Antimicrobial activity and membrane-active mechanism of tryptophan zipper-like β-hairpin antimicrobial peptides

Novel design of simplified β-hairpin antimicrobial peptide as a potential food preservative based on Trp-pocket backbone

Food contamination from microbial deterioration requires the development of potent antimicrobial peptides (AMPs). The deployment of approved AMPs as dietary preservatives is limited due to barriers such as instability, toxicity, and high synthetic costs. This exploration utilizes the primary structural elements of the Trp-pocket backbone to engineer a series of β-hairpin AMPs (XWRWRPGXKXXR-NH2, X representing I, V, F, and/or L). Peptides WpLF, with Phe as X and Leu arranged at the 11th position, demonstrated exceptional selectivity index (SI = 123.08) and sterilization effects both in vitro and in vivo. WpLF consistently exhibited stable bacteriostasis, regardless of physiological salts, serum, and extreme pH. Mechanistic analysis indicated that the peptide penetrates microbial cell membranes, inducing membrane disruption, thereby impeding drug resistance evolution. Conclusively, AMPs engineered by the Trp-pocket skeleton hold substantial potential as innovative biological preservatives in food preservation, providing valuable insights for sustainable and safe peptide-based food preservatives.

Important structural features of antimicrobial peptides towards specific activity: Trends in the development of efficient therapeutics

Proteins and peptides, as polypeptide chains, have usually got unique conformational structures for effective biological activity. Antimicrobial peptides (AMPs) are a group of bioactive peptides, which have been increasingly studied during recent years for their promising antibacterial, antifungal, antiviral and anti-inflammatory activity, as well as, other esteemed bioactivities. Numerous AMPs have been separated from a wide range of natural resources, or produced in vitro through chemical synthesis and recombinant protein expression. Natural AMPs have had limited clinical application due to several drawbacks, such as their short half-life due to protease degradation, lack of activity at physiological salt concentrations, toxicity to mammalian cells, and the absence of suitable methods of delivery for the AMPs that are targeted and sustained. The creation of synthetic analogs of AMPs would both avoid the drawbacks of the natural analogs and maintain or even increase the antimicrobial effectiveness. The structure-activity relationship of discovered AMPs or their derivatives facilitates the development of synthetic AMPs. This review discovered that the relationship between the activity of AMPs and their positive net charge, hydrophobicity, and amino acid sequence and the relationship between AMPs' function and other features like their topology, glycosylation, and halogenation.

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.

Novel β-Hairpin Antimicrobial Peptide Containing the β-Turn Sequence of -NG- and the Tryptophan Zippers Facilitate Self-Assembly into Nanofibers, Exhibiting Excellent Antimicrobial Performance

Antimicrobial peptides (AMPs) have emerged as promising agents to combat the antibiotic resistance crisis due to their rapid bactericidal activity and low propensity for drug resistance. However, AMPs face challenges in terms of balancing enhanced antimicrobial efficacy with increased toxicity during modification processes. In this study, de novo d-type β-hairpin AMPs are designed. The conformational transformation of self-assembling peptide W-4 in the environment of the bacterial membrane and the erythrocyte membrane affected its antibacterial activity and hemolytic activity and finally showed a high antibacterial effect and low toxicity. Furthermore, W-4 displays remarkable stability, minimal occurrence of drug resistance, and synergistic effects when combined with antibiotics. The in vivo studies confirm its high safety and potent wound-healing properties at the sites infected by bacteria. This study substantiates that nanostructured AMPs possess enhanced biocompatibility. These advances reveal the superiority of self-assembled AMPs and contribute to the development of nanoantibacterial materials.

Development and Antibacterial Properties of 4-[4-(Anilinomethyl)-3-phenylpyrazol-1-yl]benzoic Acid Derivatives as Fatty Acid Biosynthesis Inhibitors

A number of novel pyrazole derivatives have been synthesized, and several of these compounds are potent antibacterial agents with minimum inhibitory concentrations as low as 0.5 μg/mL. Human cell lines were tolerant to these lead compounds, and they showed negligible hemolytic effects at high concentrations. These bactericidal compounds are very effective against bacterial growth in both planktonic and biofilm contexts. Various techniques were applied to show the inhibition of biofilm growth and eradication of preformed biofilms by lead compounds. Potent compounds are more effective against persisters than positive controls. In vivo studies revealed that lead compounds are effective in rescuing C. elegans from bacterial infections. Several methods were applied to determine the mode of action including membrane permeability assay and SEM micrograph studies. Furthermore, CRISPRi studies led to the determination of these compounds as fatty acid biosynthesis (FAB) inhibitors.

"In vitro antimicrobial and synergistic effect of fermented Indian zebu (Sahiwal) cow colostrum whey derived peptides with Lactobacillus rhamnosus against pathogenic bacteria"

Bioactive peptides (BAPs) have been found to promote health through various mechanisms. Among them, antimicrobial peptides are gaining recognition as promising novel treatments. This study aims to generate BAPs from bovine colostrum whey using the proteolytic activity of Lactobacillus rhamnosus C25 and to evaluate their potential antibacterial efficacy, including their ability to synergistic efficacy against resistant bacteria. Bioactive peptides were successfully generated from lactobacillus culture proteases that were cultivated through batch fermentation. The resulting peptide fractions were then evaluated for their antibacterial efficacy against a selection of strains, including E. coli ATCC25922, S. aureus MTCC1144, Acinetobacter baumannii ATCC 17978, as well as clinically isolated resistant strains of E. coli (ESBL 1384), Acinetobacter 1379, and S. aureus (MRSA 1418). Notably, the peptide fractions with a molecular weight of < 10 kDa (0-10 kDa) significantly increased the membrane permeability of both E. coli (70.30 ± 0.41%) and S. aureus (63.04 ± 0.31%) as assessed by the crystal violet assay. The checkerboard method was utilized to perform synergistic tests with peptides and antibiotics. The peptide fractions with a molecular weight of (< 10 kDa) demonstrated synergistic effects with several antibiotics, including gentamycin, Rifampicin, Levofloxacin, Ciprofloxacin, and Chloramphenicol, against the resistant ESBL 1384 strain, as indicated by ΣFICI values of 0.55, 0.53, 0.52, 0.54, and 0.52, respectively. Furthermore, the HT-29 cell line remained completely unaffected by both peptide fractions. These findings suggest that the < 10 kDa peptide fraction possesses significant antibacterial efficacy against both reference and ESBL 1384 resistant bacterial strain. Additionally, both MRSA 1418 and Acinetobacter 1379 displayed resistance to all fractions tested. To summarize the findings of this study, colostrum whey peptides with a broad spectrum of antimicrobial activity can be efficiently produced through fermentation. This method could prove valuable for both the pharmaceutical and food industries.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05776-2.

