Strategies employed in the design of antimicrobial peptides with enhanced proteolytic stability

Homo and Hetero-Branched Lipopeptide Dendrimers: Synthesis and Antimicrobial Activity

Di-branched and tetra-branched versions of a previously reported analogue of the lipopeptide battacin were successfully synthesised using thiol-maleimide click and 1, 2, 3-triazole click chemistry. Antimicrobial studies against drug resistant clinical isolates of Escherichia coli (ESBL E. coli Ctx-M14), Pseudomonas aeruginosa (P. aeruginosa Q502), and Methicillin resistant Staphylococcus aureus (MRSA ATCC 33593), as well as clinically isolated Acinetobacter baumannii (A. baumannii ATCC 19606), and P. aeruginosa (ATCC 27853), revealed that the dendrimeric peptides have antimicrobial activity in the low micromolar range (0.5 -- 4 μM) which was 10 times more potent than the monomer peptides. Under high salt concentrations (150 mM NaCl, 2 mM MgCl2, and 2.5 mM CaCl2) the di-branched lipopeptides retained their antimicrobial activity while the monomer peptides were not active (>100 μM). The di-branched triazole click lipopeptide, Peptide 12, was membrane lytic, showed faster killing kinetics, and exhibited antibiofilm activity against A. baumannii and MRSA and eradicated > 85 % preformed biofilms at low micromolar concentrations. The di-branched analogues were > 30-fold potent than the monomers against Candida albicans. Peptide 12 was not haemolytic (HC10 = 932.12 μM) and showed up to 40-fold higher selectivity against bacteria and fungi than the monomer peptide. Peptide 12 exhibited strong proteolytic stability (>80 % not degraded) in rat serum over 24 h whereas > 95 % of the thiol-maleimide analogue (Peptide 10) was degraded. The tetra-branched peptides showed comparable antibacterial potency to the di-branched analogues. These findings indicate that dual branching using triazole click chemistry is a promising strategy to improve the antimicrobial activity and proteolytic stability of battacin based lipopeptides. The information gathered can be used to build effective antimicrobial dendrimeric peptides as new peptide antibiotics.

Medium-sized peptides from microbial sources with potential for antibacterial drug development

Covering: 1993 to the end of 2022As the rapid development of antibiotic resistance shrinks the number of clinically available antibiotics, there is an urgent need for novel options to fill the existing antibiotic pipeline. In recent years, antimicrobial peptides have attracted increased interest due to their impressive broad-spectrum antimicrobial activity and low probability of antibiotic resistance. However, macromolecular antimicrobial peptides of plant and animal origin face obstacles in antibiotic development because of their extremely short elimination half-life and poor chemical stability. Herein, we focus on medium-sized antibacterial peptides (MAPs) of microbial origin with molecular weights below 2000 Da. The low molecular weight is not sufficient to form complex protein conformations and is also associated to a better chemical stability and easier modifications. Microbially-produced peptides are often composed of a variety of non-protein amino acids and terminal modifications, which contribute to improving the elimination half-life of compounds. Therefore, MAPs have great potential for drug discovery and are likely to become key players in the development of next-generation antibiotics. In this review, we provide a detailed exploration of the modes of action demonstrated by 45 MAPs and offer a concise summary of the structure-activity relationships observed in these MAPs.

Detection strategies of infectious diseases via peptide-based electrochemical biosensors

Infectious diseases have threatened human life for as long as humankind has existed. One of the most crucial aspects of fighting against these infections is diagnosis to prevent disease spread. However, traditional diagnostic methods prove insufficient and time-consuming in the face of a pandemic. Therefore, studies focusing on detecting viruses causing these diseases have increased, with a particular emphasis on developing rapid, accurate, specific, user-friendly, and portable electrochemical biosensor systems. Peptides are used integral components in biosensor fabrication for several reasons, including various and adaptable synthesis protocols, long-term stability, and specificity. Here, we discuss peptide-based electrochemical biosensor systems that have been developed over the last decade for the detection of infectious diseases. In contrast to other reports on peptide-based biosensors, we have emphasized the following points i) the synthesis methods of peptides for biosensor applications, ii) biosensor fabrication approaches of peptide-based electrochemical biosensor systems, iii) the comparison of electrochemical biosensors with other peptide-based biosensor systems and the advantages and limitations of electrochemical biosensors, iv) the pros and cons of peptides compared to other biorecognition molecules in the detection of infectious diseases, v) different perspectives for future studies with the shortcomings of the systems developed in the past decade.

D-Peptide cell culture scaffolds with enhanced antibacterial and controllable release properties

The development of peptide-based hydrogels characterized by both high biostability and potent antimicrobial activity, aimed at combating multidrug-resistant bacterial infections and providing scaffolds for cell cultures, continues to pose a significant challenge. The susceptibility of antimicrobial peptides (AMPs) to degradation by cations, serum, and proteases restricted their applications in clinical environments. In this study, we designed a peptide sequence (termed D-IK1) entirely consisting of D-amino acids, an enantiomer of a previously reported AMP IK1. Our results demonstrated remarkably improved antibacterial and anticancer activities of D-IK1 as compared to IK1. D-IK1 self-assembled into hydrogels that effectively inhibited bacterial and cancer cell growth by the controlled and sustained release of D-IK1. Importantly, D-IK1 was extremely stable in salt solutions and resisted serum and protease degradation. In addition, the D-IK1 hydrogel fostered cell adhesion and proliferation, proving its viability as a 3D scaffold for cell culture applications. Our research presents a versatile, highly stable antibacterial hydrogel scaffold with potential widespread applications in cell culture, wound healing, and the eradication of multidrug-resistant bacterial infections.

Design of Proteolytic-Resistant Antifungal Peptides by Utilizing Minimum d-Amino Acid Ratios

Antifungal peptides are an appealing alternative to standard antifungal medicines due to their unique mechanism of action and low-level resistance. However, their susceptibility to protease degradation keeps hindering their future development. Herein, a library was established to design peptides with protease resistance and high antifungal activity. The peptides were incorporated with minimal D-amino acids to further improve the protease stability. The most active peptide, IR3, demonstrated good antifungal activity and low toxicity, and its molecular integrity was maintained after protease hydrolysis for 8 h at 2 mg/mL. Furthermore, IR3 could permeate the fungal cell wall, disrupt the cell membrane, produce reactive oxygen species, and induce apoptosis in fungal cells. In vivo experiments confirmed that IR3 could effectively treat fungal keratitis. Collectively, these findings suggest that IR3 is a promising antifungal agent and may be beneficial in the design and development of protease-resistant antifungal peptides.

Nanofiber Peptides for Bacterial Trapping: A Novel Approach to Antibiotic Alternatives in Wound Infections

The pervasive employment of antibiotics has engendered the advent of drug-resistant bacteria, imperiling the well-being and health of both humans and animals. Infections precipitated by such multi-resistant bacteria, especially those induced by methicillin-resistant Staphylococcus aureus (MRSA), pervade hospital settings, constituting a grave menace to patient vitality. Antimicrobial peptides (AMPs) have garnered considerable attention as a potent countermeasure against multidrug resistant bacteria. In preceding research endeavors, an insect-derived antimicrobial peptide is identified that, while possessing antimicrobial attributes, manifested suboptimal efficacy against drug-resistant Gram-positive bacteria. To ameliorate this issue, this work enhances the antimicrobial capabilities of the initial β-hairpin AMPs by substituting the structural sequence of the original AMPs with variant lengths of hydrophobic amino acid-hydrophilic amino acid repeat units. Throughout this endeavor, this work has identified a number of peptides that possess highly effective antibacterial characteristics against a wide range of bacteria. Additionally, some of these peptides have the ability to self-assemble into nanofibers, which then build networks in a distinctive manner to capture bacteria. Consequently, they represent prospective antibiotic alternatives for addressing wound infections engendered by drug-resistant bacteria.

