D- and Unnatural Amino Acid Substituted Antimicrobial Peptides With Improved Proteolytic Resistance and Their Proteolytic Degradation Characteristics

Structure-Activity Relationships of the Antimicrobial Peptide Natural Product Apidaecin

With the growing crisis of antimicrobial resistance, it is critical to continue to seek out new sources of novel antibiotics. This need has led to renewed interest in natural product antimicrobials, specifically antimicrobial peptides. Nonlytic antimicrobial peptides are highly promising due to their unique mechanisms of action. One such peptide is apidaecin (Api), which inhibits translation termination through stabilization of the quaternary complex of the ribosome-apidaecin-tRNA-release factor. Synthetic derivatives of apidaecin have been developed, but structure-guided modifications have yet to be considered. In this work, we have focused on modifying key residues in the Api sequence that are responsible for the interactions that stabilize the quaternary complex. We present one of the first examples of a highly modified Api peptide that maintains its antimicrobial activity and interaction with the translation complex. These findings establish a starting point for further structure-guided optimization of Api peptides.

Non-Canonical Amino Acids in Analyses of Protease Structure and Function

All known organisms encode 20 canonical amino acids by base triplets in the genetic code. The cellular translational machinery produces proteins consisting mainly of these amino acids. Several hundred natural amino acids serve important functions in metabolism, as scaffold molecules, and in signal transduction. New side chains are generated mainly by post-translational modifications, while others have altered backbones, such as the β- or γ-amino acids, or they undergo stereochemical inversion, e.g., in the case of D-amino acids. In addition, the number of non-canonical amino acids has further increased by chemical syntheses. Since many of these non-canonical amino acids confer resistance to proteolytic degradation, they are potential protease inhibitors and tools for specificity profiling studies in substrate optimization and enzyme inhibition. Other applications include in vitro and in vivo studies of enzyme kinetics, molecular interactions and bioimaging, to name a few. Amino acids with bio-orthogonal labels are particularly attractive, enabling various cross-link and click reactions for structure-functional studies. Here, we cover the latest developments in protease research with non-canonical amino acids, which opens up a great potential, e.g., for novel prodrugs activated by proteases or for other pharmaceutical compounds, some of which have already reached the clinical trial stage.

Automated solid-phase synthesis of metabolically stabilized triazolo-peptidomimetics

The use of 1,4-disubstituted 1,2,3-triazoles as trans-amide bond surrogates has become an important tool for the synthesis of metabolically stabilized peptidomimetics. These heterocyclic bioisosters are generally incorporated into the peptide backbone by applying a diazo-transfer reaction followed by CuAAC (click chemistry) with an α-amino alkyne. Even though the manual synthesis of backbone-modified triazolo-peptidomimetics has been reported by us and others, no procedure has yet been described for an automated synthesis using peptide synthesizers. In order to efficiently adapt these reactions to an automated setup, different conditions were explored, putting special emphasis on the required long-term stability of both the diazo-transfer reagent and the Cu(I) catalyst in solution. ISA·HCl is the reagent of choice to accomplish the diazo-transfer reaction; however, it was found instable in DMF, the most commonly used solvent for SPPS. Thus, an aqueous solution of ISA·HCl was used to prevent its degradation over time, and the composition in the final diazo-transfer reaction was adjusted to preserve suitable swelling conditions of the resins applied. The CuAAC reaction was performed without difficulties using [Cu (CH3 CN)4 ]PF6 as a catalyst and TBTA as a stabilizer to prevent oxidation to Cu(II). The optimized automated two-step procedure was applied to the synthesis of structurally diverse triazolo-peptidomimetics to demonstrate the versatility of the developed methodology. Under the optimized conditions, five triazolo-peptidomimetics (8-5 amino acid residues) were synthesized efficiently using two different resins. Analysis of the crude products by HPLC-MS revealed moderate to good purities of the desired triazolo-peptidomimetics (70-85%). The synthesis time ranged between 9 and 12.5 h.

Peptide-Based Agents for Cancer Treatment: Current Applications and Future Directions

Peptide-based strategies have received an enormous amount of attention because of their specificity and applicability. Their specificity and tumor-targeting ability are applied to diagnosis and treatment for cancer patients. In this review, we will summarize recent advancements and future perspectives on peptide-based strategies for cancer treatment. The literature search was conducted to identify relevant articles for peptide-based strategies for cancer treatment. It was performed using PubMed for articles in English until June 2023. Information on clinical trials was also obtained from ClinicalTrial.gov. Given that peptide-based strategies have several advantages such as targeted delivery to the diseased area, personalized designs, relatively small sizes, and simple production process, bioactive peptides having anti-cancer activities (anti-cancer peptides or ACPs) have been tested in pre-clinical settings and clinical trials. The capability of peptides for tumor targeting is essentially useful for peptide-drug conjugates (PDCs), diagnosis, and image-guided surgery. Immunomodulation with peptide vaccines has been extensively tested in clinical trials. Despite such advantages, FDA-approved peptide agents for solid cancer are still limited. This review will provide a detailed overview of current approaches, design strategies, routes of administration, and new technological advancements. We will highlight the success and limitations of peptide-based therapies for cancer treatment.

Peptidomimetic Oligomers Targeting Membrane Phosphatidylserine Exhibit Broad Antiviral Activity

The development of durable new antiviral therapies is challenging, as viruses can evolve rapidly to establish resistance and attenuate therapeutic efficacy. New compounds that selectively target conserved viral features are attractive therapeutic candidates, particularly for combating newly emergent viral threats. The innate immune system features a sustained capability to combat pathogens through production of antimicrobial peptides (AMPs); however, these AMPs have shortcomings that can preclude clinical use. The essential functional features of AMPs have been recapitulated by peptidomimetic oligomers, yielding effective antibacterial and antifungal agents. Here, we show that a family of AMP mimetics, called peptoids, exhibit direct antiviral activity against an array of enveloped viruses, including the key human pathogens Zika, Rift Valley fever, and chikungunya viruses. These data suggest that the activities of peptoids include engagement and disruption of viral membrane constituents. To investigate how these peptoids target lipid membranes, we used liposome leakage assays to measure membrane disruption. We found that liposomes containing phosphatidylserine (PS) were markedly sensitive to peptoid treatment; in contrast, liposomes formed exclusively with phosphatidylcholine (PC) showed no sensitivity. In addition, chikungunya virus containing elevated envelope PS was more susceptible to peptoid-mediated inactivation. These results indicate that peptoids mimicking the physicochemical characteristics of AMPs act through a membrane-specific mechanism, most likely through preferential interactions with PS. We provide the first evidence for the engagement of distinct viral envelope lipid constituents, establishing an avenue for specificity that may enable the development of a new family of therapeutics capable of averting the rapid development of resistance.