Cell selectivity and antibiofilm and anti-inflammatory activities and antibacterial mechanism of symmetric-end antimicrobial peptide centered on D-Pro-Pro

This study aimed to develop a new symmetric-end antimicrobial peptide (AMP) with cell selectivity, antibiofilm, and anti-inflammatory activities. Two symmetric-end AMPs, Lf6-pP and Lf6-GG, were designed based on the sequence RRWQWRzzRWQWRR, which contains two symmetric repeat sequences connected by a β-turn-promoting sequence (zz) that can be a rigid turn by D-Pro-Pro (pP) or a flexible turn by Gly-Gly (GG). Both Lf6-pP and Lf6-GG exhibited potent antibacterial activity without causing hemolysis, but Lf6-pP exhibited better cell selectivity, likely due to the more significant impact of the rigid pP turn. Compared to Lf6-GG, Lf6-pP demonstrated approximately three times higher antimicrobial activity against drug-resistant bacteria, had a low incidence of drug resistance, and maintained its activity in the presence of physiological salts and human serum. Additionally, Lf6-pP was more effective than Lf6-GG in inhibiting biofilm formation and eradicating mature biofilms. The BODIPY-cadaverine assay indicated that the potent anti-inflammatory activity of Lf6-pP may be attributed to its direct interaction with LPS, resulting in decreased TNF-α and IL-6 levels in LPS-stimulated macrophages. Mechanistic studies, including membrane depolarization, outer/inner membrane permeation, and membrane integrity change, demonstrated that Lf6-pP exerts its antibacterial action through an intracellular-target mechanism. Overall, we propose that Lf6-pP has potential as a novel antibacterial, antibiofilm, and anti-inflammatory agent against drug-resistant bacterial infections.

Structural Analysis and Antimicrobial Mechanism of a Protein GBSPI-A from Ginkgo Biloba Seed

Ginkgo biloba seed has antimicrobial activity. In this study, ginkgo biloba seed protein was prepared, identified, and named GBSPI-A, finding its construction was similar to 11-S globulin. Then, the influence of GBSPI-A on the cell membrane and physiological metabolism of K. pneumoniae and S. aureus were investigated. The results showed that GBSPI-A (20 mg/mL) destroyed the cell membrane, causing leakage of intracellular material and inhibited bacterial growth with an inhibition rate of approximately 80%. In addition, the GBSPI-A (10 mg/mL) caused the decreasing activity of ATPase and respiratory rate, and the respiratory depression rate was 7.24%. Furthermore, the decreasing ATP synthesis and intracellular β-galactosidase activity led to an insufficient supply of physiological metabolic energy. Therefore, the results showed that GBSPI-A could be used as a natural bacteriostatic agent to replace related drugs and also provide a new insight into the application of GBSPI-A in food safety.

Short, mirror-symmetric antimicrobial peptides centered on "RRR" have broad-spectrum antibacterial activity with low drug resistance and toxicity

The increasingly severe bacterial resistance worldwide pushes people to discover and design potential antibacterial drugs unavoidably. In this work, a series of short, mirror-symmetric peptides were designed and successfully synthesized, centered on "RRR" and labeled with hydrophobic amino acids at both ends. Based on the structure-activity relationship analysis, LWWR (LWWRRRWWL-NH₂) was screened as a desirable mirror-symmetric peptide for further study. As expected, LWWR displayed broad-spectrum antibacterial activity against the standard bacteria and antibiotic-resistant strains. Undoubtedly, the high stability of LWWR in a complex physiological environment was an essential guarantee to maximizing its antibacterial activity. Indeed, LWWR also exhibited a rapid bactericidal speed and a low tendency to develop bacterial resistance, based on the multiple actions of non-receptor-mediated membrane actions and intra-cellular mechanisms. Surprisingly, although LWWR showed similar in vivo antibacterial activity compared with Polymyxin B and Melittin, the in vivo safety of LWWR was far higher than that of them, so LWWR had better therapeutic potential. In summary, the desirable mirror-symmetric peptide LWWR was promised as a potential antibacterial agent to confront the antibiotics resistance crisis. STATEMENT OF SIGNIFICANCE: Witnessing the growing problem of antibiotic resistance, a series of short, mirror-symmetric peptides based on the symmetric center "RRR" and hydrophobic terminals were designed and synthesized in this study. Among, LWWR (LWWRRRWWL-NH₂) presented broad-spectrum antibacterial activity both in vitro and in vivo due to its multiple mechanisms and good stability. Meanwhile, the low drug resistance and toxicity of LWWR also suggested its potential for clinical application. The findings of this study will provide some inspiration for the design and development of potential antibacterial agents, and contribute to the elimination of bacterial infections worldwide as soon as possible.

Antibacterial Peptide NP-6 Affects Staphylococcus aureus by Multiple Modes of Action

Our previous study extracted and identified an antibacterial peptide that was named NP-6. Herein, we investigated the physicochemical properties of NP-6, and elucidated the mechanisms underlying its antimicrobial activity against Staphylococcus aureus. The results showed that the hemolysis activity of NP-6 was 2.39 ± 0.13%, lower than Nisin A (3.91 ± 0.43%) at the same concentration (512 µg/mL). Negligible cytotoxicity towards RAW264.7 cells was found when the concentration of NP-6 was lower than 512 µg/mL. In addition, it could keep most of its activity in fetal bovine serum. Moreover, transmission electron microscopy, confocal laser scanning microscopy, and flow cytometry results showed that NP-6 can destroy the integrity of the bacterial cell membrane and increase the membrane permeability. Meanwhile, NP-6 had binding activity with bacterial DNA and RNA in vitro and strongly inhibited the intracellular β-galactosidase activity of S. aureus. Our findings suggest that NP-6 could be a promising candidate against S. aureus.