Enhancing the stability and therapeutic potential of the antimicrobial peptide Feleucin-K3 against Multidrug-Resistant a. Baumannii through rational utilization of a D-amino acid substitution strategy

Antimicrobial peptides (AMPs), which have a low probability of developing resistance, are considered the most promising antimicrobial agents for combating antibiotic resistance. Feleucin-K3 is an amphiphilic cationic AMP that exhibits broad-spectrum antimicrobial activity. In our previous research, the first phenylalanine residue was identified as the critical position affecting its biological activity. Here, a series of Feleucin-K3 analogs containing hydrophobic D-amino acids were developed, leveraging the low sensitivity of proteases to unnatural amino acids and the regulatory effect of hydrophobicity on antimicrobial activity. Among them, K-1dF, which replaced the phenylalanine of Feleucin-K3 with its enantiomer (D-phenylalanine), exhibited potent antimicrobial activity with a therapeutic index of 46.97 and MICs between 4 to 8 μg/ml against both sensitive and multidrug-resistant Acinetobacter baumannii. The introduction of D-phenylalanine increased the salt tolerance and serum stability of Feleucin-K3. Moreover, K-1dF displayed a rapid bactericidal effect, a low propensity to develop resistance, and a synergistic effect when combined with antibiotics. More importantly, it exhibited considerable or superior efficacy to imipenem against pneumonia and skin abscess infection. In brief, the K-1dF obtained by simple and effective modification strategy has emerged as a promising candidate antimicrobial agent for tackling multidrug-resistant Acinetobacter baumannii infections.

Unearthing naturally-occurring cyclic antibacterial peptides and their structural optimization strategies

Natural products with antibacterial activity are highly desired globally to combat against multidrug-resistant (MDR) bacteria. Antibacterial peptide (ABP), especially cyclic ABP (CABP), is one of the abundant classes. Most of them were isolated from microbes, demonstrating excellent bactericidal effects. With the improved proteolytic stability, CABPs are normally considered to have better druggability than linear peptides. However, most clinically-used CABP-based antibiotics, such as colistin, also face the challenges of drug resistance soon after they reached the market, urgently requiring the development of next-generation succedaneums. We present here a detail review on the novel naturally-occurring CABPs discovered in the past decade and some of them are under clinical trials, exhibiting anticipated application potential. According to their chemical structures, they were broadly classified into five groups, including (i) lactam/lactone-based CABPs, (ii) cyclic lipopeptides, (iii) glycopeptides, (iv) cyclic sulfur-rich peptides and (v) multiple-modified CABPs. Their chemical structures, antibacterial spectrums and proposed mechanisms are discussed. Moreover, engineered analogs of these novel CABPs are also summarized to preliminarily analyze their structure-activity relationship. This review aims to provide a global perspective on research and development of novel CABPs to highlight the effectiveness of derivatives design in identifying promising antibacterial agents. Further research efforts in this area are believed to play important roles in fighting against the multidrug-resistance crisis.

Development of a Selective and Stable Antimicrobial Peptide

Antimicrobial peptides (AMPs) are presented as potential scaffolds for antibiotic development due to their desirable qualities including broad-spectrum activity, rapid action, and general lack of susceptibility to current resistance mechanisms. However, they often lose antibacterial activity under physiological conditions and/or display mammalian cell toxicity, which limits their potential use. Identification of AMPs that overcome these barriers will help develop rules for how this antibacterial class can be developed to treat infection. Here we describe the development of our novel synthetic AMP, from discovery through in vivo application. Our evolved AMP, DTr18-dab, has broad-spectrum antibacterial activity and is nonhemolytic. It is active against planktonic bacteria and biofilm, is unaffected by colistin resistance, and importantly is active in both human serum and a Galleria mellonella infection model. Several modifications, including the incorporation of noncanonical amino acids, were used to arrive at this robust sequence. We observed that the impact on antibacterial activity with noncanonical amino acids was dependent on assay conditions and therefore not entirely predictable. Overall, our results demonstrate how a relatively weak lead can be developed into a robust AMP with qualities important for potential therapeutic translation.

Carboxymethyl cellulose and carboxymethyl chitosan-based composite nanogel as a stable delivery vehicle for oyster peptides: Characterization, absorption and transport mechanism

An oyster peptide (OPs)-loaded composite nanogel based on carboxymethyl cellulose and carboxymethyl chitosan (CMC@CMCS@OPs) was prepared, and the characterization, absorption and transport mechanism were further investigated. CMC@CMCS@OPs, a dense spherical microstructure with a diameter of ∼64 nm, which enhanced the thermal and digestive stabilities of individual OPs and improved its retention rate of hypoglycemic activity in vitro. The swelling response and in-vitro release profiles showed that CMC@CMCS@OPs could help OPs achieve targeted and controlled release in the intestine. In addition, CMC@CMCS@OPs had no cytotoxicity on Caco-2 cells, and its apparent permeability coefficients increased 4.70-7.45 times compared with OPs, with the absorption rate increased by 129.38 %. Moreover, the transcytosis of CMC@CMCS@OPs nanogel occurred primarily through the macropinocytosis pathway, endocytosis pathway and intestinal efflux transporter-mediated efflux. Altogether, these results suggested that CMC@CMCS@OPs nanogel could be as an effective OPs delivery device for enhancing its stability and absorption.

Computer-aided molecular design and optimization of potent inhibitors disrupting APC‒Asef interaction

Colorectal cancer (CRC) is the second leading cause of cancer mortality worldwide. At initial diagnosis, approximately 20% of patients are diagnosed with metastatic CRC (mCRC). Although the APC‒Asef interaction is a well-established target for mCRC therapy, the discovery and development of effective and safe drugs for mCRC patients remains an urgent and challenging endeavor. In this study, we identified a novel structural scaffold based on MAI inhibitors, the first-in-class APC‒Asef inhibitors we reported previously. ONIOM model-driven optimizations of the N-terminal cap and experimental evaluations of inhibitory activity were performed, and 24-fold greater potency was obtained with the best inhibitor compared to the parental compound. In addition, the cocrystal structure validated that the two-layer π‒π stacking interactions were essential for inhibitor stabilization in the bound state. Furthermore, in vitro and in vivo studies have demonstrated that novel inhibitors suppressed lung metastasis in CRC by disrupting the APC‒Asef interaction. These results provide an intrinsic structural basis to further explore drug-like molecules for APC‒Asef-mediated CRC therapy.

Development strategies and application of antimicrobial peptides as future alternatives to in-feed antibiotics

The use of in-feed antibiotics has been widely restricted due to the significant environmental pollution and food safety concerns they have caused. Antimicrobial peptides (AMPs) have attracted widespread attention as potential future alternatives to in-feed antibiotics owing to their demonstrated antimicrobial activity and environment friendly characteristics. However, the challenges of weak bioactivity, immature stability, and low production yields of natural AMPs impede practical application in the feed industry. To address these problems, efforts have been made to develop strategies for approaching the AMPs with enhanced properties. Herein, we summarize approaches to improving the properties of AMPs as potential alternatives to in-feed antibiotics, mainly including optimization of structural parameters, sequence modification, selection of microbial hosts, fusion expression, and industrially fermentation control. Additionally, the potential for application of AMPs in animal husbandry is discussed. This comprehensive review lays a strong theoretical foundation for the development of in-feed AMPs to achieve the public health globally.

