Antimicrobial properties and membrane-active mechanism of a potential α-helical antimicrobial derived from cathelicidin PMAP-36

Antimicrobial Peptide JH-3 Effectively Kills Salmonella enterica Serovar Typhimurium Strain CVCC541 and Reduces Its Pathogenicity in Mice

Salmonella is an important zoonotic pathogen and is a major cause of gastrointestinal diseases worldwide. The current serious problem of antibiotic abuse has prompted the search for new substitutes for antibiotics. JH-3 is a small antimicrobial peptide with broad-spectrum bactericidal activity. In this study, we showed that JH-3 has good bactericidal activity towards the clinical isolate Salmonella enterica serovar Typhimurium strain CVCC541. The minimum inhibitory concentration (MIC) of JH-3 against this bacterium was determined to be 100 μg/mL, which could decrease the number of CVCC541 cells by 1000-fold in vitro within 5 h. The transmission electron microscopy (TEM) results showed that JH-3 can damage the cell wall and membrane of CVCC541, leading to the leakage of cell contents and subsequent cell death. To measure the bactericidal activity of CVCC541-infected mice were treated intraperitoneally 40 or 10 mg/kg JH-3 at 2 h or 3 days postinfection. Our results showed that treatment with 40 mg/kg JH-3 at 2 h postinfection had the best therapeutic effect and could significantly protect mice from a lethal dose of CVCC541. Furthermore, the clinical symptoms, bacterial burden in blood and organs, and intestinal pathological changes were all decreased and were close to normal. This study examined the therapeutic effect of the antimicrobial peptide JH-3 against S. enterica CVCC541 infection for the first time and determined the therapeutic effect of different JH-3 doses and treatment times, laying the foundation for studies of new antimicrobial agents.

Protective efficacy of the recombinant lysozyme-PMAP36 fusion protein-inactivated Salmonella Typhimurium vaccine candidate via oral immunization in a murine model

The aim of this study was to evaluate protective efficacy of S. Typhimurium ghost vaccine candidate lysed by the recombinant lysozyme-PMAP36 fusion protein via oral immunization in a murine model. Sixty BALB/c mice were equally divided into 4 groups. Group A mice were inoculated with 20 μL of sterile phosphate-buffered saline (PBS). Groups B-D mice were immunized with approximately 1 × 10⁷, 1 × 10⁸, and 1 × 10⁹ cells of the vaccine candidate, respectively, in 20 μL of PBS. Salmonella-outermembrane-proteins-specific serum IgG was considerably higher in groups B-D than in group A. The interleukin-10 and interferon-γ levels in groups B-D were significantly higher than in group A. Following challenge with wild-type S. Typhimurium, all immunized groups showed a significant level of protection compared with group A. The highest protection was shown in group D. Overall, these results show that oral immunization with the candidate vaccine can effectively protect mice from S. Typhimurium infection.

Synthesis and Antibacterial Analysis of Analogues of the Marine Alkaloid Pseudoceratidine

In an effort to gain more understanding on the structure activity relationship of pseudoceratidine 1, a di-bromo pyrrole spermidine alkaloid derived from the marine sponge Pseudoceratina purpurea that has been shown to exhibit potent biofouling, anti-fungal, antibacterial, and anti-malarial activities, a large series of 65 compounds that incorporated several aspects of structural variation has been synthesised through an efficient, divergent method that allowed for a number of analogues to be generated from common precursors. Subsequently, all analogues were assessed for their antibacterial activity against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. Overall, several compounds exhibited comparable or better activity than that of pseudoceratidine 1, and it was found that this class of compounds is generally more effective against Gram-positive than Gram-negative bacteria. Furthermore, altering several structural features allowed for the establishment of a comprehensive structure activity relationship (SAR), where it was concluded that several structural features are critical for potent anti-bacterial activity, including di-halogenation (preferable bromine, but chlorine is also effective) on the pyrrole ring, two pyrrolic units in the structure and with one or more secondary amines in the chain adjoining these units, with longer chains giving rise to better activities.

A novel, rationally designed, hybrid antimicrobial peptide, inspired by cathelicidin and aurein, exhibits membrane-active mechanisms against Pseudomonas aeruginosa

Antimicrobial peptides (AMPs) are promising alternatives to classical antibiotics for the treatment of drug-resistant infections. Due to their versatility and unlimited sequence space, AMPs can be rationally designed by modulating physicochemical determinants to favor desired biological parameters and turned into novel therapeutics. In this study, we utilized key structural and physicochemical parameters, in combination with rational engineering, to design novel short α-helical hybrid peptides inspired by the well-known natural peptides, cathelicidin and aurein. By comparing homologous sequences and abstracting the conserved residue type, sequence templates of cathelicidin (P0) and aurein (A0) were obtained. Two peptide derivatives, P7 and A3, were generated by amino acid substitution based on their residue composition and distribution. In order to enhance antimicrobial activity, a hybrid analog of P7A3 was designed. The results demonstrated that P7A3 had higher antibacterial activity than the parental peptides with unexpectedly high hemolytic activity. Strikingly, C-terminal truncation of hybrid peptides containing only the α-helical segment (PA-18) and shorter derivatives confer potent antimicrobial activity with reduced hemolytic activity in a length‐dependent manner. Among all, PA-13, showed remarkable broad-spectrum antibacterial activity, especially against Pseudomonas aeruginosa with no toxicity. PA-13 maintained antimicrobial activity in the presence of physiological salts and displayed rapid binding and penetration activity which resulted in membrane depolarization and permeabilization. Moreover, PA-13 showed an anti-inflammatory response via lipopolysaccharide (LPS) neutralization with dose-dependent, inhibiting, LPS-mediated Toll-like receptor activation. This study revealed the therapeutic potency of a novel hybrid peptide, and supports the use of rational design in development of new antibacterial agents.

