Peptide Dendrimer-Based Antibacterial Agents: Synthesis and Applications

Pathogenic bacteria cause the deaths of millions of people every year. With the development of antibiotics, hundreds and thousands of people's lives have been saved. Nevertheless, bacteria can develop resistance to antibiotics, rendering them insensitive to antibiotics over time. Peptides containing specific amino acids can be used as antibacterial agents; however, they can be easily degraded by proteases in vivo. To address these issues, branched peptide dendrimers are now being considered as good antibacterial agents due to their high efficacy, resistance to protease degradation, and low cytotoxicity. The ease with which peptide dendrimers can be synthesized and modified makes them accessible for use in various biological and nonbiological fields. That is, peptide dendrimers hold a promising future as antibacterial agents with prolonged efficacy without bacterial resistance development. Their in vivo stability and multivalence allow them to effectively target multi-drug-resistant strains and prevent biofilm formation. Thus, it is interesting to have an overview of the development and applications of peptide dendrimers in antibacterial research, including the possibility of employing machine learning approaches for the design of AMPs and dendrimers. This review summarizes the synthesis and applications of peptide dendrimers as antibacterial agents. The challenges and perspectives of using peptide dendrimers as the antibacterial agents are also discussed.

The sequencing of the key genes and end products in the TLR4 signaling pathway from the kidney of Rana dybowskii exposed to Aeromonas hydrophila

Infectious diseases caused by Aeromonas hydrophila (AH) have reduced the populations of Rana dybowskii). However, little is known about the immune response of R. dybowskii against AH infections. The toll-like receptor (TLR) signaling pathway has been identified as a critical component in innate immunity, responsible for identifying pathogen-associated molecular patterns in pathogens. Our study used the next-generation sequencing technique and single-molecule long-read sequencing to determine the structures of transcript isoforms and functions of genes in the kidneys of R. dybowskii, as well as identify and validate the related genes in the TLR4 signaling pathway. In total, 628,774 reads of inserts were identified, including 300,053 full-length non-chimeric reads and 233,592 non-full-length reads. Among the transcriptome sequences, 124 genes were identified as homologs of known genes in the TLR4 pathway especially inflammatory cytokines and receptors. Our findings shed light on the structures and functions of R. dybowskii genes exposed to AH and confirm the presence of both MyD88-dependent and independent pathways in R. dybowskii. Our work reveals how various functional proteins in amphibians at the initial stage of immune response are activated and complete their corresponding functions in a short time.

Inspiration from cruzioseptin-1: membranolytic analogue with improved antibacterial properties

Peptide engineering has gained attraction as a source of new cationicity-enhanced analogues with high potential for the design of next-generation antibiotics. In this context, cruzioseptin-1 (CZS-1), a peptide identified from Cruziohyla calcarifer, is recognized for its antimicrobial potency. However, this amidated-peptide is moderately hemolytic. In order to reduce toxicity and increase antimicrobial potency, 3 peptide analogues based on cruzioseptin-1 were designed and evaluated. [K4K15]CZS-1, an analogue with increased cationicity and reduced hydrophobicity, showed antibacterial, antifungal and antiproliferative properties. In addition, [K4K15]CZS-1 is less hemolytic than CZS-1. The in silico and scanning electron microscopy analysis reveal that [K4K15]CZS-1 induces a membranolytic effect on bacteria. Overall, these results confirm the potential of CZS-1 as source of inspiration for design new selective antimicrobial analogues useful for development of new therapeutic agents.

Tuning the Activity of Anoplin by Dendrimerization of Lysine and Lipidation of the N-Terminal

Dendrimeric antimicrobial peptides or lipopeptides have strong transmembrane ability and antibacterial activity. To obtain some ideal antimicrobial peptides, anoplin, a natural antimicrobial peptide with weak antimicrobial activity, was modified by C-terminal dendrimerization using lysine and N-terminal lipidation using fatty acids. 2K-3A-C4, a trimer of anoplin, was dendrimerized by two lysines at the C-terminal and was lipidated by n-butyric acid at the N-terminal, and thus exhibited the best antibacterial activity. However, the trimer had high hemolytic activity. Finally, A-C8, a simple structural lipopeptide, which is not a dendrimer, was obtained following the lipidation of anoplin using octanoic acid; it exhibited the highest therapeutic index, which makes it a probable antibiotic and thus was screened out.