Hepcidin contributes to largemouth bass (Micropterus salmoides) against bacterial infections

The increasing emergence and dissemination of bacterial pathogens in largemouth bass culture accelerate the desire for new treatment measures. Antimicrobial peptides as the host's antimicrobial source dominate the preferred molecules for discovering antibacterial agents. Here, the potential of Hepcidin-1 from largemouth bass (Micropterus salmoides) (MsHep-1) against bacterial infection is demonstrated. MsHep-1 not only improved the survival rate in infection experiments involving Nocardia seriolae (12 %) and Aeromonas hydrophila (18 %) but also coped with iron overload conditions in vivo. Moreover, the antibacterial activity of MsHep-1 in vitro was identified against both gram-negative and gram-positive bacteria. Mechanistic studies show MsHep-1 leads to bacterial death by changing the bacterial membrane potential and disrupting the bacterial membrane structure. These findings demonstrate that MsHep-1 may play an important role in the host response to bacterial infection. It provides promising strategies in the application of immunosuppression prevention and control in fish. AMPs may be a promising and available reservoir for treating the current bacterial diseases.

Effect of tryptophan position and lysine/arginine substitution in antimicrobial peptides on antifungal action

Every year, the overprescription, misuse, and improper disposal of antibiotics have led to the rampant development of drug-resistant pathogens and, in turn, a significant increase in the number of patients who die of drug-resistant fungal infections. Recently, researchers have begun investigating the use of antimicrobial peptides (AMPs) as next-generation antifungal agents to inhibit the growth of drug-resistant fungi. The antifungal activity of alpha-helical peptides designed using the cationic amino acids containing lysine and arginine and the hydrophobic amino acids containing isoleucine and tryptophan were evaluated using 10 yeast and mold fungi. Among these peptides, WIK-14, which is composed of a 14-mer with tryptophan sequences at the amino terminus, showed the best antifungal activity via transient pore formation and ROS generation. In addition, the in vivo antifungal effects of WIK-14 were investigated in a mouse model infected with drug-resistant Candida albicans. The results demonstrate the potential of AMPs as antifungal agents.

Design of a Novel Lysine Isopeptide 1018KI11 with Potent Antimicrobial Activity as a Safe and Stable Food Preservative Candidate

Antimicrobial peptides are potent food additive candidates, but most of them are sensitive to proteases, which limits their application. Therefore, we substituted arginine for lysine and introduced a lysine isopeptide bond to peptide IDR-1018 in order to improve its enzymatic stability. Subsequently, the protease stability and antimicrobial/antibiofilm activity of the novel peptides (1018K2-1018KI11) were investigated. The data revealed that the antienzymatic potential of 1018KI11 to bromelain and papain increased by 2-8 folds and 16 folds, respectively. The minimum inhibitory concentration (MIC) of 1018KI11 against methicillin-resistant Staphylococcus aureus (MRSA) ATCC43300 and Escherichia coli (E. coli) ATCC25922 was reduced 2-fold compared to 1018K11. Mechanism exploration suggested that 1018KI11 was more effective than 1018K11 in disrupting the cell barrier and damaging genomic DNA. Additionally, 1018KI11 at certain concentration conditions (2-64 μg/mL) reduced biofilm development of MRSA ATCC43300 by 4.9-85.9%. These data indicated that novel peptide 1018KI11 is a potential food preservative candidate.

Tryptophan End-Tagging Confers Antifungal Activity on a Tick-Derived Peptide by Triggering Reactive Oxygen Species Production

WHO has identified several Candida species including Candida albicans as critical priority fungal pathogens due to greater infection prevalence and formation of recalcitrant biofilms. Novel antifungal agents are urgently needed, and antimicrobial peptides (AMPs) are being considered as potential alternatives, but inactivity in physiological salt environments, serum, and plasma often limits further therapeutic development. Tryptophan end-tagging is a strategy to overcome these limitations and is thought to selectively enhance membrane permeabilization in both fungal and bacterial plasma membranes. Here, we show that C-terminal tryptophan end-tagging of the tick-derived peptide Os-C transforms an inactive peptide into Os-C(W5), an antifungal peptide capable of preventing the formation of C. albicans biofilms. Mechanistic insight is provided by circular dichroism spectroscopy and molecular dynamics simulations, which demonstrate that tryptophan end-tagging alters the secondary structure of Os-C, while the latter reveals that end-tagging reduces interactions with, and insertion into, a model C. albicans membrane but promotes peptide aggregation on its surface. Interestingly, this leads to the induction of reactive oxygen species production rather than membrane permeabilization, and consequently, oxidative stress leads to cell wall damage. Os-C(W5) does not induce the hemolysis of human erythrocytes. Reduced cell adhesion and viability contribute to decreased biofilm extracellular matrix formation which, although reduced, is retained in the serum-containing medium. In this study, tryptophan end-tagging was identified as a promising strategy for enhancing the antifungal activity, including the biofilm inhibitory activity of Os-C against C. albicans in physiological salt environments.

Three Rounds of Stability-Guided Optimization and Systematical Evaluation of Oncolytic Peptide LTX-315

Oncolytic peptides represent promising novel candidates for anticancer treatments. In our efforts to develop oncolytic peptides possessing both high protease stability and durable anticancer efficiency, three rounds of optimization were conducted on the first-in-class oncolytic peptide LTX-315. The robust synthetic method, in vitro and in vivo anticancer activity, and anticancer mechanism were investigated. The D-type peptides represented by FXY-12 possessed significantly improved proteolytic stability and sustained anticancer efficiency. Strikingly, the novel hybrid peptide FXY-30, containing one FXY-12 and two camptothecin moieties, exhibited the most potent in vitro and in vivo anticancer activities. The mechanism explorations indicated that FXY-30 exhibited rapid membranolytic effects and induced severe DNA double-strand breaks to trigger cell apoptosis. Collectively, this study not only established robust strategies to improve the stability and anticancer potential of oncolytic peptides but also provided valuable references for the future development of D-type peptides-based hybrid anticancer chemotherapeutics.

Effective ciprofloxacin cationic antibacterial agent against persister bacteria with low hemolytic toxicity

With the widespread use of antibiotics, bacterial resistance has developed rapidly. To make matters worse, infections caused by persistent bacteria and biofilms often cannot be completely eliminated, which brings great difficulties to clinical medication. In this work, three series of quinolone pyridinium quaternary ammonium small molecules were designed and synthesized. Most of the compounds showed good antibacterial activity against Gram-positive bacteria (S. aureus and E. faecalis) and Gram-negative bacteria (E. coli and S. maltophilia). The activity of the para-pyridine quaternary ammonium salt was better than that of the meta-pyridine. 3f was the optimal compound with good stability in body fluids and was unlikely to induce bacterial resistance. The hemolysis rate of erythrocytes at 1280 μg/mL for 3f was only 5.1%. Encouragingly, 3f rapidly killed bacteria within 4 h at 4 × MIC concentration and was effective in killing persistent bacteria in biofilms. The antibacterial mechanism experiments showed that 3f could cause disorder of bacterial membrane potential, increase bacterial membrane permeability, dissolve and destroy the membrane. Incomplete bacterial membranes lead to leakage of bacterial genetic material, concomitant production of ROS, and bacterial death due to these multiple effects.