Synergistic action of substituted indole derivatives and clinically used antibiotics against drug-resistant bacteria

Aim: The current report describes the discovery of indole derivatives that synergize with standard antibiotics. Materials & methods: The antibacterial activities were determined using an optimized time-kill method, while viability of mammalian cells was assessed using the (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Results: The synergy is observed with methicillin- and vancomycin-resistant Staphylococcus aureus bacterial strains, against which the standard antibiotics show no activities of their own. Our indole derivatives in combination with antibiotics lack toxicity toward mammalian cells, do not promote the evolution of resistance of S. aureus in comparison to clinically established antibiotics, and likely work by permeabilizing bacterial cell membranes. Conclusion: The above-mentioned findings demonstrate the potential clinical applications of our indole derivatives.

Hybridization with Insect Cecropin A (1-8) Improve the Stability and Selectivity of Naturally Occurring Peptides

Antimicrobial peptides (AMPs) offer great hope and a promising opportunity to overcome the rapid development of drug-resistant pathogenic microbes. However, AMPs often lack the stability required for a successful systemic drug. Hybridizing different AMPs is a simple and effective strategy to obtain novel peptides. N-terminal fragment of cecropin A (CA (1-8)) is often used to hybridize with other AMPs to reduce cytotoxicity. However, hybridizing with CA (1-8) in improving the stability of AMPs is not clear. Therefore, a series of peptides were designed by combining with CA (1–8) and their antibacterial activity and stability in the presence of salts and human serum were evaluated. The resultant α-helical hybrid peptide CA-FO composed of CA (1-8) and the most potent region of Fowlicidin-2 (FO (1–15)) exhibited excellent antibacterial activity (2-8 μM) and cell selectivity toward bacterial over mammalian cells. Moreover, CA-FO still retained vigorous antimicrobial activity in the presence of human serum and salts at physiological concentrations. CA-FO exhibited effective antibacterial activity by increasing membrane permeability and damaging membrane integrity. In conclusion, these results indicated the success of hybridization in designing and optimizing AMPs with improved stability and selectivity and the peptide CA-FO can be further evaluated as peptide-therapy to treat bacterial infections.

The Antifungal Peptide MCh-AMP1 Derived From Matricaria chamomilla Inhibits Candida albicans Growth via Inducing ROS Generation and Altering Fungal Cell Membrane Permeability

The rise of antifungal drug resistance in Candida species responsible for life threatening candidiasis is considered as an increasing challenge for the public health. MCh-AMP1 has previously been reported as a natural peptide from Matricaria chamomilla L. flowers with broad-spectrum antifungal activity against human pathogenic molds and yeasts. In the current study, the mode of action of synthetic MCh-AMP1 was investigated against Candida albicans, the major etiologic agent of life-threatening nosocomial candidiasis at cellular and molecular levels. Candida albicans ATCC 10231 was cultured in presence of various concentrations of MCh-AMP1 (16-64 μg/mL) and its mode of action was investigated using plasma membrane permeabilization assays, reactive oxygen species (ROS) induction, potassium ion leakage and ultrastructural analyses by electron microscopy. MCh-AMP1 showed fungicidal activity against Candida albicans at the concentrations of 32 and 64 μg/mL. The peptide increased fungal cell membrane permeability as evidenced by elevating of PI uptake and induced potassium leakage from the yeast cells. ROS production was induced by the peptide inside the fungal cells to a maximum of 64.8% at the concentration of 64 μg/mL. Scanning electron microscopy observations showed cell deformation as shrinkage and folding of treated yeast cells. Transmission electron microscopy showed detachment of plasma membrane from the cell wall, cell depletion and massive destruction of intracellular organelles and cell membrane of the fungal cells. Our results demonstrated that MCh-AMP1 caused Candida albicans cell death via increasing cell membrane permeability and inducing ROS production. Therefore, MCh-AMP1 could be considered as a promising therapeutic agent to combat Candida albicans infections.

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.

Fluorescence Imaging of Bacterial Killing by Antimicrobial Peptide Dendrimer G3KL

We recently discovered that peptide dendrimers such as G3KL ((KL)₈(KKL)₄(KKL)₂KKL, K = branching l-lysine) exert strong activity against Gram-negative bacteria including Pseudomonas aeruginosa, Acinetobacter baumannii, and Escherichia coli. Herein, we report a detailed mechanistic study using fluorescence labeled analogs bearing fluorescein (G3KL-Fluo) or dansyl (G3KL-Dansyl), which show a similar bioactivity profile as G3KL. Imaging bacterial killing by super-resolution stimulated emission depletion (STED) microscopy, time-lapse imaging, and transmission electron microscopy (TEM) reveals that the dendrimer localizes at the bacterial membrane, induces membrane depolarization and permeabilization, and destroys the outer leaflet and the inner membrane. G3KL accumulates in bacteria against which it is active; however, it only weakly penetrates into eukaryotic cells without inducing significant toxicity. G3KL furthermore binds to lipopolysaccharide (LPS) and inhibits the LPS induced release of TNF-α by macrophages, similarly to polymyxin B. Taken together, these experiments show that G3KL behaves as a potent membrane disruptive antimicrobial peptide.