Plant defensin MtDef4-derived antifungal peptide with multiple modes of action and potential as a bio-inspired fungicide

Chemical fungicides have been instrumental in protecting crops from fungal diseases. However, increasing fungal resistance to many of the single-site chemical fungicides calls for the development of new antifungal agents with novel modes of action (MoA). The sequence-divergent cysteine-rich antifungal defensins with multisite MoA are promising starting templates for design of novel peptide-based fungicides. Here, we experimentally tested such a set of 17-amino-acid peptides containing the γ-core motif of the antifungal plant defensin MtDef4. These designed peptides exhibited antifungal properties different from those of MtDef4. Focused analysis of a lead peptide, GMA4CG_V6, showed that it was a random coil in solution with little or no secondary structure elements. Additionally, it exhibited potent cation-tolerant antifungal activity against the plant fungal pathogen Botrytis cinerea, the causal agent of grey mould disease in fruits and vegetables. Its multisite MoA involved localization predominantly to the plasma membrane, permeabilization of the plasma membrane, rapid internalization into the vacuole and cytoplasm, and affinity for the bioactive phosphoinositides phosphatidylinositol 3-phosphate (PI3P), PI4P, and PI5P. The sequence motif RRRW was identified as a major determinant of the antifungal activity of this peptide. While topical spray application of GMA4CG_V6 on Nicotiana benthamiana and tomato plants provided preventive and curative suppression of grey mould disease symptoms, the peptide was not internalized into plant cells. Our findings open the possibility that truncated and modified defensin-derived peptides containing the γ-core sequence could serve as promising candidates for further development of bio-inspired fungicides.

Impact of Peptide Structure on Colonic Stability and Tissue Permeability

Most marketed peptide drugs are administered parenterally due to their inherent gastrointestinal (GI) instability and poor permeability across the GI epithelium. Several molecular design techniques, such as cyclisation and D-amino acid (D-AA) substitution, have been proposed to improve oral peptide drug bioavailability. However, very few of these techniques have been translated to the clinic. In addition, little is known about how synthetic peptide design may improve stability and permeability in the colon, a key site for the treatment of inflammatory bowel disease and colorectal cancer. In this study, we investigated the impact of various cyclisation modifications and D-AA substitutions on the enzymatic stability and colonic tissue permeability of native oxytocin and 11 oxytocin-based peptides. Results showed that the disulfide bond cyclisation present in native oxytocin provided an improved stability in a human colon model compared to a linear oxytocin derivative. Chloroacetyl cyclisation increased native oxytocin stability in the colonic model at 1.5 h by 30.0%, whereas thioether and N-terminal acetylated cyclisations offered no additional protection at 1.5 h. The site and number of D-AA substitutions were found to be critical for stability, with three D-AAs at Tyr, Ile and Leu, improving native oxytocin stability at 1.5 h in both linear and cyclic structures by 58.2% and 79.1%, respectively. Substitution of three D-AAs into native cyclic oxytocin significantly increased peptide permeability across rat colonic tissue; this may be because D-AA substitution favourably altered the peptide's secondary structure. This study is the first to show how the strategic design of peptide therapeutics could enable their delivery to the colon via the oral route.

In vitro efficacy of different PEGylation designs on cathelicidin-like peptide with high antibacterial and antifungal activity

Recent reports on antibiotic resistance have highlighted the need to reduce the impact of this global health issue through urgent prevention and control. The World Health Organization currently considers antibiotic resistance as one of the most dangerous threats to global health. Therefore, Antimicrobial peptides (AMPs) are promising for the development of novel antibiotic molecules due to their high antimicrobial effects, non-inducing antimicrobial resistance (AMR) properties, and broad spectrum. Hence, in this study, we developed novel antimicrobial peptide/polymer conjugates to reduce the adverse effects of TN6 (RLLRLLLRLLR) peptide. We demonstrate how our constructs function in vitro in terms of antimicrobial activity, hemolytic activity, cytotoxicity, and protease resistance. Our findings show that our molecules are effective against different types of microorganisms such as Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, methicillin-resistant S. aureus, vancomycin-resistant Enteroccus faecium, and Candida albicans, which are known to be pathogenic and antibiotic-resistant. Our constructs generally showed low cytotoxicity relative to the peptide in HaCaT and 3T3 cells. Especially these structures are very successful in terms of hemotoxicity. In the bacteremia model with S. aureus, the naked peptide (TN6) was hemotoxic even at 1 µg/mL, while the hemotoxicity of the conjugates was considerably lower than the peptide. Remarkably in this model, the hemolytic activity of PepC-PEG-pepC conjugate decreased 15-fold from 2.36 to 31.12 µg/mL compared to the bacteria-free 60-min treatment. This is proof that in the case of bacteremia and sepsis, the conjugates specifically direct to bacterial cell membranes rather than red blood cells. In addition, the PepC-PEG-pepC conjugate is resistant to plasma proteases. Moreover, morphological and intracellular damage of the peptide/conjugates to Escherichia coli are demonstrated in SEM and TEM images. These results suggest our molecules can be considered potential next-generation broad-spectrum antibiotic molecule/drug candidates that might be used in clinical cases such as bacteremia and sepsis.

Antimicrobial Peptides and Small Molecules Targeting the Cell Membrane of Staphylococcus aureus

Clinical management of Staphylococcus aureus infections presents a challenge due to the high incidence, considerable virulence, and emergence of drug resistance mechanisms. The treatment of drug-resistant strains, such as methicillin-resistant S. aureus (MRSA), is further complicated by the development of tolerance and persistence to antimicrobial agents in clinical use. To address these challenges, membrane disruptors, that are not generally considered during drug discovery for agents against S. aureus, should be explored. The cell membrane protects S. aureus from external stresses and antimicrobial agents, but membrane-targeting antimicrobial agents are probably less likely to promote bacterial resistance. Nontypical linear cationic antimicrobial peptides (AMPs), highly modified AMPs such as daptomycin (lipopeptide), bacitracin (cyclic peptide), and gramicidin S (cyclic peptide), are currently in clinical use. Recent studies have demonstrated that AMPs and small molecules can penetrate the cell membrane of S. aureus, inhibit phospholipid biosynthesis, or block the passage of solutes between the periplasm and the exterior of the cell. In addition to their primary mechanism of action (MOA) that targets the bacterial membrane, AMPs and small molecules may also impact bacteria through secondary mechanisms such as targeting the biofilm, and downregulating virulence genes of S. aureus. In this review, we discuss the current state of research into cell membrane-targeting AMPs and small molecules and their potential mechanisms of action against drug-resistant physiological forms of S. aureus, including persister cells and biofilms.

Systematic d-Amino Acid Substitutions to Control Peptide and Hydrogel Degradation in Cellular Microenvironments

Enzymatically degradable peptides are commonly used as linkers within hydrogels for biological applications; however, controlling the degradation of these engineered peptides with different contexts and cell types can prove challenging. In this work, we systematically examined the substitution of d-amino acids (D-AAs) for different l-amino acids in a peptide sequence commonly utilized in enzymatically degradable hydrogels (VPMS↓MRGG) to create peptide linkers with a range of different degradation times, in solution and in hydrogels, and investigated the cytocompatibility of these materials. We found that increasing the number of D-AA substitutions increased the resistance to enzymatic degradation both for free peptide and peptide-linked hydrogels; yet, this trend also was accompanied by increased cytotoxicity in cell culture. This work demonstrates the utility of D-AA-modified peptide sequences to create tunable biomaterials platforms tempered by considerations of cytotoxicity, where careful selection and optimization of different peptide designs is needed for specific biological applications.