Diversity and Mechanisms of Action of Plant, Animal, and Human Antimicrobial Peptides

Antimicrobial peptides (AMPs) are usually made up of fewer than 100 amino acid residues. They are found in many living organisms and are an important factor in those organisms' innate immune systems. AMPs can be extracted from various living sources, including bacteria, plants, animals, and even humans. They are usually cationic peptides with an amphiphilic structure, which allows them to easily bind and interact with the cellular membranes of viruses, bacteria, fungi, and other pathogens. They can act against both Gram-negative and Gram-positive pathogens and have various modes of action against them. Some attack the pathogens' membranes, while others target their intracellular organelles, as well as their nucleic acids, proteins, and metabolic pathways. A crucial area of AMP use is related to their ability to help with emerging antibiotic resistance: some AMPs are active against resistant strains and are susceptible to peptide engineering. This review considers AMPs from three key sources-plants, animals, and humans-as well as their modes of action and some AMP sequences.

Antimicrobial peptides: An alternative to traditional antibiotics

As antibiotic-resistant bacteria and genes continue to emerge, the identification of effective alternatives to traditional antibiotics has become a pressing issue. Antimicrobial peptides are favored for their safety, low residue, and low resistance properties, and their unique antimicrobial mechanisms show significant potential in combating antibiotic resistance. However, the high production cost and weak activity of antimicrobial peptides limit their application. Moreover, traditional laboratory methods for identifying and designing new antimicrobial peptides are time-consuming and labor-intensive, hindering their development. Currently, novel technologies, such as artificial intelligence (AI) are being employed to develop and design new antimicrobial peptide resources, offering new opportunities for the advancement of antimicrobial peptides. This article summarizes the basic characteristics and antimicrobial mechanisms of antimicrobial peptides, as well as their advantages and limitations, and explores the application of AI in antimicrobial peptides prediction amd design. This highlights the crucial role of AI in enhancing the efficiency of antimicrobial peptide research and provides a reference for antimicrobial drug development.

Novel antimicrobial peptides modified with fluorinated sulfono-γ-AA having high stability and targeting multidrug-resistant bacteria infections

The emergence and increasing prevalence of multidrug-resistant (MDR) bacteria have posed an urgent demand for novel antibacterial drugs. Currently, antimicrobial peptides (AMPs), potential novel antimicrobial agents with rare antimicrobial resistance, represent an available strategy to combat MDR bacterial infections but suffer the limitation of protease degradation. In this study, we developed a highly effective method for optimizing the stability of AMPs by introducing fluorinated sulfono-γ-AApeptides, and successfully synthesized novel Feleucin-K3-analogs. The results demonstrated that the incorporation of fluorinated sulfono-γ-AA into Feleucin-K3 effectively improved stability and afforded optimal peptides, such as CF3-K11, which exhibited 8-9 times longer half-lives than Feleucin-K3. Moreover, CF3-K11 displayed potent antimicrobial activity against clinically isolated Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA), excellent biosafety, low resistance propensity, and possessed powerful antimicrobial efficacy for both local skin infection and pneumonia infection. The optimal CF3-K11 exhibited strong therapeutic potential and offered a superior approach for treating MDR bacterial infections.

New N-Terminal Fatty-Acid-Modified Melittin Analogs with Potent Biological Activity

Melittin, a natural antimicrobial peptide, has broad-spectrum antimicrobial activity. This has resulted in it gaining increasing attention as a potential antibiotic alternative; however, its practical use has been limited by its weak antimicrobial activity, high hemolytic activity, and low proteolytic stability. In this study, N-terminal fatty acid conjugation was used to develop new melittin-derived lipopeptides (MDLs) to improve the characteristics of melittin. Our results showed that compared with native melittin, the antimicrobial activity of MDLs was increased by 2 to 16 times, and the stability of these MDLs against trypsin and pepsin degradation was increased by 50 to 80%. However, the hemolytic activity of the MDLs decreased when the length of the carbon chain of fatty acids exceeded 10. Among the MDLs, the newly designed analog Mel-C8 showed optimal antimicrobial activity and protease stability. The antimicrobial mechanism studied revealed that the MDLs showed a rapid bactericidal effect by interacting with lipopolysaccharide (LPS) or lipoteichoic acid (LTA) and penetrating the bacterial cell membrane. In conclusion, we designed and synthesized a new class of MDLs with potent antimicrobial activity, high proteolytic stability, and low hemolytic activity through N-terminal fatty acid conjugation.

Protective effect and mechanism of nanoantimicrobial peptide ND-C14 against Streptococcus pneumoniae infection

BACKGROUND

Streptococcus pneumoniae (S. pneumoniae) is a common pathogen that causes bacterial pneumonia. However, with increasing bacterial resistance, there is an urgent need to develop new drugs to treat S. pneumoniae infections. Nanodefensin with a 14-carbon saturated fatty acid (ND-C14) is a novel nanoantimicrobial peptide designed by modifying myristic acid at the C-terminus of human α-defensin 5 (HD5) via an amide bond. However, it is unclear whether ND-C14 is effective against lung infections caused by S. pneumoniae.

METHODS

In vitro, three groups were established, including the control group, and the HD5 and ND-C14 treatment groups. A virtual colony-count assay was used to evaluate the antibacterial activity of HD5 and ND-C14 against S. pneumoniae. The morphological changes of S. pneumoniae treated with HD5 or ND-C14 were observed by scanning electron microscopy. In vivo, mice were divided into sham, vehicle, and ND-C14 treatment groups. Mice in the sham group were treated with 25 μL of phosphate-buffered saline (PBS). Mice in the vehicle and ND-C14 treatment groups were treated with intratracheal instillation of 25 μL of bacterial suspension with 2×108 CFU/mL (total bacterial count: 5×106 CFU), and then the mice were given 25 μL PBS or intratracheally injected with 25 μL of ND-C14 (including 20 μg or 50 μg), respectively. Survival rates were evaluated in the vehicle and ND-C14 treatment groups. Bacterial burden in the blood and bronchoalveolar lavage fluid were counted. The lung histology of the mice was assessed. A propidium iodide uptake assay was used to clarify the destructive effect of ND-C14 against S. pneumoniae.

RESULTS

Compared with HD5, ND-C14 had a better bactericidal effect against S. pneumoniae because of its stronger ability to destroy the membrane structure of S. pneumoniae in vitro. In vivo, ND-C14 significantly delayed the death time and improved the survival rate of mice infected with S. pneumoniae. ND-C14 reduced bacterial burden and lung tissue injury. Moreover, ND-C14 had a membrane permeation effect on S. pneumoniae, and its destructive ability increased with increasing ND-C14 concentration.

CONCLUSION

The ND-C14 may improve bactericidal effects on S. pneumoniae both in vitro and in vivo.

Discovery of a novel antifungal agent: All-hydrocarbon stapling modification of peptide Aurein1.2

Aurein1.2 is secreted by the Australian tree frog Litoria aurea and is active against a broad range of infectious microbes including bacteria, fungi, and viruses. Its antifungal potency has garnered considerable interest in developing novel classes of natural antifungal agents to fight pathogenic infection by fungi. However, serious pharmacological hurdles remain, hindering its clinical translation. To alleviate its susceptibility to proteolytic degradation and improve its antifungal activity, six conformationally locked peptides were synthesized through hydrocarbon stapling modification and evaluated for their physicochemical and antifungal parameters. Among them, SAU2-4 exhibited significant improvement in helicity levels, protease resistance, and antifungal activity compared to the template linear peptide Aurein1.2. These results confirmed the prominent role of hydrocarbon stapling modification in the manipulation of peptide pharmacological properties and enhanced the application potential of Aurein1.2 in the field of antifungal agent development.