Design of therapeutically improved analogue of the antimicrobial peptide, indolicidin, using a glycosylation strategy

Indolicidin is a member of cathelicidin family which displays broad spectrum antimicrobial activity. Severe toxicity and aggregation propensity associated with indolicidin pose a huge limitation to its probable therapeutic application. We are reporting the use of glycosylation strategy to design an analogue of indolicidin and subsequently explore structural and functional effects of sugar on it. Our study led to the design of a potent antibacterial glycosylated peptide, [βGlc-T9,K7]indolicidin, which showed decreased toxicity against erythrocytes and macrophage cells and thus a higher therapeutic selectivity. The incorporation of sugar also increased the solubility of the peptide. The mode of bacterial killing, functional stability, LPS binding, and cytokine inhibitory potential of the peptide, however, seemed unaffected upon glycosylation. Absence of significant changes in structure upon glycosylation accounts for the possibly retained functions and mode of action of the peptide. Our report thus presents the designing of an indolicidin analogue with improved therapeutic potential by substituting aromatic amino acid with glycosylated amino acid as a promising strategy for the first time.

Self-Assembled Peptide Nanofibers Display Natural Antimicrobial Peptides to Selectively Kill Bacteria without Compromising Cytocompatibility

One of the major hurdles in the development of antimicrobial peptide (AMP)-based materials is their poor capacity in selectively killing bacteria without harming nearby mammalian cells. Namely, they are antimicrobial but cytotoxic. Current methods of nanoparticle-encapsulated AMPs to target bacteria selectively still have not yet overcome this hurdle. Here, we demonstrate a simple yet effective method to address this daunting challenge by associating a natural AMP with a β-sheet-forming synthetic peptide. The integrated peptides self-assembled to form a supramolecular nanofiber, resulting in the presentation of the AMP at the nanofiber-solvent interface in a precisely controlled manner. Using melittin as a model natural AMP, we found that the conformation of melittin changed dramatically when presented on the nanofiber surface, which, in turn, modulated the induced membrane permeability of the bacterial and mammalian cell membranes. Specifically, the presentation of melittin on the nanofiber restricted its hydrophobic residues, leading to a reduction of the hydrophobic interaction with lipids in the cell membranes. Compellingly, the reduced hydrophobic interaction led to a considerable decrease of melittin's induced permeability of the mammalian cell membrane than that of the bacterial cell membrane. As a result, the AMP-displaying nanofiber preferentially permeabilized and disrupted the membrane of the bacteria without compromising the mammalian cells. Such improved membrane selectivity and cytocompatibility were confirmed in a cell-based membrane localization and live-dead assay. Our new strategy holds great promise for fabricating cytocompatible antimicrobial assemblies that offer safer and more effective administration of therapeutic AMPs. These assemblies, with intrinsic antimicrobial activity and cytocompatibility, can also serve as building blocks for the construction of higher-ordered scaffolds for other biomedical applications such as tissue engineering and regenerative medicine.

Design of Trp-Rich Dodecapeptides with Broad-Spectrum Antimicrobial Potency and Membrane-Disruptive Mechanism

There has recently been great concern regarding antibiotics due to potential drug resistance and the impact of antibiotics on the environment. Antimicrobial peptides are believed to have potential as novel antimicrobial agents to address the problems of antibiotics. Herein, we report a set of Trp-rich dodecapeptides derived from PMAP-36 that are based on the peptide folding principle and the amino acid characteristics. An effective peptide design template, (WXYX)₃, where X represents Arg or Lys and Y represents hydrophobic or neutral amino acid, was summarized with the distribution of Trp at H-bond formation sites along the α-helical structure. The template peptide 6 (3W-2), with low amphipathicity, displayed strong antimicrobial activity against laboratory strains and clinical isolates while showing no cytotoxicity. Furthermore, 6 was able to suppress the emergence of antimicrobial resistance. Membrane permeabilization assays and microscope observations revealed the potent membrane-disruptive mechanism of 6. Overall, this study diminishes the randomness in peptide design and provides a strategy for generating effective antibiotic alternatives to overcome antibiotic resistance.

Comparative mode of action of the antimicrobial peptide melimine and its derivative Mel4 against Pseudomonas aeruginosa

Melimine and Mel4 are chimeric cationic peptides with broad-spectrum antimicrobial activity. They have been shown to be highly biocompatible in animal models and human clinical trials. The current study examined the mechanism of action of these two antimicrobial peptides against P. aeruginosa. The effect of the peptides of endotoxin neutralization, and their interactions with cytoplasmic membranes using DiSC(3)-5 and Sytox green, Syto-9 and PI dyes were analysed. Release of ATP and DNA/RNA were determined using ATP luminescence and increase in OD_260 nm. The bacteriolytic ability of the peptides was determined by measuring decreases in OD_620 nm. Both the peptides neutralized LPS suggesting their interaction with lipid A. Cytoplasmic membrane was disrupted within 30 seconds, which correlated with reductions in cellular viability. At 2 minutes melimine or Mel4, released 75% and 36% cellular ATP respectively (P < 0.001). Membrane permeabilization started 5 minutes with simultaneous release of DNA/RNA. Flow cytometry demonstrated 52% and 18% bacteria were stained with PI after 30 minutes. Overall, melimine showed higher capacity for membrane disruption compared to Mel4 (P < 0.001). The findings of this study have been summarized as a timeline of bactericidal activity, suggesting that the peptides permeabilized P. aeruginosa within 5 minutes, started lysis within 2 hours of exposure.

Hollow selenium nanoparticles from potato extract and investigation of its biological properties and developmental toxicity in zebrafish embryos

A facile synthesis of hollow selenium nanoparticles (hSeNPs) was prepared using potato starch as a reducing and capping agent. The morphological and structural characters of the hSeNPs were characterised by ultraviolet-visible spectroscopy (UV), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) and zeta potential analyser. The optical characteristics of hSeNPs were confirmed by UV. The presence of various functional groups in the hSeNPs suspension was confirmed by FTIR. The SEM results suggested that the synthesised hSeNPs were uniformly distributed and circular in shape with a hollow. The average size of the hSeNPs was found to be around 115 nm. The EDX analysis also confirmed the presence of hSeNPs in the sample. The zebrafish embryos were treated with hSeNPs of various concentrations ranging from 10 to 50 µg/ml. Abnormalities such as improper heartbeat, embryo sac oedema, ocular oedema and head oedema were noted at higher concentrations (30-50 µg/ml). A concentration-dependent antioxidant activity of hSeNPs was observed. The hSeNPs showed good antibacterial activity against gram-positive Bacillus subtilis and gram-negative Escherichia coli. The results of this study indicate that potato extract reduces the toxicity of hSeNPs and lower concentrations of hSeNPs could be used for various biomedical applications in near future.

