was evaluated. Interestingly, the incorporation of a D-amino acid into lipopeptides bearing a butanoyl or a hexanoyl chain led to less hemolytic sequences and, in general, that were as active or more active than the corresponding all L-lipopeptides. The best lipopeptides were BP475 and BP485, both incorporating a D-Phe at position 4 and a butanoyl group, with MIC values between 0.8 and 6.2 µM, low hemolysis (0 and 24% at 250 µM, respectively), and low phytotoxicity. Characterization by NMR of the secondary structure of BP475 revealed that the D-Phe at position 4 disrupts the α-helix and that residues 6 to 10 are able to fold in an α-helix. This secondary structure would be responsible for the high antimicrobial activity and low hemolysis of this lipopeptide.

Short and Robust Anti-Infective Lipopeptides Engineered Based on the Minimal Antimicrobial Peptide KR12 of Human LL-37

This study aims to push the frontier of the engineering of human cathelicidin LL-37, a critical antimicrobial innate immune peptide that wards off invading pathogens. By sequential truncation of the smallest antibacterial peptide (KR12) of LL-37 and conjugation with fatty acids, with varying chain lengths, a library of lipopeptides is generated. These peptides are subjected to antibacterial activity and hemolytic assays. Candidates (including both forms made of l- and d-amino acids) with the optimal cell selectivity are subsequently fed to the second layer of in vitro filters, including salts, pH, serum, and media. These practices lead to the identification of a miniature LL-37 like peptide (d-form) with selectivity, stability, and robust antimicrobial activity in vitro against both Gram-positive and negative bacteria. Proteomic studies reveal far fewer serum proteins that bind to the d-form than the l-form peptide. C10-KR8d targets bacterial membranes to become helical, making it difficult for bacteria to develop resistance in a multiple passage experiment. In vivo, C10-KR8d is able to reduce bacterial burden of methicillin-resistant Staphylococcus aureus (MRSA) USA300 LAC in neutropenic mice. In addition, this designer peptide prevents bacterial biofilm formation in a catheter-associated mouse model. Meanwhile, C10-KR8d also recruits cytokines to the vicinity of catheters to clear infection. Thus, based on the antimicrobial region of LL-37, this study succeeds in identifying the smallest anti-infective peptide C10-KR8d with both robust antimicrobial, antibiofilm, and immune modulation activities.

Novel temporin L antimicrobial peptides: promoting self-assembling by lipidic tags to tackle superbugs

The rapid development of antimicrobial resistance is pushing the search in the discovering of novel antimicrobial molecules to prevent and treat bacterial infections. Self-assembling antimicrobial peptides, as the lipidated peptides, are a novel and promising class of molecules capable of meeting this need. Based on previous work on Temporin L analogs, several new molecules lipidated at the N- or and the C-terminus were synthesised. Our goal is to improve membrane interactions through finely tuning self-assembly to reduce oligomerisation in aqueous solution and enhance self-assembly in bacterial membranes while reducing toxicity against human cells. The results here reported show that the length of the aliphatic moiety is a key factor to control target cell specificity and the oligomeric state of peptides either in aqueous solution or in a membrane-mimicking environment. The results of this study pave the way for the design of novel molecules with enhanced activities.

Aggregation and Its Influence on the Bioactivities of a Novel Antimicrobial Peptide, Temporin-PF, and Its Analogues

