The Mechanism of Killing by the Proline-Rich Peptide Bac7(1-35) against Clinical Strains of Pseudomonas aeruginosa Differs from That against Other Gram-Negative Bacteria

Polyproline peptide targets Klebsiella pneumoniae polysaccharides to collapse biofilms

Hypervirulent Klebsiella pneumoniae is known for its increased extracellular polysaccharide production. Biofilm matrices of hypervirulent K. pneumoniae have increased polysaccharide abundance and are uniquely susceptible to disruption by peptide bactenecin 7 (bac7 (1-35)). Here, using confocal microscopy, we show that polysaccharides within the biofilm matrix collapse following bac7 (1-35) treatment. This collapse led to the release of cells from the biofilm, which were then killed by the peptide. Characterization of truncated peptide analogs revealed that their interactions with polysaccharide were responsible for the biofilm matrix changes that accompany bac7 (1-35) treatment. Ultraviolet photodissociation mass spectrometry with the parental peptide or a truncated analog bac7 (10-35) reveal the important regions for bac7 (1-35) complexing with polysaccharides. Finally, we tested bac7 (1-35) using a murine skin abscess model and observed a significant decrease in the bacterial burden. These findings unveil the potential of bac7 (1-35) polysaccharide interactions to collapse K. pneumoniae biofilms.

Designing New Hybrid Antibiotics: Proline-Rich Antimicrobial Peptides Conjugated to the Aminoglycoside Tobramycin

Resistance to aminoglycoside antibiotics is a serious problem, typically arising from inactivating enzymes, reduced uptake, or increased efflux in the important pathogens for which they are used as treatment. Conjugating aminoglycosides to proline-rich antimicrobial peptides (PrAMPs), which also target ribosomes and have a distinct bacterial uptake mechanism, might mutually benefit their individual activities. To this aim we have developed a strategy for noninvasively modifying tobramycin to link it to a Cys residue and through this covalently link it to a Cys-modified PrAMP by formation of a disulfide bond. Reduction of this bridge in the bacterial cytosol should release the individual antimicrobial moieties. We found that the conjugation of tobramycin to the well-characterized N-terminal PrAMP fragment Bac7(1-35) resulted in a potent antimicrobial capable of inactivating not only tobramycin-resistant bacterial strains but also those less susceptible to the PrAMP. To a certain extent, this activity also extends to the shorter and otherwise poorly active fragment Bac7(1-15). Although the mechanism that allows the conjugate to act when its individual components do not is as yet unclear, results are very promising and suggest this may be a way of resensitizing pathogens that have developed resistance to the antibiotic.

Bactericidal Properties of Proline-Rich Aedes aegypti Trypsin Modulating Oostatic Factor (AeaTMOF)

The antimicrobial properties of proline-rich Aedes aegypti decapeptide TMOF (AeaTMOF) and oncocin112 (1-13) were compared. Incubations with multidrug-resistant Escherichia coli cells showed that AeaTMOF (5 mM) was able to completely inhibit bacterial cell growth, whereas oncocin112 (1-13) (20 mM) partially inhibited bacterial growth as compared with bacterial cells that were not multidrug-resistant cells. AeaTMOF (5 mM) was very effective against Acinetobacter baumannii and Pseudomonas aeruginosa, completely inhibiting cell growth during 15 h incubations. AeaTMOF (5 mM) completely inhibited the Gram-positive bacteria Staphylococcus aureus and Bacillus thurengiensis sups. Israelensis cell growth, whereas oncocin112 (1-13) (10 and 20 mM) failed to affect bacterial cell growth. E. coli cells that lack the SbmA transporter were inhibited by AeaTMOF (5 mM) and not by oncocin112 (1-13) (10 to 20 mM), indicating that AeaTMOF can use other bacterial transporters than SbmA that is mainly used by proline-rich antimicrobial peptides. Incubation of E. coli cells with NaAzide showed that AeaTMOF does not use ABC-like transporters that use ATP hydrolysis to import molecules into bacterial cells. Three-dimensional modeling and docking of AeaTMOF to SbmA and MdtM transporters showed that AeaTMOF can bind these proteins, and the binding location of AeaTMOF inside these protein transporters allows AeaTMOF to be transported into the bacterial cytosol. These results show that AeaTMOF can be used as a future antibacterial agent against both multidrug-resistant Gram-positive and -negative bacteria.