Novel Broad-Spectrum Antimicrobial Peptide Derived from Anoplin and Its Activity on Bacterial Pneumonia in Mice

The emergence of multidrug-resistant bacteria has major issues for treating bacterial pneumonia. Currently, anoplin (GLLKRIKTLL-NH₂) is a natural antimicrobial candidate derived from wasp venom. In this study, a series of new antimicrobial peptide (AMP) anoplin analogues were designed and synthesized. The relationship between their biological activities and their positive charge, hydrophobicity, amphipathicity, and secondary structure are described. The characteristic shared by these peptides is that positively charged amino acids and hydrophobic amino acids are severally arranged on the hydrophilic and hydrophobic surface of the α-helix to form a completely amphiphilic structure. To achieve ideal AMPs, below the range of the threshold of the cytotoxicity and hemolytic activity, their charges and hydrophobicity were increased as much. Among the new analogues, A-21 (KWWKKWKKWW-NH₂) exhibited the greatest antimicrobial activity (geometric mean of minimum inhibitory concentrations = 4.76 μM) against all the tested bacterial strains, high bacterial cell selectivity in vitro, high effectiveness against bacterial pneumonia in mice infected with Klebsiella pneumoniae, and low toxicity in mice (LD₅₀ = 82.01 mg/kg). A-21 exhibited a potent bacterial membrane-damaging mechanism and lipopolysaccharide-binding ability. These data provide evidence that A-21 is a promising antimicrobial candidate for the treatment of bacterial pneumonia.

The Antimicrobial Peptide Melectin Shows Both Antimicrobial and Antitumor Activity via Membrane Interference and DNA Binding

Purpose

Increasingly complex diseases require novel drugs for their treatment. Antimicrobial peptides (AMPs) are promising candidate treatments due to their broad existence and special characteristics. However, the current understanding of AMPs is not sufficient to allow them to be produced commercially for clinical use.

Materials and Methods

Melectin, from the venom of the cleptoparasitic bee Melecta albifrons, does not exhibit sequence homology with other wasp venom peptides. To investigate this more deeply, we explored the antibacterial and antitumor activities of Melectin and related mechanisms.

Results

Our results demonstrate that Melectin possesses antimicrobial properties against standard sensitive/clinical drug-resistant bacteria strains as well as antitumor activity. It has an α-helix form and exhibits moderate cytotoxicity. Its action mechanisms are involved with membrane interfering and DNA binding. The membrane interfering effect was distinct between different phospholipid compositions.

Conclusion

We found that Melectin may serve as a new potential template in the battle against multidrug resistance, and our study indicated that there are promising prospects for medically applicable drugs based on AMPs.

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

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

Two antimicrobial genes from Aegilops tauschii Cosson identified by the Bacillus subtilis expression system

Antimicrobial genes play an important role as a primary defense mechanism in all multicellular organisms. We chose Bacillus subtilis as a target pathogen indicator and transferred the Aegilops tauschii Cosson cDNA library into B. subtilis cells. Expression of the candidate antimicrobial gene can inhibit B. subtilis cell growth. Using this strategy, we screened six genes that have an internal effect on the indicator bacteria. Then, the secreted proteins were extracted and tested; two genes, AtR100 and AtR472, were found to have strong external antimicrobial activities with broad-spectrum resistance against Xanthomonas oryzae pv. oryzicola, Clavibacter fangii, and Botrytis cinerea. Additionally, thermal stability tests indicated that the antimicrobial activities of both proteins were thermostable. Furthermore, these two proteins exhibited no significant hemolytic activities. To test the feasibility of application at the industrial level, liquid fermentation and spray drying of these two proteins were conducted. Powder dilutions were shown to have significant inhibitory effects on B. cinerea. Fluorescence microscopy and flow cytometry results showed that the purified protein impaired and targeted the cell membranes. This study revealed that these two antimicrobial peptides could potentially be used for replacing antibiotics, which would provide the chance to reduce the emergence of drug resistance.