Antimicrobial peptides as drugs with double response against Mycobacterium tuberculosis coinfections in lung cancer

Tuberculosis and lung cancer are, in many cases, correlated diseases that can be confused because they have similar symptoms. Many meta-analyses have proven that there is a greater chance of developing lung cancer in patients who have active pulmonary tuberculosis. It is, therefore, important to monitor the patient for a long time after recovery and search for combined therapies that can treat both diseases, as well as face the great problem of drug resistance. Peptides are molecules derived from the breakdown of proteins, and the membranolytic class is already being studied. It has been proposed that these molecules destabilize cellular homeostasis, performing a dual antimicrobial and anticancer function and offering several possibilities of adaptation for adequate delivery and action. In this review, we focus on two important reason for the use of multifunctional peptides or peptides, namely the double activity and no harmful effects on humans. We review some of the main antimicrobial and anti-inflammatory bioactive peptides and highlight four that have anti-tuberculosis and anti-cancer activity, which may contribute to obtaining drugs with this dual functionality.

Mycobacterium tuberculosis cell-wall and antimicrobial peptides: a mission impossible?

Mycobacterium tuberculosis (Mtb) is one of the most important infectious agents worldwide and causes more than 1.5 million deaths annually. To make matters worse, the drug resistance among Mtb strains has risen substantially in the last few decades. Nowadays, it is not uncommon to find patients infected with Mtb strains that are virtually resistant to all antibiotics, which has led to the urgent search for new molecules and therapies. Over previous decades, several studies have demonstrated the efficiency of antimicrobial peptides to eliminate even multidrug-resistant bacteria, making them outstanding candidates to counterattack this growing health problem. Nevertheless, the complexity of the Mtb cell wall makes us wonder whether antimicrobial peptides can effectively kill this persistent Mycobacterium. In the present review, we explore the complexity of the Mtb cell wall and analyze the effectiveness of antimicrobial peptides to eliminate the bacilli.

Advances in Antimicrobial Peptide Discovery via Machine Learning and Delivery via Nanotechnology

Antimicrobial peptides (AMPs) have been investigated for their potential use as an alternative to antibiotics due to the increased demand for new antimicrobial agents. AMPs, widely found in nature and obtained from microorganisms, have a broad range of antimicrobial protection, allowing them to be applied in the treatment of infections caused by various pathogenic microorganisms. Since these peptides are primarily cationic, they prefer anionic bacterial membranes due to electrostatic interactions. However, the applications of AMPs are currently limited owing to their hemolytic activity, poor bioavailability, degradation from proteolytic enzymes, and high-cost production. To overcome these limitations, nanotechnology has been used to improve AMP bioavailability, permeation across barriers, and/or protection against degradation. In addition, machine learning has been investigated due to its time-saving and cost-effective algorithms to predict AMPs. There are numerous databases available to train machine learning models. In this review, we focus on nanotechnology approaches for AMP delivery and advances in AMP design via machine learning. The AMP sources, classification, structures, antimicrobial mechanisms, their role in diseases, peptide engineering technologies, currently available databases, and machine learning techniques used to predict AMPs with minimal toxicity are discussed in detail.

Boosting stability and therapeutic potential of proteolysis-resistant antimicrobial peptides by end-tagging β-naphthylalanine

Recently, much emphasis has been placed on solving the intrinsic defects of antimicrobial peptides (AMPs), especially their susceptibility to protease digestion for the systemic application of antibacterial biomaterials. Although many strategies have increased the protease stability of AMPs, antimicrobial activity was severely compromised, thereby substantially weakening their therapeutic effect. To address this issue, we introduced hydrophobic group modifications at the N-terminus of proteolysis-resistant AMPs D1 (AArIIlrWrFR) through end-tagging with stretches of natural amino acids (W and I), unnatural amino acid (Nal) and fatty acids. Of these peptides, N1 tagged with a Nal at N-terminus showed the highest selectivity index (GM_SI = 19.59), with a 6.73-fold improvement over D1. In addition to potent broad-spectrum antimicrobial activity, N1 also exhibited high antimicrobial stability toward salts, serum and proteases in vitro and ideal biocompatibility and therapeutic efficacy in vivo. Furthermore, N1 killed bacteria through multiple mechanisms, involving disruption of bacterial membranes and inhibition of bacterial energy metabolism. Indeed, appropriate terminal hydrophobicity modification opens up new avenues for developing and applying high-stability peptide-based antibacterial biomaterials. STATEMENT OF SIGNIFICANCE: To improve the potency and stability of proteolysis-resistant antimicrobial peptides (AMPs) without increasing toxicity, we constructed a convenient and tunable platform based on different compositions and lengths of hydrophobic end modifications. By tagging an Nal at the N-terminal, the obtained target compound N1 exhibited strong antimicrobial activity and desirable stability under multifarious environments in vitro (protease, salts and serum), and also showed favorable biocompatibility and therapeutic efficacy in vivo. Notably, N1exerted its bactericidal effect by damaging bacterial cell membranes and inhibiting bacterial energy metabolism in a dual mode. The findings provide a potential method for designing or optimizing proteolysis-resistant AMPs thus promoting the development and application of peptide-based antibacterial biomaterial.

Biodegradable Polymers and Polymer Composites with Antibacterial Properties

Antibiotic resistance is one of the greatest threats to global health and food security today. It becomes increasingly difficult to treat infectious disorders because antibiotics, even the newest ones, are becoming less and less effective. One of the ways taken in the Global Plan of Action announced at the World Health Assembly in May 2015 is to ensure the prevention and treatment of infectious diseases. In order to do so, attempts are made to develop new antimicrobial therapeutics, including biomaterials with antibacterial activity, such as polycationic polymers, polypeptides, and polymeric systems, to provide non-antibiotic therapeutic agents, such as selected biologically active nanoparticles and chemical compounds. Another key issue is preventing food from contamination by developing antibacterial packaging materials, particularly based on degradable polymers and biocomposites. This review, in a cross-sectional way, describes the most significant research activities conducted in recent years in the field of the development of polymeric materials and polymer composites with antibacterial properties. We particularly focus on natural polymers, i.e., polysaccharides and polypeptides, which present a mechanism for combating many highly pathogenic microorganisms. We also attempt to use this knowledge to obtain synthetic polymers with similar antibacterial activity.