Temporin is an antimicrobial peptide (AMP) family discovered in the skin secretion of ranid frog that has become a promising alternative for conventional antibiotic therapy. Herein, a novel temporin peptide, Temporin-PF (TPF), was successfully identified from Pelophylax fukienensis. It exhibited potent activity against Gram-positive bacteria, but no effect on Gram-negative bacteria. Additionally, TPF exhibited aggregation effects in different solutions. Three analogs were further designed to study the relationship between the aggregation patterns and bioactivities, and the MD simulation was performed for revealing the pattern of the peptide assembly. As the results showed, all peptides were able to aggregate in the standard culture media and salt solutions, especially CaCl2 and MgCl2 buffers, where the aggregation was affected by the concentration of the salts. MD simulation reported that all peptides were able to form oligomers. The parent peptide assembly depended on the hydrophobic interaction via the residues in the middle domain of the sequence. However, the substitution of Trp/D-Trp resulted in an enhanced inter-peptide interaction in the zipper-like domain and eliminated overall biological activities. Our study suggested that introducing aromaticity at the zipper-like domain for temporin may not improve the bioactivities, which might be related to the formation of aggregates via the inter-peptide contacts at the zipper-like motif domain, and it could reduce the binding affinity to the lipid membrane of microorganisms.

Antimicrobial Bombinin-like Peptide 3 Selectively Recognizes and Inserts into Bacterial Biomimetic Bilayers in Multiple Steps

Bombinins are a wide family of antimicrobial peptides from Xenopus skin. By sequence clustering, we highlighted at least three families named A, B, and H, which might exert antibacterial activity by different modes of action. In this work, we study bombinin-like peptide 3 (BLP-3) as a nonhemolytic representative of the quite unexplored class A due to its appealing activity toward WHO-priority-list bacteria such as Neisseria, Pseudomonas aeruginosa, and Staphylococcus aureus. A marked preference for cardiolipin and phosphatidylglycerol head groups, typically found in bacteria, is proven with biomimetic membranes studied by liquid and solid NMR and MD simulations. BLP-3 gets structured upon interaction and penetrates deeply into the bilayer in two steps involving a superficial insertion of key side chains and subsequent internalization. All along the pathway, a fundamental role is played by lysine residues in the conserved region 11-19, which act in synergy with other key residues.

α,β-Unsaturated γ-Peptide Foldamers

Despite their concomitant emergence in the 1990s, γ-peptide foldamers have not developed as fast as β-peptide foldamers and to date, only a few γ-oligomer structures have been reported, and with sparse applications. Among these examples, sequences containing α,β-unsaturated γ-amino acids have recently drawn attention since the Z/E configurations of the double bond provide opposite planar restrictions leading to divergent conformational behaviors, from helix to extended structures. In this Review, we give a comprehensive overview of the developments of γ-peptide foldamers containing α,β-unsaturated γ-amino acids with examples of applications for health and catalysis, as well as materials science.

Advances of peptides for antibacterial applications

In the past few decades, peptide antibacterial products with unique antibacterial mechanisms have attracted widespread interest. They can effectively reduce the probability of drug resistance of bacteria and are biocompatible, so they possess tremendous development prospects. This review provides recent research and analysis on the basic types of antimicrobial peptides (including poly (amino acid)s, short AMPs, and lipopeptides) and factors to optimize antimicrobial effects. It also summarizes the two most important modes of action of antimicrobial peptides and the latest developments in the application of AMPs, including antimicrobial agent, wound healing, preservative, antibacterial coating and others. Finally, we discuss the remaining challenges to improve the antibacterial peptides and propose prospects in the field.

The ecological roles of microbial lipopeptides: Where are we going?

Lipopeptides (LPs) are secondary metabolites produced by a diversity of bacteria and fungi. Their unique chemical structure comprises both a peptide and a lipid moiety. LPs are of major biotechnological interest owing to their emulsification, antitumor, immunomodulatory, and antimicrobial activities. To date, these versatile compounds have been applied across multiple industries, from pharmaceuticals through to food processing, cosmetics, agriculture, heavy metal, and hydrocarbon bioremediation. The variety of LP structures and the diversity of the environments from which LP-producing microorganisms have been isolated suggest important functions in their natural environment. However, our understanding of the ecological role of LPs is limited. In this review, the mode of action and the role of LPs in motility, antimicrobial activity, heavy metals removal and biofilm formation are addressed. We include discussion on the need to characterise LPs from a diversity of microorganisms, with a focus on taxa inhabiting 'extreme' environments. We introduce the use of computational target fishing and molecular dynamics simulations as powerful tools to investigate the process of interaction between LPs and cell membranes. Together, these advances will provide new understanding of the mechanism of action of novel LPs, providing greater insights into the roles of LPs in the natural environment.