Spermine-Conjugated Short Proline-Rich Lipopeptides as Broad-Spectrum Intracellular Targeting Antibacterial Agents

Toward the design of new proline-rich peptidomimetics, a short peptide segment, present in several proline-rich antimicrobial peptides (AMPs), was selected. Fatty acids of varying lengths and spermine were conjugated at the N- and C-terminals of the peptide, respectively. Spermine-conjugated lipopeptides, C₁₀-PR-Spn and C₁₂-PR-Spn, exhibited minimum inhibitory concentrations within 1.5-6.2 μM against the tested pathogens including resistant bacteria and insignificant hemolytic activity against human red blood cells up to 100 μM concentrations and demonstrated resistance against trypsin digestion. C₁₀-PR-Spn and C₁₂-PR-Spn showed synergistic antimicrobial activity against multidrug-resistant methicillin-resistant Staphylococcus aureus with several tested antibiotics. These lipopeptides did not permeabilize bacterial membrane-mimetic lipid vesicles or damage the Escherichia coli membrane like the nonmembrane-lytic AMP, buforin-II. The results suggested that C₁₀-PR-Spn and C₁₂-PR-Spn could interact with the 70S ribosome of E. coli and inhibit its protein synthesis. C₁₀-PR-Spn and C₁₂-PR-Spn demonstrated superior clearance of bacteria from the spleen, liver, and kidneys of mice, infected with S. aureus ATCC 25923 compared to levofloxacin.

Mammals' humoral immune proteins and peptides targeting the bacterial envelope: from natural protection to therapeutic applications against multidrug‐resistant Gram ‐negatives

Mammalian innate immunity employs several humoral ‘weapons’ that target the bacterial envelope. The threats posed by the multidrug‐resistant ‘ESKAPE’ Gram‐negative pathogens (Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) are forcing researchers to explore new therapeutic options, including the use of these immune elements. Here we review bacterial envelope‐targeting (peptidoglycan and/or membrane‐targeting) proteins/peptides of the mammalian immune system that are most likely to have therapeutic applications. Firstly we discuss their general features and protective activity against ESKAPE Gram‐negatives in the host. We then gather, integrate, and discuss recent research on experimental therapeutics harnessing their bactericidal power, based on their exogenous administration and also on the discovery of bacterial and/or host targets that improve the performance of this endogenous immunity, as a novel therapeutic concept. We identify weak points and knowledge gaps in current research in this field and suggest areas for future work to obtain successful envelope‐targeting therapeutic options to tackle the challenge of antimicrobial resistance.

Translocation of non-lytic antimicrobial peptides and bacteria penetrating peptides across the inner membrane of the bacterial envelope

The increase in multidrug-resistant pathogenic bacteria has become a problem worldwide. Currently there is a strong focus on the development of novel antimicrobials, including antimicrobial peptides (AMP) and antimicrobial antisense agents. While the majority of AMP have membrane activity and kill bacteria through membrane disruption, non-lytic AMP are non-membrane active, internalize and have intracellular targets. Antimicrobial antisense agents such as peptide nucleic acids (PNA) and phosphorodiamidate morpholino oligomers (PMO), show great promise as novel antibacterial agents, killing bacteria by inhibiting translation of essential target gene transcripts. However, naked PNA and PMO are unable to translocate across the cell envelope of bacteria, to reach their target in the cytosol, and are conjugated to bacteria penetrating peptides (BPP) for cytosolic delivery. Here, we discuss how non-lytic AMP and BPP-PMO/PNA conjugates translocate across the cytoplasmic membrane via receptor-mediated transport, such as the cytoplasmic membrane transporters SbmA, MdtM/YjiL, and/or YgdD, or via a less well described autonomous process.

The proline-rich myticalins from Mytilus galloprovincialis display a membrane-permeabilizing antimicrobial mode of action

Bivalve mollusks are continuously exposed to potentially pathogenic microorganisms living in the marine environment. Not surprisingly, these filter-feeders developed a robust innate immunity to protect themselves, which includes a broad panel of antimicrobial peptides. Among these, myticalins represent a recently discovered family of linear cationic peptides expressed in the gills of Mytilus galloprovincialis. Even though myticalins and insect and mammalian proline-rich antimicrobial peptides (PrAMPs) share a similar amino acid composition, we here show that none of the tested mussel peptides use a non-lytic mode of action relying on the bacterial transporter SbmA. On the other hand, all the tested myticalins perturbed and permeabilized the membranes of E. coli BW25113, as shown by flow-cytometry and atomic force microscopy. Circular dichroism spectra revealed that most myticalins did not adopt recognizable secondary structures in the presence of amphipathic environments, such as biological membranes. To explore possible uses of myticalins for biotech, we assessed their biocompatibility with a human cell line. Non-negligible cytotoxic effects displayed by myticalins indicate that their optimization would be required before their further use as lead compounds in the development of new antibiotics.