Structure-activity relationships of antibacterial peptides

Antimicrobial peptides play a crucial role in innate immunity, whose components are mainly peptide-based molecules with antibacterial properties. Indeed, the exploration of the immune system over the past 40 years has revealed a number of natural peptides playing a pivotal role in the defence mechanisms of vertebrates and invertebrates, including amphibians, insects, and mammalians. This review provides a discussion regarding the antibacterial mechanisms of peptide-based agents and their structure-activity relationships (SARs) with the aim of describing a topic that is not yet fully explored. Some growing evidence suggests that innate immunity should be strongly considered for the development of novel antibiotic peptide-based libraries. Also, due to the constantly rising concern of antibiotic resistance, the development of new antibiotic drugs is becoming a priority of global importance. Hence, the study and the understanding of defence phenomena occurring in the immune system may inspire the development of novel antibiotic compound libraries and set the stage to overcome drug-resistant pathogens. Here, we provide an overview of the importance of peptide-based antibacterial sources, focusing on accurately selected molecular structures, their SARs including recently introduced modifications, their latest biotechnology applications, and their potential against multi-drug resistant pathogens. Last, we provide cues to describe how antibacterial peptides show a better scope of action selectivity than several anti-infective agents, which are characterized by non-selective activities and non-targeted actions toward pathogens.

Design and Discovery of Novel Cyclic Peptides as EDPs-EBP Interaction Inhibitors for the Treatment of Liver Fibrosis

Liver fibrosis is the undesirable result of excessive deposition of the extracellular matrix (ECM), and elastin is known as one of the key ECM components. Under specific pathological conditions, elastin undergoes degradation to produce elastin-derived peptides (EDPs), which bind to elastin-binding protein (EBP) to activate corresponding signal pathways, thus accelerating fibrosis progression. Herein, we describe the discovery of novel cyclic peptides that function as potent and stable inhibitors to interfere with the peptide-protein interaction between EDPs and EBP. Remarkably, C_XJ-2 exhibited potent activities to inhibit the PI3K/ERK pathway and decrease hepatic stellate cell proliferation and migration. The subsequent in vivo study demonstrated that C_XJ-2 possessed potent antifibrotic efficacy in ameliorating CCl₄-induced liver fibrosis. This work provides a successful pharmacological strategy for the development of novel inhibitors of EDPs-EBP interaction, which sheds new light on how cyclic peptides disrupt peptide-protein interaction and may also provide new structure-oriented therapeutic candidates in liver fibrosis.

Myxinidin-Derived Peptide against Biofilms Caused by Cystic Fibrosis Emerging Pathogens

Chronic lung infections in cystic fibrosis (CF) patients are triggered by multidrug-resistant bacteria such as Pseudomonas aeruginosa, Achromobacter xylosoxidans, and Stenotrophomonas maltophilia. The CF airways are considered ideal sites for the colonization and growth of bacteria and fungi that favor the formation of mixed biofilms that are difficult to treat. The inefficacy of traditional antibiotics reinforces the need to find novel molecules able to fight these chronic infections. Antimicrobial peptides (AMPs) represent a promising alternative for their antimicrobial, anti-inflammatory, and immunomodulatory activities. We developed a more serum-stable version of the peptide WMR (WMR-4) and investigated its ability to inhibit and eradicate C. albicans, S. maltophilia, and A. xylosoxidans biofilms in both in vitro and in vivo studies. Our results suggest that the peptide is able better to inhibit than to eradicate both mono and dual-species biofilms, which is further confirmed by the downregulation of some genes involved in biofilm formation or in quorum-sensing signaling. Biophysical data help to elucidate its mode of action, showing a strong interaction of WMR-4 with lipopolysaccharide (LPS) and its insertion in liposomes mimicking Gram-negative and Candida membranes. Our results support the promising therapeutic application of AMPs in the treatment of mono- and dual-species biofilms during chronic infections in CF patients.

Peptide-based drug discovery: Current status and recent advances

The progressive development of peptides from reaction vessels to life-saving drugs via rigorous preclinical and clinical assessments is fascinating. Peptide therapeutics have gained momentum with the evolution of techniques in peptide chemistry, such as microwave irradiation in solid- and solution-phase synthesis, ligation chemistry, recombinant synthesis, and amalgamation with synthetic tools, including metal catalysis. Diverse emerging technologies, such as DNA-encoded libraries (DELs) and display techniques, are changing the status quo in the discovery of peptide therapeutics. In this review, we analyzed US Food and Drug Administration (FDA)-approved peptide drugs and those in clinical trials, highlighting recent advances in peptide-based drug discovery.

Incorporation of Non-Canonical Amino Acids into Antimicrobial Peptides: Advances, Challenges, and Perspectives

The emergence of antimicrobial resistance is a global health concern and calls for the development of novel antibiotic agents. Antimicrobial peptides seem to be promising candidates due to their diverse sources, mechanisms of action, and physicochemical characteristics, as well as the relatively low emergence of resistance. The incorporation of noncanonical amino acids into antimicrobial peptides could effectively improve their physicochemical and pharmacological diversity. Recently, various antimicrobial peptides variants with improved or novel properties have been produced by the incorporation of single and multiple distinct noncanonical amino acids. In this review, we summarize strategies for the incorporation of noncanonical amino acids into antimicrobial peptides, as well as their features and suitabilities. Recent applications of noncanonical amino acid incorporation into antimicrobial peptides are also presented. Finally, we discuss the related challenges and prospects.

In Vitro Antimycobacterial Activity of Human Lactoferrin-Derived Peptide, D-hLF 1-11, against Susceptible and Drug-Resistant Mycobacterium tuberculosis and Its Synergistic Effect with Rifampicin

Tuberculosis is a highly contagious disease caused by the Mycobacterium tuberculosis complex (MTBC). Although TB is treatable, multidrug-resistant, extensively drug-resistant, and totally drug-resistant forms of M. tuberculosis have become a new life-threatening concern. New anti-TB drugs that are capable of curing these drug-resistant strains are urgently needed. The purpose of this study is to determine the antimycobacterial activity of D-enantiomer human lactoferricin 1-11 (D-hLF 1-11) against mycobacteria in vitro using a 3-(4,5-dimethylthiazol-2-yl)-2,5-dephenyltetrazolium bromide colorimetric assay, resazurin microplate assay, and microscopic observation drug susceptibility assay. Three previously described antimicrobial peptides, protegrin-1, AK 15-6, and melittin, with potent anti-TB activity, were included in this study. The findings suggest that D-hLF 1-11 can inhibit the growth of M. tuberculosis with a minimum inhibitory concentration of 100−200 µg/mL in susceptible, isoniazid (INH)-monoresistant, rifampicin (RF)-monoresistant, and MDR strains. The peptide can also inhibit some nontuberculous mycobacteria and other MTBC in similar concentrations. The antibiofilm activity of D-hLF 1-11 against the biofilm-forming M. abscessus was determined by crystal violet staining, and no significant difference is observed between the treated and untreated biofilm control. The checkerboard assay was subsequently carried out with M. tuberculosis H37Rv and the results indicate that D-hLF 1-11 displays an additive effect when combined with INH and a synergistic effect when combined with RF, with fractional inhibitory concentration indices of 0.730 and 0.312, respectively. The red blood cell hemolytic assay was initially applied for the toxicity determination of D-hLF 1-11, and negligible hemolysis (<1%) was observed, despite a concentration of up to 4 mg/mL being evaluated. Overall, D-hLF 1-11 has potential as a novel antimycobacterial agent for the future treatment of drug-sensitive and drug-resistant M. tuberculosis infections.