A Global Review on Short Peptides: Frontiers and Perspectives

Peptides are fragments of proteins that carry out biological functions. They act as signaling entities via all domains of life and interfere with protein-protein interactions, which are indispensable in bio-processes. Short peptides include fundamental molecular information for a prelude to the symphony of life. They have aroused considerable interest due to their unique features and great promise in innovative bio-therapies. This work focusing on the current state-of-the-art short peptide-based therapeutical developments is the first global review written by researchers from all continents, as a celebration of 100 years of peptide therapeutics since the commencement of insulin therapy in the 1920s. Peptide "drugs" initially played only the role of hormone analogs to balance disorders. Nowadays, they achieve numerous biomedical tasks, can cross membranes, or reach intracellular targets. The role of peptides in bio-processes can hardly be mimicked by other chemical substances. The article is divided into independent sections, which are related to either the progress in short peptide-based theranostics or the problems posing challenge to bio-medicine. In particular, the SWOT analysis of short peptides, their relevance in therapies of diverse diseases, improvements in (bio)synthesis platforms, advanced nano-supramolecular technologies, aptamers, altered peptide ligands and in silico methodologies to overcome peptide limitations, modern smart bio-functional materials, vaccines, and drug/gene-targeted delivery systems are discussed.

Molecular Basis of the Anticancer and Antibacterial Properties of CecropinXJ Peptide: An In Silico Study

Esophageal cancer is an aggressive lethal malignancy causing thousands of deaths every year. While current treatments have poor outcomes, cecropinXJ (CXJ) is one of the very few peptides with demonstrated in vivo activity. The great interest in CXJ stems from its low toxicity and additional activity against most ESKAPE bacteria and fungi. Here, we present the first study of its mechanism of action based on molecular dynamics (MD) simulations and sequence-property alignment. Although unstructured in solution, predictions highlight the presence of two helices separated by a flexible hinge containing P24 and stabilized by the interaction of W2 with target biomembranes: an amphipathic helix-I and a poorly structured helix-II. Both MD and sequence-property alignment point to the important role of helix I in both the activity and the interaction with biomembranes. MD reveals that CXJ interacts mainly with phosphatidylserine (PS) but also with phosphatidylethanolamine (PE) headgroups, both found in the outer leaflet of cancer cells, while salt bridges with phosphate moieties are prevalent in bacterial biomimetic membranes composed of PE, phosphatidylglycerol (PG) and cardiolipin (CL). The antibacterial activity of CXJ might also explain its interaction with mitochondria, whose phospholipid composition recalls that of bacteria and its capability to induce apoptosis in cancer cells.

Curbing gastrointestinal infections by defensin fragment modifications without harming commensal microbiota

The occurrence and spread of multidrug-resistant pathogens, especially bacteria from the ESKAPE panel, increases the risk to succumb to untreatable infections. We developed a novel antimicrobial peptide, Pam-3, with antibacterial and antibiofilm properties to counter this threat. The peptide is based on an eight-amino acid carboxyl-terminal fragment of human β-defensin 1. Pam-3 exhibited prominent antimicrobial activity against multidrug-resistant ESKAPE pathogens and additionally eradicated already established biofilms in vitro, primarily by disrupting membrane integrity of its target cell. Importantly, prolonged exposure did not result in drug-resistance to Pam-3. In mouse models, Pam-3 selectively reduced acute intestinal Salmonella and established Citrobacter infections, without compromising the core microbiota, hence displaying an added benefit to traditional broad-spectrum antibiotics. In conclusion, our data support the development of defensin-derived antimicrobial agents as a novel approach to fight multidrug-resistant bacteria, where Pam-3 appears as a particularly promising microbiota-preserving candidate.