Novel pyrazoles as potent growth inhibitors of staphylococci, enterococci and Acinetobacter baumannii bacteria

Background: Methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci and Acinetobacter baumannii cause serious antibiotic-resistant infections. Finding new antibiotics to treat these infections is imperative for combating this worldwide menace. Methods & Results: In this study, the authors designed and synthesized potent antimicrobial agents using 4-trifluoromethylphenyl-substituted pyrazole derivatives. In addition to their potency against planktonic bacteria, potent compounds effectively eradicated S. aureus and Enterococcus faecalis biofilms. Human cells tolerated these compounds with good selectivity factors. Furthermore, the authors provide evidence for the mode of action of compounds based on time-kill kinetics, flow cytometry analysis of propidium iodide-treated bacteria and oxygen uptake studies. Conclusion: This study demonstrated 20 novel compounds with potent antibacterial activity that are tolerated by human cell lines.

Manipulating turn residues on de novo designed β-hairpin peptides for selectivity against drug-resistant bacteria

Synthetic β-hairpin antimicrobial peptides (AMPs) offer a useful source for the development of novel antimicrobial agents. β-hairpin peptides generally consist of two side strands bridged by a reverse turn. In literature, most studies focused on the modifications of the side strands to manipulate the stability and activity of β-hairpin peptides, and much less is known about the impact of the turn region. By designing a series of de novo β-hairpin peptides with identical side strands but varied turns, we demonstrated that mutations of only 2 to 4 amino acids at the turn region could impart a wide range of antimicrobial profiles among synthetic β-hairpin AMPs. BTT2-4 and BTT6 displayed selective potency against Gram-negative bacteria, with minimum inhibitory concentrations (MICs) of 4-8 µM. In contrast, BTT1 exhibited broad-spectrum activity, with MICs of 4-8 µM against both Gram-positive and Gram-negative strains. Additionally, BTT1 was potent against methicillin-resistant Staphylococcus aureus (MRSA) and colistin-resistant Enterobacterales. The antimicrobial potency of BTT1 persisted after 14 days of serial passage. Mechanistic studies revealed that interactions between lipopolysaccharide (LPS) and the peptides were critical to their membranolytic activity against the bacterial inner membrane. Aside from folding stability, we observed that a degree of conformational flexibility was required for disruptive membrane interactions. STATEMENT OF SIGNIFICANCE: By examining the significance of the turn region of β-hairpin peptides, we present valuable knowledge to the design toolkit of novel antimicrobial peptides as alternative therapeutics to overcome antibiotic resistance. Our de novo designed synthetic peptides displayed selective activity against Gram-negative bacteria and potent activity against clinically relevant antibiotic-resistant strains (e.g. colistin-resistant Enterobacterales and methicillin-resistant Staphylococcus aureus). The bactericidal activity of our peptides was shown to be robust in the presence of proteolytic trypsin and saline, conditions that could suppress peptide activity. Our peptides were also determined to be non-cytotoxic against a human cell line.

CpACpP: In Silico Cell-Penetrating Anticancer Peptide Prediction Using a Novel Bioinformatics Framework

Cell-penetrating anticancer peptides (Cp-ACPs) are considered promising candidates in solid tumor and hematologic cancer therapies. Current approaches for the design and discovery of Cp-ACPs trust the expensive high-throughput screenings that often give rise to multiple obstacles, including instrumentation adaptation and experimental handling. The application of machine learning (ML) tools developed for peptide activity prediction is importantly of growing interest. In this study, we applied the random forest (RF)-, support vector machine (SVM)-, and eXtreme gradient boosting (XGBoost)-based algorithms to predict the active Cp-ACPs using an experimentally validated data set. The model, CpACpP, was developed on the basis of two independent cell-penetrating peptide (CPP) and anticancer peptide (ACP) subpredictors. Various compositional and physiochemical-based features were combined or selected using the multilayered recursive feature elimination (RFE) method for both data sets. Our results showed that the ACP subclassifiers obtain a mean performance accuracy (ACC) of 0.98 with an area under curve (AUC) ≈ 0.98 vis-à-vis the CPP predictors displaying relevant values of ∼0.94 and ∼0.95 via the hybrid-based features and independent data sets, respectively. Also, the predicting evaluation of Cp-ACPs gave accuracies of ∼0.79 and 0.89 on a series of independent sequences by applying our CPP and ACP classifiers, respectively, which leaves the performance of our predictors better than the earlier reported ACPred, mACPpred, MLCPP, and CPPred-RF. The described consensus-based fusion method additionally reached an AUC of 0.94 for the prediction of Cp-ACP (http://cbb1.ut.ac.ir/CpACpP/Index).

Effects of the antimicrobial peptide WK3 on diarrhea, growth performance and intestinal health of weaned piglets challenged with enterotoxigenic Escherichia coli K88

Background

Antibiotics are very effective for treating diarrhea in weaned pigs, but the global prohibition of antibiotics makes it urgent to find an alternative to antibiotics.

Objective

An experiment was conducted to determine the antimicrobial activity of a linear trpzip-like β-hairpin antimicrobial peptide WK3 in vivo and to assess its effects on growth performance and intestinal health.

Design

Thirty-two piglets were weaned at 21 days and housed in individual metabolic cages, which were randomly divided into four groups and were maintained on a corn-soybean meal-based basal diet. Group 1 included a blank group. Groups 2, 3, and 4 were orally infected by feeding with Enterotoxigenic Escherichia coli (ETEC) K88, which was followed by saline treatment (group 2), enrofloxacin injection at a dose of 2.5 mg/kg (group 3), and WK3 injection at a dose of 2 mg/kg (group 4). The experiment lasted for 6 days, and feed and water were provided ad libitum.

Results

Both WK3 and enrofloxacin effectively attenuated diarrhea and improved growth performance of piglets. Compared with the control group, WK3 significantly improved the villus height in the ileum (P < 0.05) but did not affect the villus height in the duodenum or jejunum. Additionally, we did not observe any obvious difference in crypt depth or villus height/crypt depth among the duodenum, jejunum and ileum (P > 0.05). WK3 also reduced the numbers of Enterococcus spp (P < 0.01) in the cecal contents, and the number of Enterobacterium spp tended to decrease (0.05 < P < 0.1). Moreover, the jejunal mucosa of the WK3 group exhibited lower interleukin-1α (IL-1a; P < 0.01), toll-like receptors-4 (TLR-4; P < 0.05), and myeloid differentiation primary response 88 (MyD88; P < 0.01) messenger ribonucleic acid (mRNA) expression levels. The jejunum of the WK3 group also exhibited an increased antioxidant capacity, reduced concentration of malondialdehyde (MDA; P < 0.05), and enhanced superoxide dismutase (SOD) activity (P < 0.05).