The dual antimicrobial and immunomodulatory roles of host defense peptides and their applications in animal production

Host defense peptides (HDPs) are small molecules with broad-spectrum antimicrobial activities against infectious bacteria, viruses, and fungi. Increasing evidence suggests that HDPs can also indirectly protect hosts by modulating their immune responses. Due to these dual roles, HDPs have been considered one of the most promising antibiotic substitutes to improve growth performance, intestinal health, and immunity in farm animals. This review describes the antimicrobial and immunomodulatory roles of host defense peptides and their recent applications in animal production.

D1018 with higher stability and excellent lipopolysaccharide binding affinity has potent anti-bacterial and anti-inflammatory activity

Escherichia coli (E. coli) infection and LPS-induced inflammation are still of severe threat to human health. With the increasing problem of antibiotic resistance, there is a desperate need to develop new approaches to solve the problem. Antimicrobial peptide (AMP) IDR-1018 exhibited potential antimicrobial and immunoregulation activity. However, moderate antimicrobial efficiency and susceptibility to protease cleavage limited its therapeutic application. Therefore, the derived 1018M which has better activity against MRSA and whole sequence D-amino acids substitution peptides (D1018 and D1018M) were synthesized in this study. The resistance of D1018 and D1018M against tested proteases increased (2–4 times), particularly in D1018. The antibacterial activity of D1018 was the same as that of the parent peptide IDR-1018, but the antimicrobial activity of D1018M was slightly increased (2-fold). Though the hemolysis of IDR-1018 and D1018 was about 2%, at the concentration of 8×MIC, the cytotoxicity of IDR-1018, D1018, and 1018M was negligible. The peptides could interact with E. coli cell wall and cytoplasmic membrane, penetrate the membrane, cause leakage of contents, and disrupt genomic DNA. Among them, D1018 is the most prominent one. In addition, IDR-1018 and D1018 showed potent binding ability to LPS, thus leading to excellent inhibition capacity to LPS-induced proinflammation response. Taken together, these data demonstrate that D1018 is a promising peptide candidate for the treatment of E. coli infection.

Expanding the chemical repertoire of protein-based polymers for drug-delivery applications

Expanding the chemical repertoire of natural and artificial protein-based polymers (PBPs) can enable the production of sequence-defined, yet chemically diverse, biopolymers with customized or new properties that cannot be accessed in PBPs composed of only natural amino acids. Various approaches can enable the expansion of the chemical repertoire of PBPs, including chemical and enzymatic treatments or the incorporation of unnatural amino acids. These techniques are employed to install a wide variety of chemical groups-such as bio-orthogonally reactive, cross-linkable, post-translation modifications, and environmentally responsive groups-which, in turn, can facilitate the design of customized PBP-based drug-delivery systems with modified, fine-tuned, or entirely new properties and functions. Here, we detail the existing and emerging technologies for expanding the chemical repertoire of PBPs and review several chemical groups that either demonstrate or are anticipated to show potential in the design of PBP-based drug delivery systems. Finally, we provide our perspective on the remaining challenges and future directions in this field.

Advances in the stability challenges of bioactive peptides and improvement strategies

Bioactive peptides are widely used in functional foods due to their remarkable efficacy, selectivity, and low toxicity. However, commercially produced bioactive peptides lack quality stability between batches. Furthermore, the efficacies of bioactive peptides cannot be guaranteed in vivo due to gastrointestinal digestion and rapid plasma, liver, and kidney metabolism. The problem of poor stability has restricted the development of peptides. Bioactive peptide stability assessments use different stability assays, so the results of different studies are not always comparable. This review summarizes the quality stability challenges in the enzymatic hydrolysis production of bioactive peptides and the metabolism stability challenges after oral administration. Future directions on the strategies for improving their stability are provided. It was proposed that we use fingerprinting as a quality control measure using qualitative and quantitative characteristic functional peptide sequences. The chemical modification and encapsulation of bioactive peptides in microcapsules and liposomes are widely used to improve the digestive and metabolic stability of bioactive peptides. Additionally, the establishment of a universal stability test and a unified index would greatly improve uniformity and comparability in research into bioactive peptides. In summary, the reliable evaluation of stability is an essential component of peptide characterization, and these ideas may facilitate further development and utilization of bioactive peptides.

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Stability challenges encountered by bioactive peptides were summarized.

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Strategies to improve the stability of bioactive peptides were provided.

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A universal stability test and unified index would improve uniformity and comparability in research into bioactive peptides.

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It was proposed that we use a method of traditional Chinese medicine fingerprinting as a quality control measure.

Biomimetic antimicrobial polymers—Design, characterization, antimicrobial, and novel applications

Biomimetic antimicrobial polymers have been an area of great interest as the need for novel antimicrobial compounds grows due to the development of resistance. These polymers were designed and developed to mimic naturally occurring antimicrobial peptides in both physicochemical composition and mechanism of action. These antimicrobial peptide mimetic polymers have been extensively investigated using chemical, biophysical, microbiological, and computational approaches to gain a deeper understanding of the molecular interactions that drive function. These studies have helped inform SARs, mechanism of action, and general physicochemical factors that influence the activity and properties of antimicrobial polymers. However, there are still lingering questions in this field regarding 3D structural patterning, bioavailability, and applicability to alternative targets. In this review, we present a perspective on the development and characterization of several antimicrobial polymers and discuss novel applications of these molecules emerging in the field. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease.

Engineering Approaches for the Development of Antimicrobial Peptide-Based Antibiotics

Antimicrobial peptides (AMPs) have received increasing attention as potential alternatives for future antibiotics because of the rise of multidrug-resistant (MDR) bacteria. AMPs are small cationic peptides with broad-spectrum antibiotic activities and different action mechanisms to those of traditional antibiotics. Despite the desirable advantages of developing peptide-based antimicrobial agents, the clinical applications of AMPs are still limited because of their enzymatic degradation, toxicity, and selectivity. In this review, structural modifications, such as amino acid substitution, stapling, cyclization of peptides, and hybrid AMPs with conventional antibiotics or other peptides, will be presented. Additionally, nanodelivery systems using metals or lipids to deliver AMPs will be discussed based on the structural properties and action mechanisms of AMPs.