CPF-C1 analog with effective antimicrobial and antibiofilm activities against Staphylococcus aureus including MRSA

The evolution of Staphylococcus aureus (S. aureus) with the ability to acquire and develop resistance to antibiotics has been described as a distinct strain emergence event. Methicillin-resistant S. aureus (MRSA) is responsible for most global S. aureus bacteremia cases. Bacterial biofilms are one of the primary reasons for drug resistance. Biofilms formed by S. aureus are the most common cause of biofilm-associated infections, which increase the difficulty of treatment. Antimicrobial peptides (AMPs) represent promising candidates for the future treatment of antibiotic-resistant bacterial and biofilm-associated infections. In this study, we designed and synthesized a series of analogs to increase the druggability of the natural antimicrobial peptide CPF-C1. Among the analogs, CPF-2 showed high antimicrobial activity against MRSA and multidrug-resistant S. aureus isolated from clinics. In the serum and physiological salt environment, CPF-2 also exhibited effective antimicrobial activity. Importantly, CPF-2 did not determine resistance and showed no hemolytic activity at the active concentration. Concerning the mechanism of action, CPF-2 produced a rapid bactericidal effect by interrupting the bacterial membranes. Even more surprisingly, CPF-2 showed an excellent ability to prevent and eradicate biofilms caused by S. aureus and MRSA not only in vitro but also in vivo. Our results suggested that CPF-2 has potential as a lead compound to treat infections caused by S. aureus and MRSA, including the associated biofilms.

Anti-bacterial activity of mutant chensinin-1 peptide against multidrug-resistant Pseudomonas aeruginosa and its effects on biofilm-associated gene expression

Nosocomial infections with Pseudomonas aeruginosa (PA) are difficult to treat due to the low outer membrane permeability of the bacterium and the development of resistance. In the present study, the anti-microbial peptide (AMP) mutant chensinin-1 (MC1) was revealed to exhibit anti-bacterial activity against a multidrug-resistant PA (MRPA) strain in vitro, and the minimum inhibitory concentration was 25 µM, which was 4-fold higher than that of the native strain. MC1 was able to disrupt the integrity of the cytoplasmic membrane in the native PA strain and MRPA and had a similar membrane depolarization ability in these strains, but the outer membrane permeability of MRPA cells was lower than that of native PA cells, as determined by a 1-N-phenylnaphthylamine assay. In addition, the abundance of the gene Psl encoding for biofilm-associated polysaccharides was detected using Congo red, and a high concentration of MC1 inhibited the formation of MRPA biofilms. Furthermore, the expression levels of biofilm-associated genes affected by the AMP, MC1, were quantified by polymerase chain reaction analysis. The results indicated that MC1 induced biofilm inhibition by downregulating the relative expression of specific biofilm polysaccharide-associated genes, including pelA, algD and pslA. The present results indicated that the AMP MC1 may be an effective antibiotic against MRPA strains.

Antimicrobial genes from Allium sativum and Pinellia ternata revealed by a Bacillus subtilis expression system

Antimicrobial genes are found in all classes of life. To efficiently isolate these genes, we used Bacillus subtilis and Escherichia coli as target indicator bacteria and transformed them with cDNA libraries. Among thousands of expressed proteins, candidate proteins played antimicrobial roles from the inside of the indicator bacteria (internal effect), contributing to the sensitivity (much more sensitivity than the external effect from antimicrobial proteins working from outside of the cells) and the high throughput ability of screening. We found that B. subtilis is more efficient and reliable than E. coli. Using the B. subtilis expression system, we identified 19 novel, broad-spectrum antimicrobial genes. Proteins expressed by these genes were extracted and tested, exhibiting strong external antibacterial, antifungal and nematicidal activities. Furthermore, these newly isolated proteins could control plant diseases. Application of these proteins secreted by engineered B. subtilis in soil could inhibit the growth of pathogenic bacteria. These proteins are thermally stable and suitable for clinical medicine, as they exhibited no haemolytic activity. Based on our findings, we speculated that plant, animal and human pathogenic bacteria, fungi or even cancer cells might be taken as the indicator target cells for screening specific resistance genes.