Rational Design of Short Peptide Variants by Using Kunitzin-RE, an Amphibian-Derived Bioactivity Peptide, for Acquired Potent Broad-Spectrum Antimicrobial and Improved Therapeutic Potential of Commensalism Coinfection of Pathogens

Commensalism coinfection of pathogens has seriously jeopardized human health. Currently, Kunitzin-RE, as an amphibian-derived bioactivity peptide, is regarded as a potential antimicrobial candidate. However, its antimicrobial properties were unsatisfactory. In this study, a set of shortened variants of Kunitzin-RE was developed by the interception of a peptide fragment and single-site mutation to investigate the effect of chain length, positive charge, hydrophobicity, amphipathicity, and secondary structure on antimicrobial properties. Among them, W8 (AARIILRWRFR) significantly broadened the antimicrobial spectrum and showed the highest antimicrobial activity (GM_all = 2.48 μM) against all the fungi and bacteria tested. Additionally, W8 showed high cell selectivity and salt tolerance in vitro, whereas it showed high effectiveness against mice keratitis cause by infection by C. albicans 2.2086. Additionally, it also had obviously lipopolysaccharide-binding ability and a potent membrane-disruptive mechanism. Overall, these findings contributed to the design of short antimicrobial peptides and to combat the serious threat of commensalism coinfection of pathogens.

Capability of iturin from Bacillus subtilis to inhibit Candida albicans in vitro and in vivo

Candida albicans is a fungal pathogen that is difficult to cure clinically. The current clinic C. albicans-inhibiting drugs are very harmful to humans. This study revealed the potential of iturin fractions from Bacillus subtilis to inhibit C. albicans in free status (MIC = 32 μg/mL) and natural biofilm in vitro. The inhibition mechanism was identified as an apoptosis pathway via the decrease of mitochondrial membrane potential, the increase of the reactive oxygen species (ROS) accumulation, and the induction of nuclear condensation. For in vivo experiments, the C. albicans infection model was constructed via intraperitoneal injection of 1 × 10⁸C. albicans cells into mice. One day after the infection, iturin was used to treat infected mice at different concentrations alone and in combination with amphotericin B (AmB) by intraperitoneal injection. The treatment with AmB alone could cause the death of infected mice, whereas treatment with 15 mg/kg iturin per day alone led to the survival of all infected mice throughout the study. After continuously treated for 6 days, all mice were sacrificed and analyzed. As results, the combination of 15 mg/kg iturin and AmB at a ratio of 2:1 had the most efficient effect to remove the fungal burden in the kidney and cure the infected mice by reversing the symptoms caused by C. albicans infection, such as the loss of body weight, change of immunology cells in blood and cytokines in serum, and damage of organ structure and functions. Overall, iturin had potential in the development of efficient and safe drugs to cure C. albicans infection.

Enhanced Expression and Functional Characterization of the Recombinant Putative Lysozyme-PMAP36 Fusion Protein

The porcine myeloid antimicrobial peptide (PMAP), one of the cathelicidin family members, contains small cationic peptides with amphipathic properties. We used a putative lysozyme originated from the bacteriophage P22 (P22 lysozyme) as a fusion partner, which was connected to the N-terminus of the PMAP36 peptide, to markedly increase the expression levels of recombinant PMAP36. The PMAP36-P22 lysozyme fusion protein with high solubility was produced in Escherichia coli. The final purified yield was approximately 1.8 mg/L. The purified PMAP36-P22 lysozyme fusion protein exhibited antimicrobial activity against both Gram-negative and Gram-positive bacteria (Staphylococcus aureus, Salmonella enterica serovar Typhimurium, Pseudomonas aeruginosa, and Bacillus subtilis). Furthermore, we estimated its hemolytic activity against pig erythrocytes as 6% at the high concentration (128 μM) of the PMAP36-P22 lysozyme fusion protein. Compared with the PMAP36 peptide (12%), our fusion protein exhibited half of the hemolytic activity. Overall, our recombinant PMAP36-P22 lysozyme fusion protein sustained the antimicrobial activity with the lower hemolytic activity associated with the synthetic PMAP36 peptide. This study suggests that the PMAP36-P22 lysozyme fusion system could be a crucial addition to the plethora of novel antimicrobials.

Symmetrical Modification of Minimized Dermaseptins to Extend the Spectrum of Antimicrobials with Endotoxin Neutralization Potency

Antimicrobial peptides (AMPs) have emerged as a promising class of antimicrobial agents that could potentially address the global antibiotic resistance. Generating mirror-like peptides by minimizing dermaseptin family sequences is an effective strategy for designing AMPs. However, the previous research still had some limitations such as lower effectiveness and a narrow spectrum of antibacterial activity. To further expand and hone this strategy, we designed a series of AMPs consisting of the WXMXW-NH₂ motif (X represents V, I, F, and W; M represents KAAAKAAAK). The peptides formed α-helices and displayed broad-spectrum antimicrobial activities against eleven types of clinical bacteria including both Gram-negative and Gram-positive bacteria. The optimized peptide WW exhibited high physical rupture by inducing membrane shrinkage, disruption, and lysis. Moreover, WW effectively neutralized endotoxins and inhibited the inflammatory response while having the highest therapeutic index. In conclusion, these results indicated that the peptide WW has potential as a broad-spectrum antimicrobial agent or preservative for overcoming the risk of multidrug resistance in localized or external therapeutic applications.