Anti-microbial activity and immunomodulation of recombinant hepcidin 2 and NK-lysin from flounder (Paralichthys olivaceus)

Infections due to pathogens impact global aquaculture economy, where diseases caused by bacteria should be in particular focus due to antibiotic resistance. Hepcidin and NK-lysin are important innate immune factors having potential to be exploited as alternatives to antibiotics due to their antimicrobial activity and immunomodulatory capacity. In this study, the recombinant proteins of hepcidin 2 and NK-lysin (rPoHep2 and rPoNKL) from flounder (Paralichthys olivaceus) were obtained via a prokaryotic expression system. The results exhibited that rPoHep2 and rPoNKL killed both gram-negative and gram-positive bacteria mainly via attachment and disruption of the membrane. Interestingly, both peptides could bind to bacterial DNA. The antiviral assay showed that both peptides have antiviral activity against hirame nonvirhabdovirus. They exhibited no cytotoxicity to the mammalian and fish cell lines. PoHep2 was found localized in G-CSFR-positive peritoneal cells. Moreover, rPoHep2 significantly enhanced the phagocytosis of flounder leukocytes in vitro. These findings suggested that neutrophils contained rPoHep2 and may respond to the immunoreaction of neutrophils. In summary, both rPoHep2 and rPoNKL possess antimicrobial activities and may be exploited to replace traditional antibiotics. rPoHep2 possess immune regulatory functions, that can be further investigated as an immunostimulant in aquaculture.

A comprehensive guide on screening and selection of a suitable AMP against biofilm-forming bacteria

Lately, antimicrobial resistance (AMR) is increasing at an exponential rate making it important to search alternatives to antibiotics in order to combat multi-drug resistant (MDR) bacterial infections. Out of the several antibacterial and antibiofilm strategies being tested, antimicrobial peptides (AMPs) have shown to give better hopes in terms of a long-lasting solution to the problem. To select a desired AMP, it is important to make right use of available tools and databases that aid in identification, classification, and analysis of the physiochemical properties of AMPs. To identify the targets of these AMPs, it becomes crucial to understand their mode-of-action. AMPs can also be used in combination with other antibacterial and antibiofilm agents so as to achieve enhanced efficacy against bacteria and their biofilms. Due to concerns regarding toxicity, stability, and bioavailability, strategizing drug formulation at an early-stage becomes crucial. Although there are few concerns regarding development of bacterial resistance to AMPs, the evolution of resistance to AMPs occurs extremely slowly. This comprehensive review gives a deep insight into the selection of the right AMP, deciding the right target and combination strategy along with the type of formulation needed, and the possible resistance that bacteria can develop to these AMPs.

Gallium-based metal-organic frameworks loaded with antimicrobial peptides for synergistic killing of drug-resistant bacteria

Increased antibiotic resistance has made bacterial infections a global concern, which requires novel non-antibiotic-dependent antibacterial strategies to address the menace. Antimicrobial peptides (AMPs) are a promising antibiotic alternative, whose antibacterial mechanism is mainly to destroy the membrane of bacteria. Gallium ions exhibit an antibacterial effect by interfering with the iron metabolism of bacteria. With the rapid development of nanotechnology, it is worth studying the potential of gallium-AMP-based nanocomposites for treating bacterial infections. Herein, novel gallium-based metal-organic frameworks (MOFs) were synthesized at room temperature, followed by in situ loading of the model AMP melittin. The obtained nanocomposites exhibited stronger antibacterial activity than pure MEL and gallium ions, achieving the effects of "one plus one is greater than two". Moreover, the nanocomposites showed favorable biocompatibility and accelerated healing of a wound infected by methicillin-resistant Staphylococcus aureus by down-regulation of inflammatory cytokines IL-6 and TNF-α. This work presents an innovative antibacterial strategy to overcome the antibiotic resistance crisis and expand the application of AMPs.

Influence of Fatty Acid Modification on the Anticancer Activity of the Antimicrobial Peptide Figainin 1

Antimicrobial peptides derived from the skin secretions of amphibians have made important progress in tumor therapy due to their unique mechanism of destroying cell membranes. Figainin 1 (F1) is an 18-amino acid antimicrobial peptide from the skin secretions of Boana raniceps frogs. In a previous study, F1 was shown to inhibit cancer cell proliferation. F1 is composed entirely of natural amino acids; therefore, it is easily degraded by a variety of proteases, resulting in poor stability and a short half-life. In the present study, we used a fatty acid modification strategy to improve the stability of Figainin 1. Among the 8 peptides synthesized, A-10 showed the strongest antiproliferative activity against K562 cells and the other four tumor cell lines, and its stability against serum and proteinase K was improved compared with F1. We found that A-10 works through two mechanisms, cell membrane destruction and apoptosis, and can arrest the cell cycle in the G0/G1 phase. Moreover, A-10 exhibited self-assembly behavior. Overall, it is necessary to select a fatty acid with a suitable length for modification to improve the stability and antiproliferative activity of antimicrobial peptides. This study provides a good reference for the development of antimicrobial peptides as effective anticancer compounds.

Conjugation of antimicrobial peptides to enhance therapeutic efficacy

The growing prevalence of antimicrobial resistance (AMR) has brought with it a continual increase in the numbers of deaths from multidrug-resistant (MDR) infections. Since the current arsenal of antibiotics has become increasingly ineffective, there exists an urgent need for discovery and development of novel antimicrobials. Antimicrobial peptides (AMPs) are considered to be a promising class of molecules due to their broad-spectrum activities and low resistance rates compared with other types of antibiotics. Since AMPs also often play major roles in elevating the host immune response, the molecules may also be called "host defense peptides." Despite the great promise of AMPs, the majority remain unsuitable for clinical use due to issues of structural instability, degradation by proteases, and/or toxicity to host cells. Moreover, AMP activities in vivo can be influenced by many factors, such as interaction with blood and serum biomolecules, physiological salt concentrations or different pH values. To overcome these limitations, structural modifications can be made to the AMP. Among several modifications, physical and chemical conjugation of AMP to other biomolecules is widely considered an effective strategy. In this review, we discuss structural modification strategies related to conjugation of AMPs and their possible effects on mode of action. The conjugation of fatty acids, glycans, antibiotics, photosensitizers, polymers, nucleic acids, nanoparticles, and immobilization to biomaterials are highlighted.

An infection-microenvironment-targeted and responsive peptide-drug nanosystem for sepsis emergency by suppressing infection and inflammation

Sepsis is a life-threatening emergency that causes millions of deaths every year due to severe infection and inflammation. Nevertheless, current therapeutic regimens are inadequate to promptly address the vast diversity of potential pathogens. Omiganan, an antimicrobial peptide, has shown promise for neutralizing endotoxins and eliminating diverse pathogens. However, its clinical application is hindered by safety and stability concerns. Herein, we present a nanoscale drug delivery system (Omi-hyd-Dex@HA NPs) that selectively targets infectious microenvironments (IMEs) and responds to specific stimuli for efficient intervention in sepsis. The system consists of omiganan-dexamethasone conjugates linked by hydrazone bonds which self-assemble into nanoparticles coated with a hyaluronic acid (HA). The HA coating not only facilitates IMEs-targeting through interaction with intercellular-adhesion-molecule-1 on inflamed endotheliocytes, but also improves the biosafety of the nanosystem and enhances drug accumulation in primary infection sites triggered by hyaluronidase. The nanoparticles release dual drugs in IMEs through pH-sensitive cleavage of hydrazone bonds to eradicate pathogens and suppress inflammation. In multiple tissue infection and sepsis animal models, Omi-hyd-Dex@HA NPs exhibited rapid source control and comprehensive inflammation reduction, thereby preventing subsequent fatal complications and significantly improving survival outcomes. The bio-responsive and self-delivering nanosystem offers a promising strategy for systemic sepsis treatment in emergencies.