Intramolecular cyclization of the antimicrobial peptide Polybia-MPI with triazole stapling: influence on stability and bioactivity

Cationic antimicrobial peptides have attracted increasing attention as a novel class of antibiotics to treat infectious diseases caused by pathogenic bacteria. However, susceptibility to protease is a shortcoming in their development. Cyclization is one approach to increase the proteolytic resistance of peptides. Therefore, to improve the proteolytic resistance of Polybia-MPI, we have synthesized the MPI cyclic analogs C-MPI-1 (i-to-i+4) and C-MPI-2 (i-to-i+6) by copper(I)-catalyzed azide-alkyne cycloaddition. Compared with MPI, C-MPI-1 displayed sustained antimicrobial activity and had enhanced anti-trypsin resistance, while C-MPI-2 displayed no antimicrobial activity. The relationship between peptide structure and bioactivity was further investigated by probing the secondary structure of the peptides by circular dichroism. This showed that C-MPI-1 adopted an α-helical structure in aqueous solution and, interestingly, had increased α-helical conformation in 30 mM sodium dodecyl sulfate and 50% trifluoroethyl alcohol compared with MPI. C-MPI-2 that was not α-helical in structure, suggesting that the propensity for α-helix conformation may play an important role in cyclic peptide design. In addition, scanning electron microscopy, propidium iodide uptake, and membrane permeabilization assays indicated that MPI and the optimized analog C-MPI-1 had membrane-active action modes, indicating that the peptides would not be susceptible to conventional resistance mechanisms. Our study provides additional insight into the influence of intramolecular cyclization at various positions on peptide structure and biological activity. In conclusion, the design and synthesis of cyclic analogs via click chemistry offer a new strategy for the development of stable antimicrobial agents. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Antibacterial activity of chensinin-1b, a peptide with a random coil conformation, against multiple-drug-resistant Pseudomonas aeruginosa

Nosocomial infections caused by Pseudomonas aeruginosa are difficult to treat due to the low permeability of its outer membrane as well as to its remarkable ability to acquire further resistance to antibiotics. Chensinin-1b exhibited antibacterial activity against the tested multiple-drug-resistant bacteria with a MIC ranging between 1.56 and 50μM, except E. cloacae strain 0320 (MREC0320), P. fluorescens strain 0322 (MRPF0322) and E. aerogenes strain 0320 (MREA0320). However, the MIC (25μM) of chensinin-1b to multiple-drug-resistant P. aeruginosa strain (MRPA 0108) was 16-fold higher than that observed to P. aeruginosa susceptible strain CGMCC 1.860 (PA1860). Chensinin-1b was able to disturb the integration of the cytoplasmic membrane of PA1860 and MRPA0108 cells similarly, but the outer membrane permeability of MRPA0108 cells was significantly lower. This low permeability was associated with increased expression of lipopolysaccharide (LPS) in the outer membrane and a decrease in negatively charged phospholipids in the outer membrane leaflet. In addition, the biofilm of MRPA0108 was responsible for the reduced susceptibility to chensinin-1b. A higher concentration of chensinin-1b (12.5µM) was required to maximally inhibit the formation of MRPA0108 biofilm. Notably, chensinin-1b inhibited the formation of MRPA0108 biofilm at concentrations below its MIC value by down-regulating the level of PelA, algD, and PslA gene transcription. Importantly, chensinin-1b had a significant antibacterial effect against MRPA0108 in vivo. Administration of chensinin-1b to mice infected with MRPA 0108 significantly increased survival by 50-70%. Moreover, chensinin-1b reduced the production of pro-inflammatory mediators and correspondingly reduced lung and liver tissue damage in the mouse model of septic shock induced by MRPA 0108. Collectively, these results suggest that chensinin-1b could be an effective antibiotic against multiple-drug-resistant bacterial strains.