Antimicrobial Peptide K11 Selectively Recognizes Bacterial Biomimetic Membranes and Acts by Twisting Their Bilayers

K11 is a synthetic peptide originating from the introduction of a lysine residue in position 11 within the sequence of a rationally designed antibacterial scaffold. Despite its remarkable antibacterial properties towards many ESKAPE bacteria and its optimal therapeutic index (320), a detailed description of its mechanism of action is missing. As most antimicrobial peptides act by destabilizing the membranes of the target organisms, we investigated the interaction of K11 with biomimetic membranes of various phospholipid compositions by liquid and solid-state NMR. Our data show that K11 can selectively destabilize bacterial biomimetic membranes and torque the surface of their bilayers. The same is observed for membranes containing other negatively charged phospholipids which might suggest additional biological activities. Molecular dynamic simulations reveal that K11 can penetrate the membrane in four steps: after binding to phosphate groups by means of the lysine residue at the N-terminus (anchoring), three couples of lysine residues act subsequently to exert a torque in the membrane (twisting) which allows the insertion of aromatic side chains at both termini (insertion) eventually leading to the flip of the amphipathic helix inside the bilayer core (helix flip and internalization).

Lipidation of Antimicrobial Peptides as a Design Strategy for Future Alternatives to Antibiotics

Multi-drug-resistant bacteria are becoming more prevalent, and treating these bacteria is becoming a global concern. One alternative approach to combat bacterial resistance is to use antimicrobial (AMPs) or host-defense peptides (HDPs) because they possess broad-spectrum activity, function in a variety of ways, and lead to minimal resistance. However, the therapeutic efficacy of HDPs is limited by a number of factors, including systemic toxicity, rapid degradation, and low bioavailability. One approach to circumvent these issues is to use lipidation, i.e., the attachment of one or more fatty acid chains to the amine groups of the N-terminus or a lysine residue of an HDP. In this review, we examined lipidated analogs of 66 different HDPs reported in the literature to determine: (i) whether there is a link between acyl chain length and antibacterial activity; (ii) whether the charge and (iii) the hydrophobicity of the HDP play a role; and (iv) whether acyl chain length and toxicity are related. Overall, the analysis suggests that lipidated HDPs with improved activity over the nonlipidated counterpart had acyl chain lengths of 8–12 carbons. Moreover, active lipidated peptides attached to short HDPs tended to have longer acyl chain lengths. Neither the charge of the parent HDP nor the percent hydrophobicity of the peptide had an apparent significant impact on the antibacterial activity. Finally, the relationship between acyl chain length and toxicity was difficult to determine due to the fact that toxicity is quantified in different ways. The impact of these trends, as well as combined strategies such as the incorporation of d- and non-natural amino acids or alternative approaches, will be discussed in light of how lipidation may play a role in the future development of antimicrobial peptide-based alternatives to current therapeutics.

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 C_x-G(IIKK)_yI-NH₂ (C_xG_y, x = 4-12 and y = 2). C_xG_y 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.

Low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, wound healing, anticancer, drug delivery, bioimaging and 3D bioprinting applications

In this review, we have focused on the design and development of low molecular weight self-assembling peptide-based materials for various applications including cell proliferation, tissue engineering, antibacterial, antifungal, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting. The first part of the review describes about stimuli and various noncovalent interactions, which are the key components of various self-assembly processes for the construction of organized structures. Subsequently, the chemical functionalization of the peptides has been discussed, which is required for the designing of self-assembling peptide-based soft materials. Various low molecular weight self-assembling peptides have been discussed to explain the important structural features for the construction of defined functional nanostructures. Finally, we have discussed various examples of low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting applications.