Optimized proteolytic resistance motif (DabW)-based U1-2WD: A membrane-induced self-aggregating peptide to trigger bacterial agglutination and death

The biggest application bottleneck of antimicrobial peptides (AMPs) is the low oral bioavailability caused by the poor stability of digestive enzymes in the gastrointestinal tract. However, the research methods and evaluation criteria of available studies about anti-proteolytic strategies are not uniform and far from the actual environment in vivo. Here, we developed a research system and evaluation criteria for proteolytic resistance and systematically evaluated the effectiveness of different strategies for improving the protease stability of AMPs on the same platform for the first time. After a comprehensive analysis, Dab modification is identified as the most effective strategy to improve the trypsin stability of AMPs. By further modulating the proteolytic resistance optimization motif (DabW)_n, U1-2WD is obtained with ideal stability and antimicrobial properties in vivo and in vitro. Notably, U1-2WD has a unique antibacterial mechanism, which forms amorphous aggregates in the bacteria environment to trigger the agglutination of bacterial cells to prevent bacterial escape. It then kills bacteria by disrupting bacterial membranes and inhibiting bacterial energy metabolism. Overall, our work has led to a new understanding of the effectiveness of proteolytic resistance strategies and accelerated the development of anti-proteolytic AMPs to combat multidrug-resistant bacterial infections. STATEMENT OF SIGNIFICANCE: We developed research system and evaluation criteria for proteolytic resistance and systematically evaluated the effectiveness of different strategies for improving protease stability of AMPs on the same platform for the first time. we found effective strategies to resist trypsin hydrolysis: modification with backbone (β-Arg), D-enantiomer (D-Arg) and L-2,4-diaminobutanoic acid (Dab). Further, the proteolytic resistance optimization motif (DabW)n was designed. When n=3, derivative U1-2WD was obtained with desirable stability and antimicrobial properties in vivo and in vitro. Notably, U1-2WD has a unique antibacterial mechanism, which can self-aggregate into amorphous aggregates in the bacteria environment to mediate the agglutination and sedimentation of bacterial cells to prevent bacterial escape, and then kill bacteria by destroying bacterial membranes and inhibiting bacterial energy metabolism.

Designed symmetrical β-hairpin peptides for treating multidrug-resistant salmonella typhimurium infections

The rapid emergence and prevalence of multidrug-resistant salmonellosis lack effective therapies, which causes epidemic health problems and stimulates the development of antimicrobials with novel modes of action. In this research, 10 short symmetrical β-hairpin peptides are synthesized by combining the β-turn of Leucocin-A with recurring hydrophobic and cationic amino acid sequences. Those designed peptides exhibited potent antibacterial activities against drug-susceptible and drug-resistant Salmonella. One of the 10 peptides, WK2 ((WK)₂CTKSGC(KW)₂), displayed best cell selectivity towards Salmonella cells over macrophages and erythrocytes in a co-culture model. Fluorescent measurements and microscopic observations reflected that WK2 exerted its antimicrobial activity through a membrane-lytic mechanism. Moreover, the β-hairpin peptides can bind to endotoxin (LPS) and suppress the production of LPS-induced proinflammatory cytokines in RAW264.7 cells, indicating as a potent anti-inflammatory activity. The preliminary in vivo studies can also demonstrate that WK2 decreased loads of Salmonella in the liver and spleen, mitigated Salmonella-caused inflammation and maintained the integrity of intestinal mucosal surfaces. Ultimately, the results highlight that WK2 is a promising therapeutic agent to prevent multidrug-resistant S. Typhimurium infections in humans and animals.

The current research status and strategies employed to modify food-derived bioactive peptides

The ability of bioactive peptides to exert biological functions has mainly contributed to their exploitation. The exploitation and utilization of these peptides have grown tremendously over the past two decades. Food-derived peptides from sources such as plant, animal, and marine proteins and their byproducts constitute a more significant portion of the naturally-occurring peptides that have been documented. Due to their high specificity and biocompatibility, these peptides serve as a suitable alternative to pharmacological drugs for treating non-communicable diseases (such as cardiovascular diseases, obesity, and cancer). They are helpful as food preservatives, ingredients in functional foods, and dietary supplements in the food sector. Despite their unique features, the application of these peptides in the clinical and food sector is to some extent hindered by their inherent drawbacks such as toxicity, bitterness, instability, and susceptibility to enzymatic degradation in the gastrointestinal tract. Several strategies have been employed to eliminate or reduce the disadvantages of peptides, thus enhancing the peptide bioactivity and broadening the opportunities for their applications. This review article focuses on the current research status of various bioactive peptides and the strategies that have been implemented to overcome their disadvantages. It will also highlight future perspectives regarding the possible improvements to be made for the development of bioactive peptides with practical uses and their commercialization.

Apolipoprotein E mimetic peptide COG1410 combats pandrug-resistant Acinetobacter baumannii

The emergence of pandrug-resistant bacteria breaks through the last line of defense and raises fear among people of incurable infections. In the post-antibiotic era, the pharmaceutical field turns to seek non-conventional anti-infective agents. Antimicrobial peptides are considered a prospective solution to the crisis of antimicrobial resistance. In this study, we evaluated the antimicrobial efficiency of an ApoE mimetic peptide, COG1410, which has been confirmed to exhibit strong neural protective activity and immunomodulatory function. COG1410 showed potent antimicrobial activity against pandrug-resistant Acinetobacter baumannii, even eliminating large inocula (108 CFU/ml) within 30 min. LC99.9 in PBS and 50% pooled human plasma was 2 μg/ml (1.4 μM) and 8 μg/ml (5.6 μM), respectively. Moreover, COG1410 exhibited biofilm inhibition and eradication activity, excellent stability in human plasma, and a low propensity to induce resistance. Although COG1410 easily entered bacterial cytoplasm and bound to DNA nonspecifically, the major mechanism of COG1410 killing was to disrupt the integrity of cell membrane and lead to leakage of cytoplasmic contents, without causing obvious pores on the cell surface or cell lysis. Additionally, transcriptome analysis showed that treatment with COG1410-enriched genes involved a series of oxidation–reduction processes. DCFH-DA probe detected an increased ROS level in the presence of COG1410, indicating ROS was another hit of this AMP. Furthermore, the action of COG1410 did not depend on the electronic interaction with the LPS layer, in contrast to polymyxin B. The strong synergistic interaction between COG1410 and polymyxin B dramatically reduced the working concentration of COG1410, expanding the safety window of the application. C. elegans infection model showed that combined therapy of COG1410 and polymyxin B was capable of significantly rescuing the infected nematodes. Taken together, our study demonstrates that COG1410 is a promising drug candidate in the battle against pandrug-resistant A. baumannii.

Gramicidin A is hydrolyzed by a d-stereospecific peptidase produced by Bacillus anthracis

Previously we described the discovery of a Bacillus spp. specific peptidase activity related to d-stereospecific peptidases (DSPs). The peptidase showed a strong preference for d-leucine and d-valine amino acids. These amino acids are present in the structure of the non-ribosomal peptide (NRP) antibiotics gramicidin A, B and C and polymyxin E. To examine if the Bacillus spp. DSP-related peptidase can hydrolyze these NRPs, the effect of gramicidin A and C and polymyxin E on peptidase activity in Bacillus anthracis culture supernatant was monitored. It was found that both gramicidins inhibited the DSP-related activity in a competitive manner. MALDI-TOF analysis revealed that upon incubation with B. anthracis culture supernatant gramicidin A hydrolyzation products appeared. This study shows that the Bacillus spp. specific DSP-like peptidase was potentially produced by the bacteria to gain intrinsic resistance against NRP antibiotics. These results are of utmost importance in research towards antimicrobial resistance, whereas transfer of DSP-related activity to other clinically relevant pathogens can be a serious threat to human health.