Cathelicidins PMAP-36, LL-37 and CATH-2 are similar peptides with different modes of action

Host defense peptides (HDPs) play a pivotal role in innate immunity and have, in addition to antimicrobial activity, also important immunomodulatory functions. Bacteria are less likely to develop resistance against HDPs because these peptides target and kill bacteria in multiple ways, as well as modulate the immune system. Therefore, HDPs, and derivatives thereof, are promising alternatives to traditional antibiotics. Hardly anything is known about the immunomodulatory functions of porcine cathelicidin PMAP-36. In this study, we aimed to determine both antibacterial and immunomodulatory activities of PMAP-36 comparing the properties of PMAP-36 analogs with two well-studied peptides, human LL-37 and chicken CATH-2. Transmission electron microscopy revealed different killing mechanisms of E. coli for PMAP-36, CATH-2 and LL-37. LL-37 binds LPS very weakly in contrast to PMAP-36, but it inhibits LPS activation of macrophages the strongest. The first 11 amino acids of the N-terminal side of PMAP-36 are dispensable for E. coli killing, LPS-neutralization and binding. Deletion of four additional amino acids resulted in a strong decrease in activity. The activity of full length PMAP-36 was not affected by monomerization, whereas the shorter analogs require dimerization for proper immunomodulatory activity but not for their antibacterial activity.

Characterization and Antimicrobial Activity of Amphiphilic Peptide AP3 and Derivative Sequences

The continued emergence of new antibiotic resistant bacterial strains has resulted in great interest in the development of new antimicrobial treatments. Antimicrobial peptides (AMPs) are one of many potential classes of molecules to help meet this emerging need. AMPs are naturally derived sequences, which act as part of the innate immune system of organisms ranging from insects through humans. We investigated the antimicrobial peptide AP3, which is originally isolated from the winter flounder Pleuronectes americanus. This peptide is of specific interest because it does not exhibit the canonical facially amphiphilic orientation of side chains when in a helical orientation. Different analogs of AP3 were synthesized in which length, charge identity, and Trp position were varied to investigate the sequence-structure and activity relationship. We performed biophysical and microbiological characterization using fluorescence spectroscopy, CD spectroscopy, vesicle leakage assays, bacterial membrane permeabilization assays, and minimal inhibitory concentration (MIC) assays. Fluorescence spectroscopy showed that the peptides bind to lipid bilayers to similar extents, while CD spectra show the peptides adopt helical conformations. All five peptides tested in this study exhibited binding to model lipid membranes, while the truncated peptides showed no measurable antimicrobial activity. The most active peptide proved to be the parent peptide AP3 with the highest degree of leakage and bacterial membrane permeabilization. Moreover, it was found that the ability to permeabilize model and bacterial membranes correlated most closely with the ability to predict antimicrobial activity.

Membrane disintegration by the antimicrobial peptide (P)GKY20: lipid segregation and domain formation

Antimicrobial peptides (AMPs) are membrane-active peptides with a broad spectrum of activity against different pathogenic organisms and they represent promising new drugs to overcome the emergence of resistance to antibiotics in bacteria. (P)GKY20 is an antimicrobial peptide with a low hemolytic effect on eukaryotic cells and a strong antimicrobial activity especially against Gram-negative bacteria. However, its mechanism of action is still unknown. Here, we use fluorescence spectroscopy and differential scanning calorimetry combined with atomic force microscopy to characterise the binding of (P)GKY20 with model biomembranes and its effect on the membrane's microstructure and thermotropic properties. We found that (P)GKY20 selectively perturbs the bacterial-like membrane via a carpet-like mechanism employing peptide conformational changes, lipid segregation and domain formation as key steps in promoting membrane disruption. These results shed a first light on the action mechanism of (P)GKY20 and could represent an important contribution to the development of new peptides serving as antimicrobial agents.

Protective efficacy of an inactivated Brucella abortus vaccine candidate lysed by GI24 against brucellosis in Korean black goats

The efficacy of GI24-lysed Brucella abortus cells as a vaccine candidate against brucellosis in goats was evaluated on 2 groups of Korean black goats. Group A goats were immunized subcutaneously (SC) with sterile phosphate-buffered saline, whereas group B goats were immunized SC with approximately 3 × 10⁹ lysed B. abortus cells. Subcutaneous immunization with the lysed cells did not cause any negative impact on the overall clinical status, such as behavior and appetite, throughout the study period. The enzyme-linked immunosorbent assay (ELISA) optical densities values for B. abortus lipopolysaccharide in serum were considerably higher in group B than those in group A. Also, the levels of the cytokines interleukin 4 (IL-4), tumor necrosis factor-alpha (TNF-α), and interferon gamma (IFN-γ) were significantly elevated in group B compared with those in group A. Following intraconjunctival challenge with B. abortus strain 544, the severity of brucellosis in terms of infection index and colonization of B. abortus in tissues was significantly lower in group B than in group A. The present study concluded that 3 of 5 goats immunized with GI24-lysed bacteria were completely protected against challenge. Future investigations are required to improve the protective efficacy offered by lysed B. abortus cells for practical applications in small ruminants.