Topoisomeric Membrane-Active Peptides: A Review of the Last Two Decades

In recent decades, bioactive peptides have been gaining recognition in various biomedical areas, such as intracellular drug delivery (cell-penetrating peptides, CPPs) or anti-infective action (antimicrobial peptides, AMPs), closely associated to their distinct mode of interaction with biological membranes. Exploiting the interaction of membrane-active peptides with diverse targets (healthy, tumoral, bacterial or parasitic cell membranes) is opening encouraging prospects for peptides in therapeutics. However, ordinary peptides formed by L-amino acids are easily decomposed by proteases in biological fluids. One way to sidestep this limitation is to use topoisomers, namely versions of the peptide made up of D-amino acids in either canonic (enantio) or inverted (retroenantio) sequence. Rearranging peptide sequences in this fashion provides a certain degree of native structure mimicry that, in appropriate contexts, may deliver desirable biological activity while avoiding protease degradation. In this review, we will focus on recent accounts of membrane-active topoisomeric peptides with therapeutic applications as CPP drug delivery vectors, or as antimicrobial and anticancer candidates. We will also discuss the most common modes of interaction of these peptides with their membrane targets.

Recent Progress in the Characterization, Synthesis, Delivery Procedures, Treatment Strategies, and Precision of Antimicrobial Peptides

Infectious diseases are constantly evolving to bypass antibiotics or create resistance against them. There is a piercing alarm for the need to improve the design of new effective antimicrobial agents such as antimicrobial peptides which are less prone to resistance and possess high sensitivity. This would guard public health in combating and overcoming stubborn pathogens and mitigate incurable diseases; however, the emergence of antimicrobial peptides' shortcomings ranging from untimely degradation by enzymes to difficulty in the design against specific targets is a major bottleneck in achieving these objectives. This review is aimed at highlighting the recent progress in antimicrobial peptide development in the area of nanotechnology-based delivery, selectivity indices, synthesis and characterization, their doping and coating, and the shortfall of these approaches. This review will raise awareness of antimicrobial peptides as prospective therapeutic agents in the medical and pharmaceutical industries, such as the sensitive treatment of diseases and their utilization. The knowledge from this development would guide the future design of these novel peptides and allow the development of highly specific, sensitive, and accurate antimicrobial peptides to initiate treatment regimens in patients to enable them to have accommodating lifestyles.

Bypassing the Need for Cell Permeabilization: Nanobody CDR3 Peptide Improves Binding on Living Bacteria

Membrane interaction constitutes to be an essential parameter in the mode of action of entities such as proteins, as well as cell-penetrating and antimicrobial peptides, resulting in noninvasive or lytic activities depending on the membrane compositions and interactions. Recently, a nanobody able to interact with the top priority, multidrug-resistant bacterial pathogen Acinetobacter baumannii was discovered, although binding took place with fixed cells only. To potentially overcome this limitation, linear peptides corresponding to the complementarity-determining regions (CDR) were synthesized and fluorescently labeled. Microscopy data indicated clear membrane interactions of the CDR3 sequence with living A. baumannii cells, indicating both the importance of the CDR3 as part of the parent nanobody paratope and the improved binding ability and thus avoiding the need for permeabilization of the cells. In addition, cyclization of the peptide with an additionally introduced rigidifying 1,2,3-triazole bridge retains its binding ability while proteolytically protecting the peptide. Overall, this study resulted in the discovery of novel peptides binding a multidrug-resistant pathogen.

A low-fouling electrochemical biosensor for biomarker detection in serum based on designed α/β-peptides with anti-enzymolysis and antifouling capabilities

The zwitterionic peptides, especially those composed of glutamic (E) and lysine (K) groups have drawn enormous attention as antifouling biomaterials owing to their strong hydration capability and biocompatibility. However, the susceptibility of α-amino acid K to the proteolytic enzymes in human serum limited the broad application of such peptides in biological media. Herein, a new multifunctional peptide with favorable stability in human serum was designed, and it was composed of three sections with immobilizing, recognizing and antifouling capabilities, respectively. The antifouling section was composed of alternating E and K amino acids, but the enzymolysis-susceptive amino acid α-K was replaced by the unnatural β-K. Compared with the conventional peptide composed of all α-amino acids, the α/β-peptide exhibited significantly enhanced stability and longer antifouling performance in human serum and blood. The electrochemical biosensor based on the α/β-peptide showed a favorable sensitivity to its target IgG, with a quite wide linear range from 100 pg mL^-1 to 10 μg mL^-1 and a low detection limit (33.7 pg mL^-1, S/N = 3), and it was promising for the detection of IgG in complex human serum. The tactic of designing antifouling α/β-peptides offered an efficient way to develop low-fouling biosensors with robust operation in complex body fluids.

A novel ESKAPE-sensitive peptide with enhanced stability and its application in controlling multiple bacterial contaminations in chilled fresh pork

The co-existence of various pathogenic bacteria on the surface of pork products exacerbates difficulties in food safety control. Developing broad-spectrum and stable antibacterial agents that are not antibiotics is an unmet need. To address this issue, all l-arginine residues of a reported peptide (IIRR)₄-NH₂ (zp80) were substituted with the corresponding D enantiomers. This novel peptide (IIrr)₄-NH₂ (zp80r) was expected to maintain favourable bioactivity against ESKAPE strains and have enhanced proteolytic stability compared with zp80. In a series of experiments, zp80r maintained favourable bioactivities against starvation-induced persisters. Electron microscopy and fluorescent dye assays were used to verify the antibacterial mechanism of zp80r. Importantly, zp80r reduced bacterial colonies in chilled fresh pork contaminated with multiple bacterial species. This newly designed peptide is a potential antibacterial candidate to combat problematic foodborne pathogens during storage of pork.

p-Chloropropynyl Phenylalanine, a Versatile Non-Canonical Amino Acid for Co-Translational Peptide Macrocyclization and Side Chain Diversification

Macrocyclization has proven to be a beneficial strategy to improve upon some of the disadvantages of peptides as therapeutics. Nevertheless, many peptide cyclization strategies are not compatible with in vitro display technologies like mRNA display. Here we describe the novel amino acid p-chloropropynyl phenylalanine (pCPF). pCPF is a substrate for a mutant phenylalanyl-tRNA synthetase and its introduction into peptides via in vitro translation leads to spontaneous peptide macrocyclization in the presence of peptides containing cysteine. Macrocyclization occurs efficiently with a wide variety of ring sizes. Moreover, pCPF can be reacted with thiols after charging onto tRNA, enabling the testing of diverse ncAAs in translation. The versatility of pCPF should facilitate downstream studies of translation and enable the creation of novel macrocyclic peptide libraries.