Lipidated Short Analogue of α-Melanocyte Stimulating Hormone Exerts Bactericidal Activity against the Stationary Phase of Methicillin-Resistant Staphylococcus aureus and Inhibits Biofilm Formation

Stationary phase Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), has been widely associated with many persistent infections as well as biofilm-associated infections, which are challenging due to their increasing antibiotic resistance. α-Melanocyte stimulating hormone (α-MSH) is an antimicrobial peptide (AMP) with well-established potent activity against S. aureus , but little is known about its antimicrobial efficacy against the stationary phase of the bacteria. We investigated the in vitro activities of two palmitoylated analogues, Pal-α-MSH(6-13) and Pal-α-MSH(11-13), of the C-terminal fragments of α-MSH against biofilm-producing strains of methicillin-sensitive S. aureus (MSSA) and MRSA. While both the peptides demonstrated anti-staphylococcal efficacy, Pal-α-MSH(11-13) emerged as the most effective AMP as palmitoylation led to a remarkable enhancement in its activity against stationary phase bacteria. Similar to α-MSH, both the designed analogues were membrane-active and exhibited improved bacterial membrane depolarization and permeabilization, as further confirmed via electron microscopy studies. Of the two peptides, Pal-α-MSH(11-13) was able to retain its activity in the presence of standard microbiological media, which otherwise is a major limiting factor toward the therapeutic use of α-MSH-based peptides. More importantly, Pal-α-MSH(11-13) was also highly effective in inhibiting the formation of biofilms. Furthermore, it did not lead to resistance development in MRSA cells even upon 18 serial passages at sub-MIC concentrations. These observations support the potential use of Pal-α-MSH(11-13) in the treatment of planktonic as well as sessile S. aureus infections.

Effects of salts on the self-assembly behavior and antibacterial activity of a surfactant-like peptide

Self-assembling peptides have become one of the most promising antibacterial agents due to their superior properties, such as simple molecular composition, favorable assembly structures, and rich designability. For maximum application in vivo, their activities in the presence of salts are desirable, however, the potent correlation between peptide nanostructures, antibacterial activity, and salt resistance behavior remains poorly explored. Previously, we have demonstrated that the potent antibacterial activity of a designed surfactant-like peptide Ac-A9K-NH2 benefited from its high self-assembly ability and appropriate size of its self-assembled nanostructures. In this study, we investigated the effect of salts on its self-assembly behavior and antibacterial activity. The results indicated that the flexible and long nanofibrils formed by Ac-A9K-NH2 in the presence of CaCl2 were adverse to its membrane insertion, leading to the reduction of antibacterial activity. Comparatively, Ac-A9K-NH2 maintained its potent antibacterial activity in the presence of NaCl due to its suitable shape and size of nanostructures. The newly formed nanofibers and nanorods facilitated the penetration of peptides into the bacterial membrane, forming nanopores and eventually leading to the lysis of bacteria. The high antibacterial activity and NaCl tolerance of Ac-A9K-NH2 make it a promising antibacterial agent at elevated salt concentrations.

Physicochemical and Biological Characterization of Novel Membrane-Active Cationic Lipopeptides with Antimicrobial Properties

We have designed and synthesized new short lipopeptides composed of tetrapeptide conjugated to fatty acids with different chain lengths. The amino acid sequence of the peptide moiety included d-phenylalanine, two residues of l-2,4-diaminobutyric acid and l-leucine. To explore the possible mechanism of lipopeptide action, we have provided a physicochemical characterization of their interactions with artificial lipid membranes. For this purpose, we have used monolayers and bilayers composed of lipids representative of Gram-negative and Gram-positive bacterial membranes. Using surface pressure measurements and atomic force microscopy, we were able to monitor the changes occurring within the films upon exposure to lipopeptides. Our experiments revealed that all lipopeptides can penetrate the lipid membranes and affect their molecular ordering. The latter results in membrane thinning and fluidization. However, the effect is stronger in the lipid films mimicking Gram-positive bacterial membranes. The results of the physicochemical characterization were compared with the biological activity of lipopeptides. The effect of lipopeptides on bacterial growth was tested on several strains of bacteria. It was revealed that lipopeptides show stronger antimicrobial activity against Gram-positive bacteria. At the same time, all tested compounds display relatively low hemolytic activity.