Antimicrobial cell penetrating peptides with bacterial cell specificity: pharmacophore modelling, quantitative structure activity relationship and molecular dynamics simulation

Current research has shown cell-penetrating peptides and antimicrobial peptides (AMPs) as probable vectors for use in drug delivery and as novel antibiotics. It has been reported that the higher the therapeutic index (TI) the higher would be the bacterial cell penetrating ability. To the best of our knowledge, no in-silico study has been performed to determine bacterial cell specificity of the antimicrobial cell penetrating peptides (aCPP's) based on their TI. The aim of this study was to develop a quantitative structure activity relationship (QSAR) model, which can estimate antimicrobial potential and cell-penetrating ability of aCPPs against S. aureus, to confirm the relationship between the TI and aCPPs and to identify specific descriptors responsible for aCPPs penetrating ability. Molecular dynamics (MD) simulation was also performed to confirm the membrane insertion of the most active aCPPs obtained from the QSAR study. The most appropriate pharmacophore was identified to predict the aCPP's activity. The statistical results confirmed the validity of the model. The QSAR model was successful in identifying the optimal aCPP with high activity prediction and provided insights into the structural requirements to correlate their TI to cell penetrating ability. MD simulation of the best aCPP with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer confirmed its interaction with the membrane and the C-terminal residues of the aCPP played a key role in membrane penetration. The strategy of combining QSAR and molecular dynamics, allowed for optimal estimation of ligand-target interaction and confirmed the importance of Trp and Lys in interacting with the POPC bilayer. Communicated by Ramaswamy H. Sarma.

Antimicrobial peptides: Promising alternatives in the post feeding antibiotic era

Antimicrobial peptides (AMPs), critical components of the innate immune system, are widely distributed throughout the animal and plant kingdoms. They can protect against a broad array of infection-causing agents, such as bacteria, fungi, parasites, viruses, and tumor cells, and also exhibit immunomodulatory activity. AMPs exert antimicrobial activities primarily through mechanisms involving membrane disruption, so they have a lower likelihood of inducing drug resistance. Extensive studies on the structure-activity relationship have revealed that net charge, hydrophobicity, and amphipathicity are the most important physicochemical and structural determinants endowing AMPs with antimicrobial potency and cell selectivity. This review summarizes the recent advances in AMPs development with respect to characteristics, structure-activity relationships, functions, antimicrobial mechanisms, expression regulation, and applications in food, medicine, and animals.

A novel bi-functional chalcone inhibits multi-drug resistant Staphylococcus aureus and potentiates the activity of fluoroquinolones

Staphylococcus aureus is the leading cause of bacteraemia and the dwindling supply of effective antibacterials has exacerbated the problem of managing infections caused by this bacterium. Isoliquiritigenin (ISL) is a plant flavonoid that displays therapeutic potential against S. aureus. The present study identified a novel mannich base derivatives of ISL, IMRG4, active against Vancomycin intermediate S. aureus (VISA). IMRG4 damages the bacterial membranes causing membrane depolarization and permeabilization, as determined by loss of salt tolerance, flow cytometric analysis, propidium idodie and fluorescent microscopy. It reduces the intracellular invasion of HEK-293 cells by S. aureus and decreases the staphylococcal load in different organs of infected mice models. In addition to anti-staphylococcal activity, IMRG4 inhibits the multidrug efflux pump, NorA, which was determined by molecular docking and EtBr efflux assays. In combination, IMRG4 significantly reduces the MIC of norfloxacin for clinical strains of S. aureus including VISA. Development of resistance against IMRG4 alone and in combination with norfloxacin was low and IMRG4 prolongs the post-antibiotic effect of norfloxacin. These virtues combined with the low toxicity of IMRG4, assessed by MTT assay and haemolysis, makes it an ideal candidate to enter drug development pipeline against S. aureus.

Brucella abortus lysed cells using GI24 induce robust immune response and provide effective protection in Beagles

The aim of the present study is to estimate the protective efficacy of Brucella abortus lysed cells by GI24 against brucellosis in Beagles. Group A was subcutaneously (sc) immunized with sterile phosphate-buffered saline, and group B was sc immunized with approximately 3 × 109 of the lysed cells. Brucella-LPS-specific serum IgG titers and IL-4, TNF-α and IFN-γ concentrations were investigated by enzyme linked immunosorbent assay. All dogs were intraconjunctivally challenged with B. abortus strain 544 at 6 weeks post-prime immunization. The serum IgG titers were considerably higher in group B than in group A. The levels of IL-4, TNF-α and IFN-γ in group B than in group A were significantly higher. Following challenge, no challenge strain was observed from all tissues of three dogs of group B. However, challenge strain was detected from spleen, uterus (except one Beagle) and inguinal and retropharyngeal lymph nodes of all group A Beagles. The results of this study demonstrated that sc immunization with the lysed cells induced robust antibody and cell-mediated immune responses in Beagles. The lysed cells also conferred protection against infection with B. abortus. These results suggest that sc immunization with B. abortus lysed cells by GI24 is a good vaccine candidate against brucellosis in dogs.

A Bowman-Birk Inhibitor from the Seeds of Luetzelburgia auriculata Inhibits Staphylococcus aureus Growth by Promoting Severe Cell Membrane Damage

Staphylococcus aureus is a multidrug-resistant bacterium responsible for several cases of hospital-acquired infections, which constitute a global public health problem. The introduction of new healthcare strategies and/or the discovery of molecules capable of inhibiting the growth or killing S. aureus would have a huge impact on the treatment of S. aureus-mediated diseases. Herein, a Bowman-Birk protease inhibitor ( LzaBBI), with strong in vitro antibacterial activity against S. aureus, was purified to homogeneity from Luetzelburgia auriculata seeds. LzaBBI in its native form is a 14.3 kDa protein and has a pI of 4.54, and its NH₂-terminal sequence has high identity with other Bowman-Birk inhibitors. LzaBBI showed a mixed-type inhibitory activity against both trypsin and chymotrypsin, respectively, and it remained stable after both boiling at 98 °C for 120 min and incubation at various pHs. Scanning electron microscopy revealed that LzaBBI disrupted the S. aureus membrane integrity, leading to bacterial death. This study suggests that LzaBBI is a powerful candidate for developing a new antimicrobial to overcome drug resistance toward reducing hospital-acquired infections caused by S. aureus.