In pursuit of next-generation therapeutics: Antimicrobial peptides against superbugs, their sources, mechanism of action, nanotechnology-based delivery, and clinical applications

Antimicrobial peptides (AMPs) attracted attention as potential source of novel antimicrobials. Multi-drug resistant (MDR) infections have emerged as a global threat to public health in recent years. Furthermore, due to rapid emergence of new diseases, there is pressing need for development of efficient antimicrobials. AMPs are essential part of the innate immunity in most living organisms, acting as the primary line of defense against foreign invasions. AMPs kill a wide range of microorganisms by primarily targeting cell membranes or intracellular components through a variety of ways. AMPs can be broadly categorized based on their physico-chemical properties, structure, function, target and source of origin. The synthetic analogues produced either with suitable chemical modifications or with the use of suitable delivery systems are projected to eliminate the constraints of toxicity and poor stability commonly linked with natural AMPs. The concept of peptidomimetics is gaining ground around the world nowadays. Among the delivery systems, nanoparticles are emerging as potential delivery tools for AMPs, amplifying their utility against a variety of pathogens. In the present review, the broad classification of various AMPs, their mechanism of action (MOA), challenges associated with AMPs, current applications, and novel strategies to overcome the limitations have been discussed.

Lights and Shadows on the Therapeutic Use of Antimicrobial Peptides

The emergence of antimicrobial-resistant infections is still a major concern for public health worldwide. The number of pathogenic microorganisms capable of resisting common therapeutic treatments are constantly increasing, highlighting the need of innovative and more effective drugs. This phenomenon is strictly connected to the rapid metabolism of microorganisms: due to the huge number of mutations that can occur in a relatively short time, a colony can “adapt” to the pharmacological treatment with the evolution of new resistant species. However, the shortage of available antimicrobial drugs in clinical use is also caused by the high costs involved in developing and marketing new drugs without an adequate guarantee of an economic return; therefore, the pharmaceutical companies have reduced their investments in this area. The use of antimicrobial peptides (AMPs) represents a promising strategy for the design of new therapeutic agents. AMPs act as immune defense mediators of the host organism and show a poor ability to induce antimicrobial resistance, coupled with other advantages such as a broad spectrum of activity, not excessive synthetic costs and low toxicity of both the peptide itself and its own metabolites. It is also important to underline that many antimicrobial peptides, due to their inclination to attack cell membranes, have additional biological activities, such as, for example, as anti-cancer drugs. Unfortunately, they usually undergo rapid degradation by proteolytic enzymes and are characterized by poor bioavailability, preventing their extensive clinical use and landing on the pharmaceutical market. This review is focused on the strength and weak points of antimicrobial peptides as therapeutic agents. We give an overview on the AMPs already employed in clinical practice, which are examples of successful strategies aimed at overcoming the main drawbacks of peptide-based drugs. The review deepens the most promising strategies to design modified antimicrobial peptides with higher proteolytic stability with the purpose of giving a comprehensive summary of the commonly employed approaches to evaluate and optimize the peptide potentialities.

The role of bacterial transport systems in the removal of host antimicrobial peptides in Gram-negative bacteria

Antibiotic resistance is a global issue that threatens our progress in healthcare and life expectancy. In recent years, antimicrobial peptides (AMPs) have been considered as promising alternatives to the classic antibiotics. AMPs are potentially superior due to their lower rate of resistance development, since they primarily target the bacterial membrane ('Achilles' heel' of the bacteria). However, bacteria have developed mechanisms of AMP resistance, including the removal of AMPs to the extracellular space by efflux pumps such as the MtrCDE or AcrAB-TolC systems, and the internalization of AMPs to the cytoplasm by the Sap transporter, followed by proteolytic digestion. In this review, we focus on AMP transport as a resistance mechanism compiling all the experimental evidence for the involvement of efflux in AMP resistance in Gram-negative bacteria and combine this information with the analysis of the structures of the efflux systems involved. Finally, we expose some open questions with the aim of arousing the interest of the scientific community towards the AMPs-efflux pumps interactions. All the collected information broadens our understanding of AMP removal by efflux pumps and gives some clues to assist the rational design of AMP-derivatives as inhibitors of the efflux pumps.

Development of the Antithrombotic Peptide LEKNSTY Targeting the Collagen Surface: II. Improvement of Plasma Stability

The development of antithrombotic peptides targeting collagen was proven effective, and an effective antithrombotic peptide LEKNSTY was obtained in part I. However, the plasma stability of LEKNSTY was found to be not good enough. In this part, the LEKNSTY was further optimized for improvement in plasma stability by substitution using d-amino acid residues. Two novel antithrombotic peptides LekNStY and lEKnsTy were designed, where lowercase letters represent d-amino acid residues. Improvements in plasma stability of both LekNStY and lEKnsTy were experimentally confirmed. Moreover, good binding of these antithrombotic peptides on the collagen surface was confirmed by molecular dynamics simulation and experimental validation. For example, a K_d of only 0.75 ± 0.10 μM was observed for lEKnsTy. Moreover, LekNStY and lEKnsTy were found to inhibit platelet adhesion on the collagen surface more effectively than LEKNSTY, and the IC₅₀ of lEKnsTy was only 2/5 of that of LEKNSTY. These results confirmed the successful design of LekNStY and lEKnsTy that had good plasma stability and could effectively inhibit arterial thrombosis, which would be helpful for the research into interfaces involved in thrombus formation and the development of antithrombotic nanomedicine.

d-Amino Acid-Based Antifouling Peptides for the Construction of Electrochemical Biosensors Capable of Assaying Proteins in Serum with Enhanced Stability

The susceptibility of peptides to proteolytic degradation in human serum significantly hindered the potential application of peptide-based antifouling biosensors for long-term assaying of clinical samples. Herein, a robust antifouling biosensor with enhanced stability was constructed based on peptides composed of d-amino acids (d-peptide) with prominent proteolytic resistance. The electrode was electropolymerized with poly(3,4-ehtylenedioxythiophene) and electrodeposited with Au nanoparticles (AuNPs), and the d-peptide was then immobilized onto the AuNPs, and a typical antibody specific for immunoglobulin M (IgM) was immobilized. Because of the effect of d-amino acids, the d-peptide-modified electrode surface showed prominent antifouling capability and high tolerance to enzymatic hydrolysis. Moreover, the d-peptide-modified electrode exhibited much stronger long-term stability, as well as antifouling ability in human serum than the electrode modified with normal peptides. The electrochemical biosensor exhibited a sensitive response to IgM linearly within the range of 100 pg mL^-1 to 1.0 μg mL^-1 and a very low detection limit down to 37 pg mL^-1, and it was able to detect IgM in human serum with good accuracy. This work provided a new strategy to develop robust peptide-based biosensors to resist the proteolytic degradation for practical application in complex clinical samples.

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.

Bacteriocins from lactic acid bacteria: purification strategies and applications in food and medical industries: a review

Background

Bacteriocins are generally defined as ribosomally synthesized peptides, which are produced by lactic acid bacteria (LAB) that affect the growth of related or unrelated microorganisms. Conventionally, the extracted bacteriocins are purified by precipitation, where ammonium sulphate is added to precipitate out the protein from the solution.