Conclusions

WK3 has the potential to replace antibiotics as a new generation feed additive.

Insights into the antibacterial activity of cottonseed protein-derived peptide against Escherichia coli

In the study, antibacterial peptides were separated and identified from cottonseed protein hydrolysates and the interactions between antibacterial peptides and Escherichia coli were further investigated. Firstly, by using a combined strategy of Amberlite CG-50 ion exchange chromatography and reversed-phase high-performance liquid chromatography, three peptides with antibacterial activity were purified and identified, including HHRRFSLY, KFMPT, and RRLFSDY. Interestingly, HHRRFSLY and RRLFSDY exhibited higher inhibition activity with the IC50 value of 0.26 mg mL-1 and 0.58 mg mL-1 (p < 0.05), respectively. Flow cytometry results showed that the incubation of antibacterial peptides with E. coli could cause damage to the integrity of the E. coli cell membrane. Transmission electron microscopy and scanning electron microscopy results revealed the damage caused to the bacterial cell surface and the leakage of cytoplasmic content by the antibacterial peptides. Molecular docking studies indicated that HHRRFSLY, KFMPT, and RRLFSDY have a good binding affinity to the active sites of the surface protein (OmpF) mainly through a hydrogen bond and salt bridge. The results here showed that the antibacterial peptides derived from cottonseed protein could be used as a good choice for functional foods or related drugs, and also shed light on further studies of antibacterial mechanism.

LHH1, a novel antimicrobial peptide with anti-cancer cell activity identified from Lactobacillus casei HZ1

Antimicrobial peptides have been attracting increasing attention for their multiple beneficial effects. In present study, a novel AMP with a molecular weight of 1875.5 Da, was identified from the genome of Lactobacillus casei HZ1. The peptide, which was named as LHH1 was comprised of 16 amino acid residues, and its α-helix content was 95.34% when dissolved in 30 mM SDS. LHH1 exhibited a broad range of antimicrobial activities against Gram-positive bacteria and fungus. It could effectively inhibit Staphylococcus aureus with a minimum inhibitory concentration of 3.5 μM and showed a low hemolytic activity. The scanning electron microscope, confocal laser scanning microscope and flow cytometry results showed that LHH1 exerted its antibacterial activity by damaging the cell membrane of Staphylococcus aureus. Meanwhile, LHH1 also exhibited anti-cancer cell activities against several cancer cells via breaking the cell membrane of MGC803, HCT116 and C666-1 cancer cells.

Proadrenomedullin N-terminal 20 peptide (PAMP) and its C-terminal 12-residue peptide, PAMP(9-20): Cell selectivity and antimicrobial mechanism

Proadrenomedullin N-terminal 20 peptide (PAMP) is a regulatory peptide that is found in various cell types. It is involved in many biological activities and is rich in basic and hydrophobic amino acids, a common feature of antimicrobial peptides (AMPs). In this study, the cell selectivity and antimicrobial mechanism of PAMP and its C-terminal peptide, PAMP(9-20), were investigated. PAMP and PAMP(9-20) displayed potent antimicrobial activity (minimum inhibitory concentration: 4-32 μM) against standard bacterial strains, but showed no hemolytic activity even at the highest tested concentration of 256 μM. PAMP(9-20) showed 2- to 4-fold increase in antimicrobial activity against gram-negative bacteria compared to PAMP. Cytoplasmic membrane depolarization, leakage of calcein dye from membrane mimic liposomes, SYTOX Green uptake, membrane permeabilization, and flow cytometry studies indicated that the major target of PAMP and PAMP(9-20) is not the microbial cell membrane. Interestingly, laser-scanning confocal microscopy demonstrated that FITC-labeled PAMP and PAMP(9-20) enter the cytoplasm of Escherichia coli similar to buforin-2, and gel retardation assay indicated that PAMP and PAMP(9-20) effectively bind to bacterial DNA. These results suggest that the intracellular target mechanism is responsible for the antimicrobial action of PAMP and PAMP(9-20). Collectively, PAMP and PAMP(9-20) could be considered promising candidates for the development of new antimicrobial agents.

Bacterial Aggregation Triggered by Fibril Forming Tryptophan-Rich Sequences: Effects of Peptide Side Chain and Membrane Phospholipids

The influence of side chain residue and phospholipid characteristics of the cytoplasmic membrane upon the fibrillation and bacterial aggregation of arginine (Arg) and tryptophan (Trp) rich antimicrobial peptides (AMPs) has not been well described to date. Here, we utilized the structural advantages of HHC-10 and ^4HarHHC-10 (Har, l-homoarginine) that are highly active Trp-rich AMPs and investigated their fibril formation and activity behavior against bacteria. The peptides revealed time-dependent self-assembly of polyproline II (PPII) α-helices, but by comparison, ^4HarHHC-10 tended to form higher ordered fibrils due to relatively strong cation-π stacking of Trp with Har residue. Both peptides rapidly killed S. aureus and E. coli at their MICs and caused aggregation of bacteria at higher concentrations. This bacterial aggregation was accompanied by the formation of morphologically distinct electron-dense nanostructures, likely including but not limited to peptides alone. Both HHC-10-derived peptides caused blebs and buds in the E. coli membrane that are rich in POPE phospholipid that promotes negative curvature. However, the main population of S. aureus cells retained their cocci structure upon treatment with HHC peptides even at concentration higher than the MICs. In contrast, the cell aggregation was not induced by HHC fibrils that were most likely stabilized through intra-/intermolecular cation-π stacking. It is proposed that masking of these interactions might have resulted in diminished membrane association/insertion of the HHC nanostructures. The peptides caused aggregation of POPC/POPG (1/3) and POPE/POPG (3/1) liposomes. Nonetheless, disaggregation of the former vesicles was observed at ratios of lipid to peptide of greater than 6 and 24 for HHC-10 and ^4HarHHC-10, respectively. Collectively, our results revealed dose-dependent bacterial aggregation mediated by Trp-rich AMPs that was profoundly influenced by the degree of peptide's self-association and the composition and intrinsic curvature of the cytoplasmic membrane lipids.