Effects of citric acid and heterofermentative inoculants on anaerobic co-fermentation of Chinese cabbage waste and wheat bran

The effects of Lactobacillus buchneri, Lactobacillus hilgardii and citric acid on organic acid production, substrate consumption, protein degradation and microbial community were investigated in this study. The results indicated that combined inoculants induced a significant increase in levels of lactic acid (43 g/kg dry matter), acetic acid (14 g/kg dry matter), butyric acid (5 g/kg dry matter), total organic acid (60 g/kg dry matter) and ammonia nitrogen (20 g/kg total nitrogen). Furthermore, citric acid addition into the combined inoculants caused a significant increase in levels of acetic acid (12 g/kg dry matter), water-soluble carbohydrate (12 g/kg dry matter) and a reduction in ammonia nitrogen formation (22 g/kg total nitrogen). Microbiologically, combining inoculants and citric acid enriched Lactobacillus buchneri and Lactobacillus hilgardii and upregulated the functional pathways related to acid production and resistance. Collectively, combining citric acid and heterofermentative inoculants was beneficial to recycle Chinese cabbage waste in producing organic acids.

Conjugates of Aminoglycosides with Stapled Peptides as a Way to Target Antibiotic-Resistant Bacteria

The misuse and overuse of antibiotics led to the development of bacterial resistance to existing aminoglycoside (AMG) antibiotics and limited their use. Consequently, there is a growing need to develop effective antimicrobials against multidrug-resistant bacteria. To target resistant strains, we propose to combine 2-deoxystreptamine AMGs, neomycin (NEO) and amikacin (AMK), with a membrane-active antimicrobial peptide anoplin and its hydrocarbon stapled derivative. The AMG-peptide hybrids were conjugated using the click chemistry reaction in solution to obtain a non-cleavable triazole linker and by disulfide bridge formation on the resin to obtain a linker cleavable in the bacterial cytoplasm. Homo-dimers connected via disulfide bridges between the N-terminus thiol analogues of anoplin and hydrocarbon stapled anoplin were also synthesized. These hybrid compounds show a notable increase in antibacterial and bactericidal activity, as compared to the unconjugated ones or their combinations, against Gram-positive and Gram-negative bacteria, especially for the strains resistant to AMK or NEO. The conjugates and disulfide peptide dimers exhibit low hemolytic activity on sheep red blood erythrocytes.

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.

Broad-spectrum hybrid antimicrobial peptides derived from PMAP-23 with potential LPS binding ability

Antimicrobial peptides, as an integral part of the innate immune system, kill bacteria through a special mechanism of action, making them less susceptible to drug resistance. However, Lipopolysaccharide (LPS) as the permeation barrier on the bacterial membrane, inhibits the antibacterial activity of antimicrobial peptides and triggers the inflammatory response. GWKRKRFG is an LPS binding sequence with a β-boomerang motif that can be linked to antimicrobial peptides to enhance their LPS affinity and reduce the possibility of LPS-induced inflammatory responses. In this study, a series of hybrid peptides were designed by conjugating the reported LPS binding sequence to the C-/N-terminal sequences of the natural porcine antimicrobial peptide PMAP-23 to increase the LPS affinity of peptides. Among all the designed hybrid peptides, 4R-PP-G8 showed the best antibacterial activity, nonhemolytic activity, and excellent cell selectivity. The presence of LPS not only induced the secondary structure transformation of 4R-PP-G8 from a random structure to an α-helical structure but also reduced the antibacterial activity of 4R-PP-G8 in a dose-dependent manner, indicating the excellent binding ability of 4R-PP-G8 to LPS. The LPS/LTA binding assay further verified the interaction between the peptide and LPS. The membrane permeability test verified that 4R-PP-G8 possessed a strong capability to penetrate the bacterial membrane after interacting with LPS. More direct membrane disruption was observed under FE-SEM and TEM. In conclusion, we provided a simple and efficient method to improve the LPS binding ability of antimicrobial peptides and enhance their antimicrobial activity, resulting in the peptide 4R-PP-G8 with clinical application potential.

Prediction and bioactivity of small-molecule antimicrobial peptides from Protaetia brevitarsis Lewis larvae

Antimicrobial peptides (AMPs) are widely recognized as promising natural antimicrobial agents. Insects, as the group of animals with the largest population, have great potential as a source of AMPs. Thus, it is worthwhile to investigate potential novel AMPs from Protaetia brevitarsis Lewis larvae, which is a saprophagous pest prevalent in China. In this study, comparing the whole-genome sequence of Protaetia brevitarsis Lewis larvae with the Antimicrobial Peptide Database (APD3) led to the identification of nine peptide templates that were potentially AMPs. Next, based on the peptide templates, 16 truncated sequences were predicted to the AMPs by bioinformatics software and then underwent structural and physicochemical property analysis. Thereafter, candidate small-molecule AMPs were artificially synthesized and their minimal inhibitory concentration (MIC) values were assessed. A candidate peptide, designated FD10, exhibited strong antimicrobial activity against both bacteria and fungi comprising Escherichia coli (MIC: 8 μg/mL), Pseudomonas aeruginosa (MIC: 8 μg/mL), Bacillus thuringiensis (MIC: 8 μg/mL), Staphylococcus aureus (MIC: 16 μg/mL), and Candida albicans (MIC: 16 μg/mL). Additionally, two other candidate peptides, designated FD12 and FD15, exhibited antimicrobial activity against both E. coli (MIC: both 32 μg/mL) and S. aureus (MIC: both 16 μg/mL). Moreover, FD10, FD12, and FD15 killed almost all E. coli and S. aureus cells within 1 h, and the hemolytic effect of FD10 (0.31%) and FD12 (0.40%) was lower than that of ampicillin (0.52%). These findings indicate that FD12, FD15, and especially FD10 are promising AMPs for therapeutic application. This study promoted the development of antibacterial drugs and provided a theoretical basis for promoting the practical application of antimicrobial peptides in the Protaetia brevitarsis Lewis larvae.

Characterization of simplified nonapeptides with broad-spectrum antimicrobial activities as potential food preservatives, and their antibacterial mechanism

Antimicrobial peptides (AMPs) have attracted attention in the field of food preservatives due to their favorable biosafety and potential antimicrobial activity. However, high synthetic cost, systemic toxicity, a narrow antimicrobial spectrum, and poor antimicrobial activity become the main bottlenecks for their practical applications. To address these questions, a set of derived nonapeptides were designed based on a previously discovered ultra-short peptide sequence template (RXRXRXRXL-NH₂) and screened to identify an optimal peptide-based food preservative with excellent antimicrobial properties. Among these nonapeptides, the designed peptides 3IW (RIRIRIRWL-NH₂) and W2IW (RWRIRIRWL-NH₂) presented a membrane-disruptive and reactive oxygen species (ROS) accumulation mechanism to execute potent and rapid broad-spectrum antimicrobial activity without observed cytotoxicity. Moreover, they exhibited favorable antimicrobial stability regardless of high ionic strength, heat, and excessive acid-base conditions, retaining potent antimicrobial effects for chicken meat preservation. Collectively, their ultra-short sequence length and potent broad-spectrum antimicrobial capacity may be beneficial for the further development of green and safe peptide-based food preservatives.

The efficacy of anti-proteolytic peptide R7I in intestinal inflammation, function, microbiota, and metabolites by multi-omics analysis in murine bacterial enteritis

Increased antibiotic resistance poses a major limitation in tackling inflammatory bowel disease and presents a large challenge for global health care. Antimicrobial peptides (AMPs) are a potential class of antimicrobial agents. Here, we have designed the potential oral route for antimicrobial peptide R7I with anti-proteolytic properties to deal with bacterial enteritis in mice. The results revealed that R7I protected the liver and gut from damage caused by inflammation. RNA-Seq analysis indicated that R7I promoted digestion and absorption in the small intestine by upregulating transmembrane transporter activity, lipid and small molecule metabolic processes and other pathways, in addition to upregulating hepatic steroid biosynthesis and fatty acid degradation. For the gut microbiota, Clostridia were significantly reduced in the R7I-treated group, and Odoribacteraceae, an efficient isoalloLCA-synthesizing strain, was the main dominant strain, protecting the gut from potential pathogens. In addition, we further discovered that R7I reduced the accumulation of negative organic acid metabolites. Overall, R7I exerted better therapeutic and immunomodulatory potential in the bacterial enteritis model, greatly reduced the risk of disease onset, and provided a reference for the in vivo application of antimicrobial peptides.