Novel antimicrobial peptide CPF-C1 analogs with superior stabilities and activities against multidrug-resistant bacteria

As numerous clinical isolates are resistant to most conventional antibiotics, infections caused by multidrug-resistant bacteria are associated with a higher death rate. Antimicrobial peptides show great potential as new antibiotics. However, a major obstacle to the development of these peptides as useful drugs is their low stability. To overcome the problem of the natural antimicrobial peptide CPF-C1, we designed and synthesized a series of analogs. Our results indicated that by introducing lysine, which could increase the number of positive charges, and by introducing tryptophan, which could increase the hydrophobicity, we could improve the antimicrobial activity of the peptides against multidrug-resistant strains. The introduction of d-amino acids significantly improved stability. Certain analogs demonstrated antibiofilm activities. In mechanistic studies, the analogs eradicated bacteria not just by interrupting the bacterial membranes, but also by linking to DNA, which was not impacted by known mechanisms of resistance. In a mouse model, certain analogs were able to significantly reduce the bacterial load. Among the analogs, CPF-9 was notable due to its greater antimicrobial potency in vitro and in vivo and its superior stability, lower hemolytic activity, and higher antibiofilm activity. This analog is a potential antibiotic candidate for treating infections induced by multidrug-resistant bacteria.

Beyond conventional antibiotics - New directions for combination products to combat biofilm

Medical device related infections are a significant and growing source of morbidity and mortality. Biofilm formation is a common feature of medical device infections that is not effectively prevented or treated by systemic antibiotics. Antimicrobial medical device combination products provide a pathway for local delivery of antimicrobial therapeutics with the ability to achieve high local concentrations while minimizing systemic side effects. In this review, we present considerations for the design of local antimicrobial delivery systems, which can be facilitated by modeling local pharmacokinetics in the context of the target device application. In addition to the need for local delivery, a critical barrier to progress in the field is the need to incorporate agents effective against biofilm. This article aims to review key properties of antimicrobial peptides that make them well suited to meet the demands of the next generation of antimicrobial medical devices, including broad spectrum activity, rapid and biocidal mechanisms of action, and efficacy against biofilm.

Design of novel antimicrobial peptide dimer analogues with enhanced antimicrobial activity in vitro and in vivo by intermolecular triazole bridge strategy

Currently, antimicrobial peptides have attracted considerable attention because of their broad-sprectum activity and low prognostic to induce antibiotic resistance. In our study, for the first time, a series of side-chain hybrid dimer peptides J-AA (Anoplin-Anoplin), J-RR (RW-RW), and J-AR (Anoplin-RW) based on the wasp peptide Anoplin and the arginine- and tryptophan-rich hexapeptide RW were designed and synthesized by click chemistry, with the intent to improve the antimicrobial efficacy of peptides against bacterial pathogens. The results showed that all dimer analogues exhibited up to a 4-16 fold increase in antimicrobial activity compared to the parental peptides against bacterial strains. Furthermore, the antimicrobial activity was confirmed by time-killing kinetics assay with two strains which showed that these dimer analogues at 1, 2×MIC were rapidly bactericidal and reduced the initial inoculum significantly during the first 2-6h. Notably, dimer peptides showed synergy and additivity effects when used in combination with conventional antibiotics rifampin or penicillin respectively against the multidrug-resistant strains. In the Escherichia coli-infected mouse model, all of hybrid dimer analogues had significantly lower degree of bacterial load than the untreated control group when injected once i.p. at 5mg/kg. In addition, the infected mice by methicillin-resistant (MRSA) strain could be effectively treated with J-RR. All of dimer analogues had membrane-active action mode. And the membrane-dependent mode of action signifies that peptides functions freely and without regard to conventional resistant mechanisms. Circular dichroism analyses of all dimer analogues showed a general predominance of α-helix conformation in 50% trifluoroethanol (TFE). Additionally, the acute toxicities study indicated that J-RR or J-AR did not show the signs of toxicity when adult mice exposed to concentration up to 120mg/kg. The 50% lethal dose (LD₅₀) of J-AA was 53.6mg/kg. In conclusion, to design and synthesize side chain-hybrid dimer analogues via click chemistry may offer a new strategy for antibacterial therapeutic option.

Preparation of an antimicrobial surface by direct assembly of antimicrobial peptide with its surface binding activity

The designed antimicrobial peptide has surface binding activity onto titanium, gold, polymethyl methacrylate and hydroxyapatite substrates.