Supramolecular Peptide Assemblies as Antimicrobial Scaffolds

Antimicrobial discovery in the age of antibiotic resistance has demanded the prioritization of non-conventional therapies that act on new targets or employ novel mechanisms. Among these, supramolecular antimicrobial peptide assemblies have emerged as attractive therapeutic platforms, operating as both the bactericidal agent and delivery vector for combinatorial antibiotics. Leveraging their programmable inter- and intra-molecular interactions, peptides can be engineered to form higher ordered monolithic or co-assembled structures, including nano-fibers, -nets, and -tubes, where their unique bifunctionalities often emerge from the supramolecular state. Further advancements have included the formation of macroscopic hydrogels that act as bioresponsive, bactericidal materials. This systematic review covers recent advances in the development of supramolecular antimicrobial peptide technologies and discusses their potential impact on future drug discovery efforts.

β-Turn Mimics by Chemical Ligation

We report a simple reductive amination protocol to ligate two peptides, while simultaneously installing a β-turn mimic at the ligation junction. This strategy uses commercially available materials, mild chemical conditions, and a chemoselective ligation reaction of unprotected peptide substrates accessed through standard solid phase methods. This system was implemented in a designed β-hairpin system, and biophysical analysis demonstrates effective mimicry of the β-turn.

Effect of sodium deoxycholate sulfate on outer membrane permeability and neutralization of bacterial lipopolysaccharides by polymyxin B formulations

We demonstrated binding interactions of polymyxin B (PMB), PMB formulations in the mole ratios of 1:2 and 1:3 of PMB:sodium deoxycholate sulfate (SDCS) and a commercial PMB formulation (CPMB) with lipopolysaccharides (LPS). The 1:2 PMB formulation (78.5-135.2 nM) exhibited a lower number of binding sites to the tested LPS compared to CPMB (112.6-140.9 nM) whereas 1:3 PMB formulation exhibited a higher number of binding sites (143.9-340.2 nM). Similarly, in the presence of LPS, the 1:2 PMB formulation (163.8-221.4 nm) exhibited smaller particle sizes compared to PMB, CPMB and 1:3 PMB formulation (248.8-603.5 nm). Molecular docking simulation suggested that the fatty acyl tails of LPS wrap together to produce a pseudo-globular structure of PMB-LPS complex, and among those 1:2 PMB formulation formed a more stable structure. The primary forces behind this complex are hydrogen bonds and salt bridges among the LPS, PMB, and SDCS. This study revealed that the PMB, CPMB, and PMB formulations inserted into the LPS micelles to disrupt the LPS membrane, whereas the SDCS may induce aggregation. The 1:2 PMB formulation also had higher bacterial uptake than other PMB formulations. The 1:2 PMB formulation neutralized the LPS micelles and was effective against Escherichia coli and Pseudomonas aeruginosa.

Study the antibacterial mechanism of cinnamaldehyde against drug-resistant Aeromonas hydrophila in vitro

Aeromonas hydrophila, a highly infectious pathogen, causes several infections in aquatic animals and huge economic losses. Antibiotics are often used to treat A. hydrophila infections. However, overuse and irrational usage of antibiotics has led to severe antibiotic residues and emergence of resistance. There is therefore an urgent need for a new sustainable drug to control bacterial infection. Cinnamaldehyde, a plant-derived ingredient, has been found to have good antibacterial activity against A. hydrophila in vitro, but its mechanism of action remains unknown. In this study, we investigated the mechanism of cinnamaldehyde against A. hydrophila by evaluating the effects of cinnamaldehyde on A. hydrophila cell growth, cell morphology, electrical conductivity, lactate dehydrogenase (LDH), protein metabolism and DNA. The minimal inhibitory concentration and minimum bactericidal concentration of cinnamaldehyde were 256 and 512 μg/mL, respectively. Microscopy results showed disrupted cell wall and membrane, loss of cytoplasm, interior cavitation and unusual binary fission in the cinnamaldehyde-treated group. Electrical conductivity, LDH activity content and DNA extravasation in cinnamaldehyde-treated A. hydrophila increased by 7.14%, 16.75% and 20.29 μg/mL, respectively. Furthermore, nucleic acid fluorescence intensity and density decreased over time in the cinnamaldehyde-treated group. Taken together, these findings suggest that cinnamaldehyde can inhibit the growth of A. hydrophila by disrupting cell membranes and affecting protein metabolism.

Conversion of Broad-Spectrum Antimicrobial Peptides into Species-Specific Antimicrobials Capable of Precisely Targeting Pathogenic Bacteria

Currently, the majority of antibiotics in clinical use have broad activity spectra, killing pathogenic and beneficial microorganisms indiscriminately. The disruption of the ecological balance of normal flora often results in secondary infections or other antibiotic-associated complications. Therefore, targeted antimicrobial therapies capable of specifically eliminating pathogenic bacteria while retaining the protective benefits of a normal microflora would be advantageous. In this study, we successfully constructed a series of Enterococcus faecalis-targeted antimicrobial peptides from wide-spectrum antimicrobial peptide precursors. These peptides are designed based on fusion of the species-specific peptide pheromone cCF10 and modification of the active region of the antimicrobial peptide. The results showed that cCF10-C4 possessed specific antimicrobial activity against E. faecalis and was not active against other types of bacteria tested. The specificity of this hybrid peptide was shown by the absence of antimicrobial effects in the pheromone-substituted derivative. Further studies indicated that cCF10-C4 and its parent peptide C4 exert their activities by damaging cytoplasmic membrane integrity. The present study reveals the application potential of these molecules as “probiotic” antimicrobials for the control of specific bacterial infections, and it also helps to elucidate the design and construction of species-specific antimicrobials with precise targeting specificity.

Characterization of an antibacterial dodecapeptide from pig as a potential food preservative and its antibacterial mechanism

A membrane-active dodecapeptide was generated from porcine antimicrobial peptide with promising antimicrobial activity for application in the food industry as a potential bio-preservative to prevent microbial spoilage.