The responses of organic acid production and microbial community to different carbon source additions during the anaerobic fermentation of Chinese cabbage waste

The effects of glucose, fructose, sucrose and molasses on organic acid levels, protein degradation, nutrient preservation and bacteriome were studied during the anaerobic fermentation of Chinese cabbage waste. The results showed that fructose and molasses additions caused a significant (p < 0.05) increase in lactic acid production (82.16-89.79 %), acetic acid production (175.41-196.93 %), ammonia nitrogen formation (15.93-37.43 %) and reduction of neutral detergent fiber level (8.17-15.87 %). However, few positive effects of glucose and sucrose additions were found on organic acid production. Furthermore, carbon source additions enriched (p < 0.05) the acid-producing bacteria, such as Lactobacillus paralimentarius and Lactobacillus heilongjiangensis, upregulated (p < 0.05) the pathways of carbohydrate and lipid metabolisms and reduced (p < 0.05) the abundances of Lactobacillus buchneri and Escherichia coli and bacteria that were mobile elements-contained and stress-tolerant. Collectively, fructose and molasses additions enhanced the recycling of Chinese cabbage waste by anaerobic fermentation, in which the desired products are organic acids.

Introducing Aliphatic Fluoropeptides: Perspectives on Folding Properties, Membrane Partition and Proteolytic Stability

A de novo designed class of peptide-based fluoropolymers composed of fluorinated aliphatic amino acids as main components is reported. Structural characterization provided insights into fluorine-induced alterations on β-strand to α-helix transition upon an increase in SDS content and revealed the unique formation of PPII structures for trifluorinated fluoropeptides. A combination of circular dichroism, fluorescence-based leaking assays and surface enhanced infrared absorption spectroscopy served to examine the insertion and folding processes into unilamellar vesicles. While partitioning into lipid bilayers, the degree of fluorination conducts a decrease in α-helical content. Furthermore, this study comprises a report on the proteolytic stability of peptides exclusively built up by fluorinated amino acids and proved all sequences to be enzymatically degradable despite the degree of fluorination. Herein presented fluoropeptides as well as the distinctive properties of these artificial and polyfluorinated foldamers with enzyme-degradable features will play a crucial role in the future development of fluorinated peptide-based biomaterials.

Novel Feleucin-K3-Derived Peptides Modified with Sulfono-γ-AA Building Blocks Targeting Pseudomonas aeruginosa and Methicillin-Resistant Staphylococcus aureus Infections

The prevalence of multidrug-resistant bacterial infections has led to dramatically increased morbidity and mortality. Antimicrobial peptides (AMPs) have great potential as new therapeutic agents to reverse this dangerous trend. Herein, a series of novel AMP Feleucin-K3 analogues modified with unnatural peptidomimetic sulfono-γ-AA building blocks were designed and synthesized. The structure-activity, structure-toxicity, and structure-stability relationships were investigated to discover the optimal antimicrobial candidates. Among them, K122 exhibited potent and broad-spectrum antimicrobial activity and high selectivity. K122 had a rapid bactericidal effect and a low tendency to induce resistance. Surprisingly, K122 showed excellent effectiveness against bacterial pneumonia. For biofilm and local skin infections, K122 significantly decreased the bacterial load and improved tissue injury at a dose of only 0.25 mg/kg, which was 160 times lower than the concentration deemed to be safe for local dermal applications. In summary, K122 is an outstanding candidate for the treatment of multidrug-resistant bacteria and biofilm infections.

Self-Assembly of Antimicrobial Peptide-Based Micelles Breaks the Limitation of Trypsin

Targeting the limitation of antimicrobial peptides (AMPs) application in vivo, self-assembled AMPs library with specific nanostructures is expected to gradually overtake monomer AMPs libraries in the future. Peptide polymers are fascinating self-assembling nanoscale structures that have great advantage in biomedical applications because of their satisfactory biocompatibility and versatile properties. Herein, we describe a strategy for inducing the self-assembly of T9W into nanostructured antimicrobial micelles with evidently improved pharmacological properties, that is, PEGylation at the C-terminal of T9W (CT9W₁₀₀₀), an antibacterial biomaterial that self-assembles in aqueous media without exogenous excipients, has been developed. Compared with parental molecular, the CT9W₁₀₀₀ is more effective against Pseudomonas aeruginosa, and its antibacterial spectrum had also been broadened. Additionally, CT9W₁₀₀₀ micelles had higher stability under salt ion, serum, and acid-base environments. Importantly, the self-assembled structure is highly resistant to trypsin degradation, probably allowing T9W to be applied in clinical settings in the future. Mechanistically, by acting on membranes and through supplementary bactericidal mechanisms, CT9W₁₀₀₀ micelles contribute to the antibacterial process. Collectively, CT9W₁₀₀₀ micelles exhibited good biocompatibility in vitro and in vivo, resulting in highly effective treatment in a mouse acute lung injury model induced by P. aeruginosa PAO1 without drug resistance. These advances may profoundly accelerate the clinical transformation of T9W and promote the development of a combination of peptide-based antibiotics and PEGylated nanotechnology.

Anaerobic fermentation featuring wheat bran and rice bran realizes the clean transformation of Chinese cabbage waste into livestock feed

Rapid aerobic decomposition and a high cost/benefit ratio restrain the transformation of Chinese cabbage waste into livestock feed. Herein, anaerobically co-fermenting Chinese cabbage waste with wheat bran and rice bran at different dry matter levels (250, 300, 350 g/kg fresh weight) was employed to achieve the effective and feasible clean transformation of Chinese cabbage waste, and the related microbiological mechanisms were revealed by high-throughput sequencing technology. The bran treatments caused an increase in pH value (4.75-77.25%) and free amino acid content (12.09-152.66%), but a reduction in lactic acid concentration (54.58-77.25%) and coliform bacteria counts (15.91-20.27%). In addition, the wheat bran treatment improved the levels of short-chain fatty acids, nonprotein nitrogen, water-soluble carbohydrates and antioxidant activity and reduced the ammonia nitrogen contents. In contrast, the rice bran treatment decreased the levels of acetic acid, water-soluble carbohydrates, nonprotein nitrogen, ammonia nitrogen, and antioxidant activities. Microbiologically, the bran treatments stimulated Pediococcus, Lactobacillus, Enterobacter, and Weissella but inhibited Lactococcus and Leuconostoc, which were the primary organic acid producers reflected by the redundancy analysis. In addition, Chinese cabbage waste fermented with wheat bran at 350 g/kg fresh weight or with rice bran at 300 g/kg fresh weight increased the scale and complexity of bacteriome, promoted commensalism or mutualism and upregulated the global metabolism pathways, including carbohydrate and amino acid metabolisms. Furthermore, the bran treatments resulted in an increase in bacterial communities that were facultatively anaerobic, biofilm-formed, Gram-negative, potentially pathogenic and stress-tolerant. Collectively, the bran treatments inhibited effluent formation and protein degradation and improved nutrient preservation but reduced organic acid production during the anaerobic fermentation, which is linked to the variations in the bacteriome, indicating that the constructed fermentation system should be further optimized.