Main text

To achieve the high purity of bacteriocins, a combination with chromatography is used where the hydrophobicity and cationic properties of bacteriocins are employed. The complexity column inside the chromatography can afford to resolve the loss of bacteriocins during the ammonium sulphate precipitation. Recently, an aqueous two-phase system (ATPS) has been widely used in bacteriocins purification due to the several advantages of its operational simplicity, mild process conditions and versatility. It reduces the operation steps and processing time yet provides high recovery products which provide alternative ways to conventional methods in downstream processing. Bacteriocins are widely approached in the food and medical industry. In food application, nisin, which is produced by Lactococcus lactis subsp. has been introduced as food preservative due to its natural, toxicology safe and effective against the gram-positive bacteria. Besides, bacteriocins provide a board range in medical industries where they are used as antibiotics and probiotics.

Short conclusion

In summary, this review focuses on the downstream separation of bacteriocins from various sources using both conventional and recent ATPS techniques. Finally, recommendations for future interesting areas of research that need to be pursued are highlighted.

Chirality-influenced antibacterial activity of methylthiazole- and thiadiazole-based supramolecular biocompatible hydrogels

Chiral stereochemistry is a unique and fundamental strategy that determines the interaction of bacteria cells with chiral biomolecules and stereochemical surfaces. The interaction between bacteria and material surface (molecular chirality or supramolecular chirality) plays a significant role in modulating antibacterial performance. Herein, we developed inherent chiral antibacterial hydrogels by modifying the carboxyl groups of our previously reported supramolecular gelator (LPF-left handed phenylalanine gelator and DPF- right handed phenylalanine gelator) with 2-amino-5-methylthiazole (MTZ) and 5-amino-1,3,4-thiadiazole-2- thiol (TDZ). The new L/D-gelator molecules initiate self-assembly to form hydrogels through non-covalent interactions (Hydrogen bonding and π-π interactions) verified by FTIR and CD spectroscopy. Morphological studies of the xerogels revealed left and right-handed chiral nanofibers for the gelators' L-form and D-form, respectively. The resulting hydrogels exhibited inherent antibacterial activity against Gram-positive (Bacillus subtilis, Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacteria, with TDZ hydrogels showing more significant antibacterial activity than MTZ hydrogels. Interestingly, the D-form (having right-handed nanofibers) of both hydrogels (MTZ and TDZ) exhibited higher antibacterial activities compared with the left-handed nanofibrous hydrogels (L-form) attributed to the stereoselective interaction of the chiral helical nanofiber. Moreover, the amplification of chirality moving from a molecular to a supramolecular level essentially improved the antibacterial action. Our results provide deep insight into the development of unique supramolecular chiral antimicrobial agents and hint at the potentiality of right-handed nanofibers (D-form) having enhanced antibacterial activity. STATEMENT OF SIGNIFICANCE: Chiral stereochemistry plays a significant role in many biological processes, which determines the interaction of bacteria cells with chiral biomolecules. The interaction between bacteria and material surface (molecular chirality or supramolecular chirality) plays a significant role in modulating antibacterial performance. Here, we deigned and synthesized unique inherent biocompatible supramolecular chiral hydrogel. From this study we concluded that the D-form (having right-handed nanofibers) of hydrogels exhibited higher antibacterial activities compared with the left-handed nanofibrous hydrogels (L-form) attributed to the stereoselective interaction of the chiral helical nanofiber. Additionally, this study also explored the amplification of chirality moving from a molecular to a supramolecular level essentially improved the antibacterial action.

Antimicrobial Peptides: From Design to Clinical Application

Infection of multidrug-resistant (MDR) bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), carbapenem-resistant Enterobacteriaceae (CRE), and extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli, brings public health issues and causes economic burden. Pathogenic bacteria develop several methods to resist antibiotic killing or inhibition, such as mutation of antibiotic function sites, activation of drug efflux pumps, and enzyme-mediated drug degradation. Antibiotic resistance components can be transferred between bacteria by mobile genetic elements including plasmids, transposons, and integrons, as well as bacteriophages. The development of antibiotic resistance limits the treatment options for bacterial infection, especially for MDR bacteria. Therefore, novel or alternative antibacterial agents are urgently needed. Antimicrobial peptides (AMPs) display multiple killing mechanisms against bacterial infections, including directly bactericidal activity and immunomodulatory function, as potential alternatives to antibiotics. In this review, the development of antibiotic resistance, the killing mechanisms of AMPs, and especially, the design, optimization, and delivery of AMPs are reviewed. Strategies such as structural change, amino acid substitution, conjugation with cell-penetration peptide, terminal acetylation and amidation, and encapsulation with nanoparticles will improve the antimicrobial efficacy, reduce toxicity, and accomplish local delivery of AMPs. In addition, clinical trials in AMP studies or applications of AMPs within the last five years were summarized. Overall, AMPs display diverse mechanisms of action against infection of pathogenic bacteria, and future research studies and clinical investigations will accelerate AMP application.

Disrupting GPCR Complexes with Smart Drug-like Peptides

G protein-coupled receptors (GPCRs) are a superfamily of proteins classically described as monomeric transmembrane (TM) receptors. However, increasing evidence indicates that many GPCRs form higher-order assemblies made up of monomers pertaining to identical (homo) or to various (hetero) receptors. The formation and structure of these oligomers, their physiological role and possible therapeutic applications raise a variety of issues that are currently being actively explored. In this context, synthetic peptides derived from TM domains stand out as powerful tools that can be predictably targeted to disrupt GPCR oligomers, especially at the interface level, eventually impairing their action. However, despite such potential, TM-derived, GPCR-disrupting peptides often suffer from inadequate pharmacokinetic properties, such as low bioavailability, a short half-life or rapid clearance, which put into question their therapeutic relevance and promise. In this review, we provide a comprehensive overview of GPCR complexes, with an emphasis on current studies using GPCR-disrupting peptides mimicking TM domains involved in multimerization, and we also highlight recent strategies used to achieve drug-like versions of such TM peptide candidates for therapeutic application.

The characteristics and roles of antimicrobial peptides as potential treatment for antibiotic-resistant pathogens: a review

The emergence of antibiotic-resistant bacteria has become a significant and ever-increasing threat to global public health, increasing both morbidity and mortality rates, and the financial burden on health services. Infection by drug-resistant bacteria is anticipated to contribute to the demise of almost 10 million people by the year 2050 unless a competent and effective response is devised to engage with this issue. The emergence and spread of resistance are commonly caused by the excessive or inappropriate use of antibiotics and substandard pharmaceuticals. It arises when pathogens adapt to different conditions and develop self-defence mechanisms. Currently, novel antimicrobial peptides (AMPs) have been reported to be the sole cure for some clinical cases of infectious diseases such as sepsis and skin infections, although these agents may, on occasion, require administration together with an adjunctive low-dose antibiotic. Although AMPs are a promising alternative form of anti-microbial therapy and easily applied in the medical sector, they still have limitations that should not be taken lightly. Hence, this review explores the characteristics, advantages and disadvantages of AMPs for their potential in treating antibiotic-resistant pathogens.