Novel 3D Structure Based Model for Activity Prediction and Design of Antimicrobial Peptides

The emergence and worldwide spread of multi-drug resistant bacteria makes an urgent challenge for the development of novel antibacterial agents. A perspective weapon to fight against severe infections caused by drug-resistant microorganisms is antimicrobial peptides (AMPs). AMPs are a diverse class of naturally occurring molecules that are produced as a first line of defense by all multi-cellular organisms. Limited by the number of experimental determinate 3D structure, most of the prediction or classification methods of AMPs were based on 2D descriptors, including sequence, amino acid composition, peptide net charge, hydrophobicity, amphiphilic, etc. Due to the rapid development of structural simulation methods, predicted models of proteins (or peptides) have been successfully applied in structure based drug design, for example as targets of virtual ligand screening. Here, we establish the activity prediction model based on the predicted 3D structure of AMPs molecule. To our knowledge, it is the first report of prediction method based on 3D descriptors of AMPs. Novel AMPs were designed by using the model, and their antibacterial effect was measured by in vitro experiments.

Antimicrobial activity of bovine NK-lysin-derived peptides on Mycoplasma bovis

Antimicrobial peptides (AMPs) are a diverse group of molecules which play an important role in the innate immune response. Bovine NK-lysins, a type of AMP, have been predominantly found in the granules of cytotoxic T-lymphocytes and NK-cells. Bovine NK-lysin-derived peptides demonstrate antimicrobial activity against various bacterial pathogens, including several involved in bovine respiratory disease complex (BRDC) in cattle; however, such studies are yet to be performed with one important contributor to the BRDC, Mycoplasma bovis. Therefore, the goal of this study was to assess the antimicrobial activity of bovine NK-lysin-derived peptides on M. bovis. Thirty-mer synthetic peptides corresponding to the functional region helices 2 and 3 of bovine NK-lysins NK1, NK2A, NK2B, and NK2C were evaluated for killing activity on M. bovis isolates. Among four peptides, NK2A and NK2C showed the highest antimicrobial activity against the M. bovis isolates tested. All four NK-lysin peptides induced rapid plasma membrane depolarization in M. bovis at two concentrations tested. However, based on propidium iodide uptake, only NK2A and NK2C appeared capable of causing structural damage to M. bovis plasma membrane. Confocal microscopy, flow cytometry, and transmission electron microscopy further suggested NK-lysin-induced damage to the plasma membrane. Taken together, the findings in this study suggest that plasma membrane depolarization alone was insufficient to induce lethality, but disruption/permeabilization of the M. bovis plasma membrane was the cause of lethality.

CGA-N12, a peptide derived from chromogranin A, promotes apoptosis of Candida tropicalis by attenuating mitochondrial functions

CGA-N12 (the amino acid sequence from the 65th to the 76th residue of the N-terminus of chromagranin A) is an antifungal peptide derived from human chromogranin A (CGA). In our previous investigation, CGA-N12 was found to have specific anti-candidal activity, though the mechanism of action remained unclear. Here, we investigated the effects of CGA-N12 on mitochondria. We found that CGA-N12 induced an over-generation of intracellular reactive oxygen species and dissipation in mitochondrial membrane potential, in which the former plays key roles in the initiation of apoptosis and the latter is a sign of the cell apoptosis. Accordingly, we assessed the apoptosis features of Candida tropicalis cells after treatment with CGA-N12 and found the following: leakage of cytochrome c and uptake of calcium ions into mitochondria and the cytosol; metacaspase activation; and apoptotic phenotypes, such as chromatin condensation and DNA degradation. In conclusion, CGA-N12 is capable of inducing apoptosis in C. tropicalis cells through mitochondrial dysfunction and metacaspase activation. Antifungal peptide CGA-N12 from human CGA exhibits a novel apoptotic mechanism as an antifungal agent.

Characterization of bactericidal efficiency, cell selectivity, and mechanism of short interspecific hybrid peptides

Facing rising global antibiotics resistance, physical membrane-damaging antimicrobial peptides (AMPs) represent promising antimicrobial agents. Various strategies to design effective hybrid peptides offer many advantages in overcoming the adverse effects of natural AMPs. In this study, hybrid peptides from different species were investigated, and three hybrid antimicrobial peptides, LI, LN, and LC, were designed by combining the typical fragment of human cathelicidin-derived LL37 with either indolicidin, pig nematode cecropin P1 (CP-1) or rat neutrophil peptide-1 (NP-1). In an aqueous solution, all hybrid peptides had an unordered conformation. In simulated membrane conditions, the hybrid peptide LI displayed more β-turn and β-hairpin structures, whereas LN and LC folded into α-helix structures. The three interspecific hybrid peptides LI, LN, and LC exhibited different levels of antimicrobial activity against Gram-positive and Gram-negative bacteria. LI demonstrated the highest antimicrobial activity and cell selectivity. The results of the swimming motility indicated that LI repressed bacterial motility in a concentration-dependent method. Endotoxin binding assay demonstrated that hybrid peptide LI conserved the binding ability to LPS (polyanionic lipopolysaccharides) of its parental peptides. Fluorescence assays, flow cytometry, and SEM further revealed that hybrid peptide LI acted through different bacteriostatic mechanisms than LL37 and indolicidin and that LI killed bacterial cells via membrane damage. In summary, this study demonstrated that hybrid peptide LI produced by interspecific hybrid synthesis possessed strong cell selectivity and is a promising therapeutic candidate for drug-resistant bacteria infection.