Functional Analysis of Plantaricin E and Its Mutant by Heterologous Expression in Escherichia coli

Plantaricins are a group of ribosomally synthesized antimicrobial peptides in Lactobacillus plantarum that exert antimicrobial activities against some foodborne pathogens. In this study, we observed that plantaricin E in L. plantarum 163 was missing 19 amino acids (plnE mutant amino acid sequence: FNRGGYNFGKSVRH, plnE amino acid sequence: FNRGGYNFGKSVRHVVDAIGSVAGIRGILKSIR). In order to study the effects of mutant plnE, plnE mutant genes with and without the signal peptide were cloned from the L. plantarum 163 genome, linked to the pET32a vector, and expressed via a fusion protein (thioredoxin) in Escherichia coli BL21 (DE3). All target proteins were purified using Ni-NTA, SP FF columns, and RP-HPLC. The purified proteins were stable in an acidic environment and at temperatures below 80 °C, but they were easily degraded under alkaline conditions and by protease treatment. They showed antimicrobial activity against gram-positive bacteria such as Micrococcus luteus, Staphylococcus epidermidis, Lactococcus lactis, Lactobacillus paracasei, and Listeria innocua. In addition, SP-plnE and PlnE exerted stronger activity than nisin. The signal peptide had a positive effect on the activities of PlnE and PlnEm. Thus, these purified proteins may have potential applications in the food industry to control foodborne pathogens.

The New Antimicrobial Peptide SpHyastatin from the Mud Crab Scylla paramamosain with Multiple Antimicrobial Mechanisms and High Effect on Bacterial Infection

SpHyastatin was first identified as a new cationic antimicrobial peptide in hemocytes of the mud crab Scylla paramamosain. Based on the amino acid sequences deduced, it was predicted that this peptide was composed of two different functional domains, a proline-rich domain (PRD) and a cysteine-rich domain (CRD). The recombinant product of SpHyastatin displayed potent antimicrobial activities against the human pathogen Staphylococcus aureus and the aquatic animal pathogens Aeromonas hydrophila and Pseudomonas fluorescens. Compared with the CRD of SpHyastatin, the PRD presented better antimicrobial and chitin binding activities, but both regions were essential for allowing SpHyastatin complete antimicrobial activity. The binding properties of SpHyastatin to different microbial surface molecules suggested that this might be an initial and crucial step for performing its antimicrobial activities. Evaluated using propidium iodide uptake assays and scanning electron microscopy images, the antimicrobial mechanism of SpHyastatin was found to be prone to disrupt cell membrane integrity. Interestingly, SpHyastatin exerted its role specifically on the surface of S. aureus and Pichia pastoris whereas it directly killed P. fluorescens through simultaneous targeting the membrane and the cytoplasm, indicating that SpHyastatin could use different antimicrobial mechanisms to kill different species of microbes. As expected, the recombinant SpHyastatin increased the survival rate of crabs challenged with Vibrio parahaemolyticus. In addition, SpHyastatin could modulate some V. parahaemolyticus-responsive genes in S. paramamosain.

New molecules and adjuvants in the treatment of infections by Acinetobacter baumannii

INTRODUCTION

The current problems of the treatment of infections by Acinetobacter baumannii are linked with the increase of multidrug- and extensive-drug resistance and the lack of development of new antimicrobial drugs for Gram-negative bacilli. For these reasons, new alternatives for the treatment and control of severe infections by A. baumannii are necessary. Several studies have reported the effect of adjuvants to restore the efficacy of existing antimicrobial agents.

AREAS COVERED

In the present review, the authors describe the main results in the development of adjuvant drugs as well as new data on antimicrobial peptides, in monotherapy or in combination therapy with existing antimicrobial agents, which have shown promising preclinical results in vitro and in vivo.

EXPERT OPINION

The preclinical evaluation of adjuvants and antimicrobial peptides, in monotherapy or in combination therapy, for A. baumannii infections has shown promising results. However, caution is needed and further extensive in vivo studies and clinical trials have to be performed to confirm the potential use of these adjuvants as true therapeutic alternatives.