Short, symmetric-helical peptides have narrow-spectrum activity with low resistance potential and high selectivity

Broad-spectrum antibiotics have, until now, been the mainstay of antibiotic therapy. However, the increasing threat of drug-resistant bacteria and the ecological imbalance of normal microbial communities have forced a reconsideration of the best strategies to treat such pathogens. Therefore, antibacterial agents with specific abilities of eliminating pathogens may provide long-term protection. Antimicrobial peptides (AMPs), which can be optimized by modifying their primary sequences, are regarded as potentially valuable in development of pathogen-specific agents. To obtain efficient narrow-spectrum AMPs, database-filtering technology, which filters the most probable amino acid composition, positive charge, sequence length and hydrophobic content of peptides against Gram-negative bacteria, was taken as the first step. Then, the filtered parameters were distributed and modified into an α-helical symmetrical structure by considering the structure-function relationship of synthesized antimicrobial peptides. Finally, short, safe and stable peptides against Escherichia coli, Salmonella pullorum and Pseudomonas aeruginosa were successfully identified. The potential peptides F1 and F4 showed low cell toxicity, low resistance potential and low salt sensitivity. CD spectroscopy of the peptides illustrated that F1 and F4 exhibited a tendency towards an α-helical structure in a membrane-mimetic environment. Indeed, fluorescence spectroscopy and electron microscopy analyses indicated that the shorter potential sequence F4 killed the bacteria by causing physical destruction of the bacterial membrane and cytosol leakage. In the mouse model test, F4 reduced the bacterial load in major organs and the cytokine (TNF-α, IL-6, and IL-1β) levels in serum significantly (P < 0.05). Collectively, this symmetric-helical distribution, dependent on database-filtering parameters, is a promising strategy for designing effective smart AMPs with high cell selectivity, and it also provides new insights into the design and optimization of pathogen-specific biomaterials.

The antibacterial mechanism of oridonin against methicillin-resistant Staphylococcus aureus (MRSA)

Context: Methicillin-resistant Staphylococcus aureus (MRSA) is a very harmful bacterium. Oridonin, a component in Rabdosia rubescens (Hemsl.) Hara (Lamiaceae), is widely used against bacterial infections in China. Objective: We evaluated oridonin effects on MRSA cell membrane and wall, protein metabolism, lactate dehydrogenase (LDH), DNA and microscopic structure. Materials and methods: Broth microdilution and flat colony counting methods were used to measure oridonin minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against USA300 strain. Electrical conductivity and DNA exosmosis were analysed to study oridonin effects (128 μg/mL) on cell membrane and wall for 0, 1, 2, 4 and 6 h. Sodium dodecyl sulphate polyacrylamide gel electrophoresis was used to detect effects on soluble protein synthesis after 6, 10 and 16 h. LDH activity was examined with an enzyme-linked immunosorbent assay. Effects of oridonin on USA300 DNA were investigated with DAPI staining. Morphological changes in MRSA following oridonin treatment were determined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results: Oridonin MIC and MBC values against USA300 were 64 and 512 μg/mL, respectively. The conductivity and DNA exosmosis level of oridonin-treated USA300 improved by 3.20±0.84% and increased by 58.63 ± 1.78 μg/mL, respectively. LDH and soluble protein levels decreased by 30.85±7.69% and 27.51 ± 1.39%, respectively. A decrease in fluorescence intensity was reported with time. Oridonin affected the morphology of USA300. Conclusions: Oridonin antibacterial mechanism was related to changes in cell membrane and cell wall permeability, disturbance in protein and DNA metabolism, and influence on bacterial morphology. Thus, oridonin may help in treating MRSA infection.

Redesigning Arenicin-1, an Antimicrobial Peptide from the Marine Polychaeta Arenicola marina, by Strand Rearrangement or Branching, Substitution of Specific Residues, and Backbone Linearization or Cyclization

Arenicin-1, a β-sheet antimicrobial peptide isolated from the marine polychaeta Arenicola marina coelomocytes, has a potent, broad-spectrum microbicidal activity and also shows significant toxicity towards mammalian cells. Several variants were rationally designed to elucidate the role of structural features such as cyclization, a certain symmetry of the residue arrangement, or the presence of specific residues in the sequence, in its membranolytic activity and the consequent effect on microbicidal efficacy and toxicity. The effect of variations on the structure was probed using molecular dynamics simulations, which indicated a significant stability of the β-hairpin scaffold and showed that modifying residue symmetry and β-strand arrangement affected both the twist and the kink present in the native structure. In vitro assays against a panel of Gram-negative and Gram-positive bacteria, including drug-resistant clinical isolates, showed that inversion of the residue arrangement improved the activity against Gram-negative strains but decreased it towards Gram-positive ones. Variants with increased symmetry were somewhat less active, whereas both backbone-cyclized and linear versions of the peptides, as well as variants with R→K and W→F replacement, showed antimicrobial activity comparable with that of the native peptide. All these variants permeabilized both the outer and the inner membranes of Escherichia coli, suggesting that a membranolytic mechanism of action was maintained. Our results indicate that the arenicin scaffold can support a considerable degree of variation while maintaining useful biological properties and can thus serve as a template for the elaboration of novel anti-infective agents.

Antibacterial Activity and Mechanism of Action of Bovine Lactoferricin Derivatives with Symmetrical Amino Acid Sequences

In recent years, the overuse of antibiotics has become very serious. Many pathogenic bacteria have become resistant to them, with serious potential health consequences. Thus, it is urgent that we develop new antibiotic drugs. Antimicrobial peptides (AMPs) are important endogenous antibacterial molecules that contribute to immunity. Most have spectral antibacterial properties and do not confer drug resistance. In this paper, an 11-residue peptide (LFcinB18⁻28) with a sequence of KCRRWQWRMKK was modified by amino acid substitution to form a symmetrical amino acid sequence. The antibacterial activities and mechanisms of action of engineered peptides including KW-WK (KWRRWQWRRWK), FP-PF (FPRRWQWRRPF), FW-WF (FWRRWQWRRWF), and KK-KK (KKRRWQWRRKK) were investigated. The four engineered peptides could more effectively inhibit bacteria than the original peptide, LFcinB18⁻28. This suggested that a symmetrical amino acid sequence might enhance the antibacterial activity of AMPs. However, only peptides KW-WK, FP-PF, and KK-KK were safe; FW-WF displayed hemolytic activity. The engineered peptides shared cationic and amphipathic characteristics that facilitated interactions with the anionic microbial membranes, leading to disruption of membrane integrity and permeabilizing microbial membranes, resulting in cell death. Therefore, a symmetrical amino acid sequence and related structural parameters offer an alternative approach to the design of AMPs. This will provide a scientific basis for the design and synthesis of new AMPs.