Protective efficacy of a Salmonella Typhimurium ghost vaccine candidate constructed with a recombinant lysozyme-PMAP36 fusion protein in a murine model

A Salmonella Typhimurium ghost vaccine was constructed with the use of a recombinant fusion protein consisting of lysozyme and porcine myeloid antimicrobial peptide 36 expressed by the Escherichia coli overexpression system. After confirmation of its effectiveness by transmission electron microscopy the vaccine was evaluated in a murine model. Of the 60 BALB/c mice equally divided into 4 groups, group A mice were intramuscularly inoculated with 100 μL of sterile phosphate-buffered saline, and the mice in groups B, C, and D were intramuscularly inoculated with approximately 1.0 × 10⁴, 1.0 × 10⁵, or 1.0 × 10⁶ cells of the S. Typhimurium ghost vaccine, respectively, in 100-μL amounts. The serum IgG titers against S. Typhimurium outer membrane proteins were significantly higher in groups B to D than in group A, as were the concentrations of interleukin-10 and interferon gamma in supernatants of harvested splenocytes. After challenge with wild-type S. Typhimurium, all the vaccinated groups showed significant protection compared with group A, notably perfect protection in groups C and D. Overall, these results show that intramuscular vaccination with 1.0 × 10⁵ cells of this ghost vaccine candidate provided efficient protection against systemic infection with virulent S. Typhimurium.

A manganese photosensitive tricarbonyl molecule [Mn(CO)3(tpa-κ3N)]Br enhances antibiotic efficacy in a multi-drug-resistant Escherichia coli

Carbon monoxide-releasing molecules (CORMs) are a promising class of new antimicrobials, with multiple modes of action that are distinct from those of standard antibiotics. The relentless increase in antimicrobial resistance, exacerbated by a lack of new antibiotics, necessitates a better understanding of how such novel agents act and might be used synergistically with established antibiotics. This work aimed to understand the mechanism(s) underlying synergy between a manganese-based photoactivated carbon monoxide-releasing molecule (PhotoCORM), [Mn(CO)3(tpa-κ³N)]Br [tpa=tris(2-pyridylmethyl)amine], and various classes of antibiotics in their activities towards Escherichia coli EC958, a multi-drug-resistant uropathogen. The title compound acts synergistically with polymyxins [polymyxin B and colistin (polymyxin E)] by damaging the bacterial cytoplasmic membrane. [Mn(CO)3(tpa-κ³N)]Br also potentiates the action of doxycycline, resulting in reduced expression of tetA, which encodes a tetracycline efflux pump. We show that, like tetracyclines, the breakdown products of [Mn(CO)3(tpa-κ³N)]Br activation chelate iron and trigger an iron starvation response, which we propose to be a further basis for the synergies observed. Conversely, media supplemented with excess iron abrogated the inhibition of growth by doxycycline and the title compound. In conclusion, multiple factors contribute to the ability of this PhotoCORM to increase the efficacy of antibiotics in the polymyxin and tetracycline families. We propose that light-activated carbon monoxide release is not the sole basis of the antimicrobial activities of [Mn(CO)3(tpa-κ³N)]Br.

Plant flavones enhance antimicrobial activity of respiratory epithelial cell secretions against Pseudomonas aeruginosa

Flavones are a class of natural plant secondary metabolites that have anti-inflammatory and anti-bacterial effects. Some flavones also activate the T2R14 bitter taste receptor, which is expressed in motile cilia of the sinonasal epithelium and activates innate immune nitric oxide (NO) production. Flavones may thus be potential therapeutics for respiratory infections. Our objective was to examine the anti-microbial effects of flavones on the common sinonasal pathogens Candida albicans, Staphylococcus aureus, and Pseudomonas aeruginosa, evaluating both planktonic and biofilm growth. Flavones had only very low-level antibacterial activity alone. They did not reduce biofilm formation, but did reduce production of the important P. aeruginosa inflammatory mediator and ciliotoxin pyocyanin. However, flavones exhibited synergy against P. aeruginosa in the presence of antibiotics or recombinant human lysozyme. They also enhanced the efficacy of antimicrobials secreted by cultured and primary human airway cells grown at air-liquid interface. This suggests that flavones may have anti-gram-negative potential as topical therapeutics when combined with antibiotics or in the context of innate antimicrobials secreted by the respiratory or other epithelia. This may have an additive effect when combined with T2R14-activated NO production. Additional studies are necessary to understand which flavone compounds or mixtures are the most efficacious.

Antibacterial Activity of AI-Hemocidin 2, a Novel N-Terminal Peptide of Hemoglobin Purified from Arca inflata

The continued emergence of antibiotic resistant bacteria in recent years is of great concern. The search for new classes of antibacterial agents has expanded to non-traditional sources such as shellfish. An antibacterial subunit of hemoglobin (Hb-I) was purified from the mantle of Arca inflata by phosphate extraction and ion exchange chromatography. A novel antibacterial peptide, AI-hemocidin 2, derived from Hb-I, was discovered using bioinformatics analysis. It displayed antibacterial activity across a broad spectrum of microorganisms, including several Gram-positive and Gram-negative bacteria, with minimal inhibitory concentration (MIC) values ranging from 37.5 to 300 μg/mL, and it exhibited minimal hemolytic or cytotoxic activities. The antibacterial activity of AI-hemocidin 2 was thermostable (25–100 °C) and pH resistant (pH 3–10). The cellular integrity was determined by flow cytometry. AI-hemocidin 2 was capable of permeating the cellular membrane. Changes in the cell morphology were observed with a scanning electron microscope. Circular dichroism spectra suggested that AI-hemocidin 2 formed an α-helix structure in the membrane mimetic environment. The results indicated that the anti-bacterial mechanism for AI-hemocidin 2 occurred through disrupting the cell membrane. AI-hemocidin 2 might be a potential candidate for tackling antibiotic resistant bacteria.