Mechanism study on a new antimicrobial peptide Sphistin derived from the N-terminus of crab histone H2A identified in haemolymphs of Scylla paramamosain

Histone H2A is known to participate in host immune defense through generating special antimicrobial peptides (AMPs), for which it has been an interesting research focus to characterize this kind of peptides in vertebrates and invertebrates. Although thousands of AMPs have been reported in variety of life species, only several AMPs are known in crabs and in particular no H2A-derived AMP has yet been reported. In the present study, a 38-amino acid peptide with antimicrobial activity was determined based on the sequence analysis of a histone H2A identified from the mud crab Scylla paramamosain. The histone H2A derived peptide was an AMP-like molecule and designated as Sphistin. Sphistin showed typical features of AMPs such as amphiphilic α-helical second structrue and positive charge net. The synthetic Sphistin exerted high antimicrobial activity against Gram-positive, Gram-negative bacteria and yeast, among which Aeromonas hydrophila, Pseudomonas fluorescens and Pseudomonas stutzeri are important aquatic pathogens. Leakage of the cell content and disruption of the cell surface were observed in bacterial cells treated with Sphistin using scanning electron microscopy. It was proved that the increasing cytoplasmic membrane permeability of Escherichia coli was caused by Sphistin. Further observation under confocal microscopy showed that Sphistin could combine onto the membrane of Staphylococcus aureus, E. coli MC1061 and Pichia pastoris but not translocate into the cytoplasm. Moreover, the affinity of Sphistin with either LPS or LTA was also testified that there was an interaction between Sphistin and cell membrane. Thus, the antimicrobial mechanism of this peptide likely exerted via adsorption and subsequently permeabilization of the bacterial cell membranes other than penetrating cell membrane. In addition, synthetic Sphistin exhibited no cytotoxicity to primary cultured crab haemolymphs and mammalian cells even at a high concentration of 100 μg/mL for 24 h. This is the first report of a histone-derived Sphistin identified from S. paramamosain with a specific antimicrobial activity and mechanism, which could be a new candidate for future application in aquaculture and veterinary medicine.

Characterization and Biological Activities of Ocellatin Peptides from the Skin Secretion of the Frog Leptodactylus pustulatus

Eight new peptides were isolated from the skin secretion of the frog Leptodactylus pustulatus and their amino acid sequences determined by de novo sequencing and by cDNA cloning. Structural similarities between them and other antimicrobial peptides from the skin secretion of Leptodactylus genus frogs were found. Ocellatins-PT1 to -PT5 (25 amino acid residues) are amidated at the C-terminus, while ocellatins-PT6 to -PT8 (32 amino acid residues) have free carboxylates. Antimicrobial activity, hemolytic tests, and cytotoxicity against a murine fibroblast cell line were investigated. All peptides, except for ocellatin-PT2, have antimicrobial activity against at least one Gram-negative strain. Ocellatin-PT8 inhibited the growth of Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, and Salmonella choleraesuis strains with MICs in the 60-240 μM range. No significant effect was observed in human erythrocytes and in a murine fibroblast cell line after exposure to the peptides at MICs. A comparison between sequences obtained by both direct HPLC-MS de novo sequencing and cDNA cloning demonstrates the secretion of mature peptides derived from a pre-pro-peptide structure.

Antimicrobial activities and action mechanism studies of transportan 10 and its analogues against multidrug-resistant bacteria

The increased emergence of multidrug-resistant bacteria is perceived as a critical public health threat, creating an urgent need for the development of novel classes of antimicrobials. Cell-penetrating peptides that share common features with antimicrobial peptides have been found to have antimicrobial activity and are currently being considered as potential alternatives to antibiotics. Transportan 10 is a chimeric cell-penetrating peptide that has been reported to transport biologically relevant cargoes into mammalian cells and cause damage to microbial membranes. In this study, we designed a series of TP10 analogues and studied their structure-activity relationships. We first evaluated the antimicrobial activities of these compounds against multidrug-resistant bacteria, which are responsible for most nosocomial infections. Our results showed that several of these compounds had potent antimicrobial and biofilm-inhibiting activities. We also measured the toxicity of these compounds, finding that Lys substitution could increase the antimicrobial activity but significantly enhanced the cytotoxicity. Pro introduction could reduce the cytotoxicity but disrupted the helical structure, resulting in a loss of activity. In the mechanistic studies, TP10 killed bacteria by membrane-active and DNA-binding activities. In conclusion, TP10 and its analogues could be developed into promising antibiotic candidates for the treatment of infections caused by multidrug-resistant bacteria.