Identification, Recombinant Expression, and Characterization of LHG2, a Novel Antimicrobial Peptide of Lactobacillus casei HZ1

L. casei HZ1 was identified from Chinese traditional fermented milk, and angiotensin converting enzyme inhibitory peptide was separated from its culture in our previous work. Here, LGH2 was a novel AMP, identified from the genome of L. casei HZ1. Altogether, roughly 52.76% of LGH2 was α-helical, with the remainder in β-strand and random coil in 50% TFE solution tested by CD. The peptide was also an amphipathic and cationic molecule, which was composed of 20 amino acid residues. The similarity of the amino acid sequence between LGH2 and Temporin-RN3 was highest. Then, the peptide successfully expressed in E. coli Rossetta (DE3) pLysS using the SUMO fusion expression system and purified by chromatography technologies. The molecular weight of the peptide was 2448 Da determined by MALDI-TOF MS. Antimicrobial tests showed that the peptide has strong activities against G+ bacteria, special for S. aureus (MIC = 4 μM). The toxicity assay showed that the peptide exhibits a low hemolytic activity against sheep red blood cells. The antimicrobial mechanisms of LGH2 against pathogens were further investigated by dye leakage, CLSM, SEM, and FCM assays. We found that LGH2 can bind to the cell membrane, and destroy its integrity. These significant results indicate that LGH2 has great potential to treat the infections caused by pathogenic bacteria such as S. aureus, and it provides a new template to improve antimicrobial peptides targeting antibiotic-resistant pathogenic bacteria.

Broad-Spectrum Antimicrobial Activity and Low Cytotoxicity against Human Cells of a Peptide Derived from Bovine αS1-Casein

The primary objective of this study was to improve our understanding of the antimicrobial mechanism of protein-derived peptides and to provide evidence for protein-derived peptides as food bio-preservatives by examining the antimicrobial activities, low cytotoxicity, stabilities, and mechanism of Cp1 (LRLKKYKVPQL). In this study, the protein-derived peptide Cp1 was synthesized from bovine αS1-casein, and its potential use as a food biopreservative was indicated by the higher cell selectivity shown by 11-residue peptide towards bacterial cells than human RBCs. It also showed broad-spectrum antimicrobial activity, with minimum inhibitory concentrations (MICs) of 64⁻640 μM against both gram-positive and gram-negative bacteria. The peptide had low hemolytic activity (23.54%, 512 μM) as well as cytotoxicity. The results of fluorescence spectroscopy, flow cytometry, and electron microscopy experiments indicated that Cp1 exerted its activity by permeabilizing the microbial membrane and destroying cell membrane integrity. We found that Cp1 had broad-spectrum antimicrobial activity, low hemolytic activity, and cytotoxicity. The results also revealed that Cp1 could cause cell death by permeabilizing the cell membrane and disrupting membrane integrity. Overall, the findings presented in this study improve our understanding of the antimicrobial potency of Cp1 and provided evidence of the antimicrobial mechanisms of Cp1. The peptide Cp1 could have potential applications as a food biopreservative.

Central β-turn increases the cell selectivity of imperfectly amphipathic α-helical peptides

Although membrane lytic antimicrobial peptides (AMPs) show enormous potential for addressing mounting global antibiotic resistance, therapeutic applications are hindered by their weak antimicrobial activity, high toxicity, salt sensitivity and poor understanding of structure-activity relationships. To investigate the effects of different parameters on the biological activities of AMPs, a rational approach was adopted to design a series of short cationic α-helical peptides comprising the Ac-WxKyWxzzyKxWyK-NH₂ sequence, where x: cationic residues (Arg or Lys), y: hydrophobic residues (Ala, Val, Ile or Leu), and zz: β-turn (rigid D-Pro-Gly turn or flexible Gly-Gly turn). The peptides showed a more helical structure as the concentration of membrane-mimetic solution increased. The peptide RL with a central D-Pro-Gly turn (x: Arg, y: Lys, zz = D-Pro-Gly) exhibited broad-spectrum antimicrobial activities (2-8 μM) against ten types of clinically relevant microorganisms and even maintained its activity in the presence of physiological salts and showed excellent selectivity toward bacterial cells over human red blood cells and mammalian cells. However, the toxicity was increased after the removal of D-Pro-Gly turn. Additionally, the bactericidal activity was reduced when the D-Pro-Gly turn was replaced by a Gly-Gly turn. Fluorescence spectroscopy and electron microscopy analyses indicated that RL and its derivatives killed microbial cells by permeabilizing the cell membrane and damaging membrane integrity. In conclusion, these findings clearly generalized a potential method for designing or optimizing AMPs, and the peptide RL is a promising therapeutic candidate to combat antibiotic resistance.

STATEMENT OF SIGNIFICANCE

We proposed a rational approach to design imperfectly amphiphilic peptides and identified RL (Ac-WRKLWRpGLKRWLK-NH₂) in particular that shows strong antibacterial properties, low toxicity and high salt resistance. The β-turn unit inserted into the central position of cationic α-helical peptides, especially the D-Pro-Gly turn, significantly increase the cell selectivity of the synthetic amphiphiles. The findings demonstrate a potential method for designing and/or optimizing AMPs, which would facilitate the development of strategies to design peptide-based antimicrobial biomaterials in a variety of biotechnological and clinical applications.

Antibacterial activities and molecular mechanism of amino-terminal fragments from pig nematode antimicrobial peptide CP-1

High manufacturing costs and weak cell selectivity have limited the clinical application of naturally occurring peptides when faced with an outbreak of drug resistance. To overcome these limitations, a set of antimicrobial peptides was synthesized with the general sequence of (WL)n, where n = 1, 2, 3, and WL was truncated from the N-terminus of Cecropin P1 without initial serine residues. The antimicrobial peptide WL3 exhibited stronger antimicrobial activity against both Gram-negative and Gram-positive microbes than the parental peptide CP-1. WL3 showed no hemolysis even at the highest test concentrations compared to the parental peptide CP-1. The condition sensitivity assays (salts, serum, and trypsin) demonstrated that WL3 had high stability in vitro. Fluorescence spectroscopy and electron microscopy indicated that WL3 killed microbes by means of penetrating the membrane and causing cell lysis. In a mouse model, WL3 was able to significantly reduce the bacteria load in major organs and cytokines (TNF-α, IL-6, and IL-1β) levels in serum. In summary, these findings suggest that WL3, which was modified from a natural antimicrobial peptide, has enormous potential for application as a novel antibacterial agent.