Effect of Disulfide Cyclization of Ultrashort Cationic Lipopeptides on Antimicrobial Activity and Cytotoxicity

Ultrashort cationic lipopeptides (USCLs) are considered to be a promising class of antimicrobials with high activity against a broad-spectrum of microorganisms. However, the majority of these compounds are characterized by significant toxicity toward human cells, which hinders their potential application. To overcome those limitations, several approaches have been advanced. One of these is disulfide cyclization that has been shown to improve drug-like characteristics of peptides. In this article the effect of disulfide cyclization of the polar head of N-palmitoylated USCLs on in vitro biological activity has been studied. Lipopeptides used in this study consisted of three or four basic amino acids (lysine and arginine) and cystine in a cyclic peptide. In general, disulfide cyclization of the lipopeptides resulted in peptides with reduced cytotoxicity. Disulfide-cyclized USCLs exhibited improved selectivity between Candida sp., Gram-positive strains and normal cells in contrast to their linear counterparts. Interactions between selected USCLs and membranes were studied by molecular dynamics simulations using a coarse-grained force field. Moreover, membrane permeabilization properties and kinetics were examined. Fluorescence and transmission electron microscopy revealed damage to Candida cell membrane and organelles. Concluding, USCLs are strong membrane disruptors and disulfide cyclization of polar head can have a beneficial effect on its in vitro selectivity between Candida sp. and normal human cells.

Antimicrobial Peptides with Enhanced Salt Resistance and Antiendotoxin Properties

A strategy was described to design antimicrobial peptides (AMPs) with enhanced salt resistance and antiendotoxin activities by linking two helical AMPs with the Ala-Gly-Pro (AGP) hinge. Among the designed peptides, KR12AGPWR6 demonstrated the best antimicrobial activities even in high salt conditions (NaCl ~300 mM) and possessed the strongest antiendotoxin activities. These activities may be related to hydrophobicity, membrane-permeability, and α-helical content of the peptide. Amino acids of the C-terminal helices were found to affect the peptide-induced permeabilization of LUVs, the α-helicity of the designed peptides under various LUVs, and the LPS aggregation and size alternation. A possible model was proposed to explain the mechanism of LPS neutralization by the designed peptides. These findings could provide a new approach for designing AMPs with enhanced salt resistance and antiendotoxin activities for potential therapeutic applications.

Antibacterial lipo-random peptide mixtures exhibit high selectivity and synergistic interactions

Random peptide mixtures (RPMs) have been recently proposed as powerful antimicrobial compounds. These are unique mixtures of peptides synthesized by random combination of a cationic and a hydrophobic amino acid. Here, we introduce a new type of antimicrobial compounds, short lipo-RPMs, which result from N-palmitoylation of RPMs. We report the characterization of 5-mer lipo-RPMs containing l-phenylalanine and d-lysine, named p-FdK5. p-FdK5 had high antibacterial activity against several bacterial strains and was able to reduce disease severity caused by a plant pathogen. We further synthesized and studied all 32 (2⁵) possible lipopeptides that compose the p-FdK5 mixture. We showed that the antibacterial activity of specific lipopeptides depends on the peptide hydrophobicity and on the location of the hydrophobic amino acids relative to the palmitic acid. Interestingly, synergism assays revealed positive interactions between different sequence-specific lipopeptides in terms of antimicrobial activity.