Membrane-Active Amphipathic Peptide WRL3 with in Vitro Antibiofilm Capability and in Vivo Efficacy in Treating Methicillin-Resistant Staphylococcus aureus Burn Wound Infections

Methicillin-resistant Staphylococcus aureus (MRSA) has become increasingly prevalent in hospitals, clinics, and the community. MRSA can cause significant and even lethal infections, especially in skin burn wounds. The currently available topical agents have largely failed to eliminate MRSA infections due to resistance. Therefore, there is an urgent need for new and effective approaches for treating MRSA. Here, we show that a novel engineered amphipathic peptide, WRL3 (WLRAFRRLVRRLARGLRR-NH2), exhibits potent antimicrobial activity against MRSA, even in the presence of various salts or serum. The cell selectivity of WRL3 was demonstrated by its ability to specifically eliminate MRSA cells over host cells in a coculture model. Additionally, WRL3 showed a synergistic effect against MRSA when combined with ceftriaxone and effectively inhibited sessile biofilm bacteria growth leading to a reduction in biomass. Fluorescent measurements and microscopic observations of live bacterial cells and artificial membranes revealed that WRL3 exerted its bactericidal activity possibly by destroying the bacterial membrane. In vivo studies indicate that WRL3 is able to control proliferation of MRSA in wound tissue and reduce bioburden and provides a more favorable environment for wound healing. Collectively, our data suggest that WRL3 has enormous potential as a novel antimicrobial agent for the treatment of clinical MRSA infections of skin burn wounds.

Novel Design of Heptad Amphiphiles To Enhance Cell Selectivity, Salt Resistance, Antibiofilm Properties and Their Membrane-Disruptive Mechanism

Coiled-coil, a basic folding pattern of native proteins, was previously demonstrated to be associated with the specific spatial recognition, association, and dissociation of proteins and can be used to perfect engineering peptide model. Thus, in this study, a series of amphiphiles composed of heptads repeats with coiled-coil structures was constructed, and the designed peptides exhibited a broad spectrum of antimicrobial activities. Circular dichroism and biological assays showed that the heptad repeats and length of the linker between the heptads largely influenced the amphiphile's helical propensity and cell selectivity. The engineered amphiphiles were also found to efficiently reduce sessile P. aeruginosa biofilm biomass, neutralize endotoxins, inhibit the inflammatory response, and remain active under physiological salt concentrations. In summary, these findings are helpful for short AMP design with a highly therapeutic index to treat bacteria-induced infection.

Capping β-hairpin with N-terminal d-amino acid stabilizes peptide scaffold

Various strategies exist to stabilize de novo designed synthetic peptide β-hairpins or β-sheets structures, especially at the non-hydrogen bonding position. However, strategies to stabilize strand termini, which are affected by fraying, are highly limited. Here, by substituting N-terminal aliphatic amino acid with its mirror image counterpart, we achieve a significant increase in scaffold stabilization, resulting from the formation of a terminal aliphatic-aromatic hydrophobic CH…pi cluster. Our extensive solution NMR studies support the incorporation of an N-terminal d-aliphatic amino acid in the design of short β-hairpins, while successfully retaining the overall structural scaffold. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 260-266, 2016.

Antimicrobial Peptides: An Introduction

The "golden era" of antibiotic discovery has long passed, but the need for new antibiotics has never been greater due to the emerging threat of antibiotic resistance. This urgency to develop new antibiotics has motivated researchers to find new methods to combat pathogenic microorganisms resulting in a surge of research focused around antimicrobial peptides (AMPs; also termed host defense peptides) and their potential as therapeutics. During the past few decades, more than 2000 AMPs have been identified from a diverse range of organisms (animals, fungi, plants, and bacteria). While these AMPs share a number of common features and a limited number of structural motifs; their sequences, activities, and targets differ considerably. In addition to their antimicrobial effects, AMPs can also exhibit immunomodulatory, anti-biofilm, and anticancer activities. These diverse functions have spurred tremendous interest in research aimed at understanding the activity of AMPs, and various protocols have been described to assess different aspects of AMP function including screening and evaluating the activities of natural and synthetic AMPs, measuring interactions with membranes, optimizing peptide function, and scaling up peptide production. Here, we provide a general overview of AMPs and introduce some of the methodologies that have been used to advance AMP research.

The C-terminal sequences of porcine thrombin are active as antimicrobial peptides

The C-terminal sequences of porcine thrombin encode a series of peptides with the characteristics of net positive charge and hydrophobicity, suggesting antimicrobial potential. In this study, we synthesized truncated C-terminal peptides to explore their antimicrobial potency and structure-activity relationship. The results showed that some peptides exerted antimicrobial activity against Gram-positive and Gram-negative bacteria, with selectivity for microbial membranes. The antimicrobial potency of the peptides increased with the extension of chain length. Considering toxicity to red blood cells, the 21-mer peptide T-6 displayed the highest therapeutic index of 43.4, suggesting its higher cell selectivity. Typical α-helical conformations were observed upon binding to a bacteria-mimicking environment. The derivatives tended to interact preferentially with negatively charged vesicles compared to zwitterionic vesicles. Flow cytometry and electron microscopy revealed that the peptides targeted bacterial cell membranes and disrupted cytoplasmic membrane integrity, thereby causing the release of cellular contents leading to cell death. Peptide-membrane interaction experiments provided evidence that the peptides killed bacteria via a membrane-mediating mechanism. In summary, the C-terminal sequence of porcine thrombin has antimicrobial functions.

Expression, purification and characterization of the recombinant cysteine-rich antimicrobial peptide snakin-1 in Pichia pastoris

Snakin-1 (SN-1) is a small cysteine-rich plant antimicrobial peptide with broad spectrum antimicrobial activity which was isolated from potato (Solanum tuberosum). Here, we carried out the expression of a recombinant SN-1 in the methylotrophic yeast Pichia pastoris, along with its purification and characterization. A DNA fragment encoding the mature SN-1 was cloned into pPIC9 vector and introduced into P. pastoris. A large amount of pure recombinant SN-1 (approximately 40 mg/1L culture) was obtained from a fed-batch fermentation culture after purification with a cation exchange column followed by RP-HPLC. The identity of the recombinant SN-1 was verified by MALDI-TOF MS, CD and (1)H NMR experiments. All these data strongly indicated that the recombinant SN-1 peptide had a folding with six disulfide bonds that was identical to the native SN-1. Our findings showed that SN-1 exhibited strong antimicrobial activity against test microorganisms and produced very weak hemolysis of mammalian erythrocytes. The mechanism of its antimicrobial action against Escherichia coli was investigated by both outer membrane permeability assay and cytoplasmic membrane depolarization assay. These assays demonstrated that SN-1 is a membrane-active antimicrobial peptide which can disrupt both outer and cytoplasmic membrane integrity. This is the first report on the recombinant expression and purification of a fully active SN-1 in P. pastoris.