Sustainable, Site-Specific Linkage of Antimicrobial Peptides to Cotton Textiles

A new general method to covalently link a peptide to cotton via thiazolidine ring formation is developed. Three different analogues of an ultrashort antibacterial peptide are synthesized to create an antibacterial fabric. The chemical ligation approach to the heterogeneous phase made up of insoluble cellulose fibers and a peptide solution in water is adapted. The selective click reaction occurs between an N-terminal cysteine on the peptide and an aldehyde on the cotton matrix. The aldehyde is generated on the primary alcohol of glucose by means of the enzyme laccase and the cocatalyst 2,2,6,6-tetramethylpiperidine-1-oxyl. This keeps the pyranose rings intact and may bring a benefit to the mechanical properties of the fabric. The presence of the peptide on cotton is demonstrated through instant colorimetric tests, UV spectroscopy, IR spectroscopy, and X-ray photoelectron spectroscopy analysis. The antibacterial activity of the peptides is maintained even after their covalent attachment to cotton fibers.

A Novel Peptide Antibiotic, Pro10-1D, Designed from Insect Defensin Shows Antibacterial and Anti-Inflammatory Activities in Sepsis Models

Owing to the challenges faced by conventional therapeutics, novel peptide antibiotics against multidrug-resistant (MDR) gram-negative bacteria need to be urgently developed. We had previously designed Pro9-3 and Pro9-3D from the defensin of beetle Protaetia brevitarsis; they showed high antimicrobial activity with cytotoxicity. Here, we aimed to develop peptide antibiotics with bacterial cell selectivity and potent antibacterial activity against gram-negative bacteria. We designed 10-meric peptides with increased cationicity by adding Arg to the N-terminus of Pro9-3 (Pro10-1) and its D-enantiomeric alteration (Pro10-1D). Among all tested peptides, the newly designed Pro10-1D showed the strongest antibacterial activity against Escherichia coli, Acinetobacter baumannii, and MDR strains with resistance against protease digestion. Pro10-1D can act as a novel potent peptide antibiotic owing to its outstanding inhibitory activities against bacterial film formation with high bacterial cell selectivity. Dye leakage and scanning electron microscopy revealed that Pro10-1D targets the bacterial membrane. Pro10-1D inhibited inflammation via Toll Like Receptor 4 (TLR4)/Nuclear factor-κB (NF-κB) signaling pathways in lipopolysaccharide (LPS)-stimulated RAW264.7 cells. Furthermore, Pro10-1D ameliorated multiple-organ damage and attenuated systemic infection-associated inflammation in an E. coli K1-induced sepsis mouse model. Overall, our results suggest that Pro10-1D can potentially serve as a novel peptide antibiotic for the treatment of gram-negative sepsis.

d-Amino acids in antimicrobial peptides: a potential approach to treat and combat antimicrobial resistance

Antimicrobial resistance is one of the leading challenges in the human healthcare segment. Advances in antimicrobial resistance have triggered exploration of natural alternatives to stabilize its seriousness. Antimicrobial peptides are small, positively charged oligopeptides that are as potent as commercially available antibiotics against a wide spectrum of organisms, such as Gram-positive bacteria, Gram-negative bacteria, viruses, and fungal strains. In addition to their antibiotic capabilities, these peptides possess anticancer activity, activate the immune response, and regulate inflammation. Peptides have distinct modes of action and fall into various categories due to their amino acid composition. Although antimicrobial peptides specifically target the bacterial cytoplasmic membrane, they can also target the cell nucleus and protein synthesis. Owing to the increasing demand for novel treatments against the threat of antimicrobial resistance, naturally synthesized peptides are a beneficial development concept. Antimicrobial peptides are pervasive and can easily be modified using de-novo synthesis technology. Antimicrobial peptides can be isolated from natural resources such as humans, plants, bacteria, and fungi. This review gives a brief overview of antimicrobial peptides and their diastereomeric composition. Other current trends, the future scope of antimicrobial peptides, and the role of d-amino acids are also discussed, with a specific emphasis on the design and development of new drugs.