Antimicrobial Peptides and Proteins: From Nature's Reservoir to the Laboratory and Beyond

Rapid rise of antimicrobial resistance against conventional antimicrobials, resurgence of multidrug resistant microbes and the slowdown in the development of new classes of antimicrobials, necessitates the urgent development of alternate classes of therapeutic molecules. Antimicrobial peptides (AMPs) are small proteins present in different lifeforms in nature that provide defense against microbial infections. They have been effective components of the host defense system for a very long time. The fact that the development of resistance by the microbes against the AMPs is relatively slower or delayed compared to that against the conventional antibiotics, makes them prospective alternative therapeutics of the future. Several thousands of AMPs have been isolated from various natural sources like microorganisms, plants, insects, crustaceans, animals, humans, etc. to date. However, only a few of them have been translated commercially to the market so far. This is because of some inherent drawbacks of the naturally obtained AMPs like 1) short half-life owing to the susceptibility to protease degradation, 2) inactivity at physiological salt concentrations, 3) cytotoxicity to host cells, 4) lack of appropriate strategies for sustained and targeted delivery of the AMPs. This has led to a surge of interest in the development of synthetic AMPs which would retain or improve the antimicrobial potency along with circumventing the disadvantages of the natural analogs. The development of synthetic AMPs is inspired by natural designs and sequences and strengthened by the fusion with various synthetic elements. Generation of the synthetic designs are based on various strategies like sequence truncation, mutation, cyclization and introduction of unnatural amino acids and synthons. In this review, we have described some of the AMPs isolated from the vast repertoire of natural sources, and subsequently described the various synthetic designs that have been developed based on the templates of natural AMPs or from de novo design to make commercially viable therapeutics of the future. This review entails the journey of the AMPs from their natural sources to the laboratory.

Fabrication of cell penetrating peptide-conjugated bacterial cellulose nanofibrils with remarkable skin adhesion and water retention performance

Bacterial cellulose nanofibrils (BCNFs), possessing excellent biocompatibility as well as hygroscopicity, are receiving high interest as a biomaterial for biomedical and healthcare treatments, since they exert various interactions with tissues after surface modification with biochemicals such as peptides, proteins, and catechols. Herein, we report a BCNF-based skin adhesion system, wherein cell penetrating peptide (CPP)-conjugated BCNFs were employed to enhance the attraction to the skin under wet conditions. For this, we conjugated Bac7, a type of CPP, with the carboxylate of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized BCNFs. We showed that Bac7-conjugated BCNFs exhibited both hydrophobic and electrostatic interactions with the cell membrane, which eventually led to the remarkable adhesion against the skin surface. Furthermore, we demonstrated that such tailored skin attraction played a key role in improving skin water retention.

Caprine Bactenecins as Promising Tools for Developing New Antimicrobial and Antitumor Drugs

Proline-rich antimicrobial peptides (PR-AMPs) having a potent antimicrobial activity predominantly toward Gram-negative bacteria and negligible toxicity toward host cells, are attracting attention as new templates for developing antibiotic drugs. We have previously isolated and characterized several bactenecins that are promising in this respect, from the leukocytes of the domestic goat Capra hircus: ChBac5, miniChBac7.5N-α, and -β, as well as ChBac3.4. Unlike the others, ChBac3.4 shows a somewhat unusual pattern of activities for a mammalian PR-AMP: it is more active against bacterial membranes as well as tumor and, to the lesser extent, normal cells. Here we describe a SAR study of ChBac3.4 (RFRLPFRRPPIRIHPPPFYPPFRRFL-NH2) which elucidates its peculiarities and evaluates its potential as a lead for antimicrobial or anticancer drugs based on this peptide. A set of designed structural analogues of ChBac3.4 was explored for antibacterial activity toward drug-resistant clinical isolates and antitumor properties. The N-terminal region was found to be important for the antimicrobial action, but not responsible for the toxicity toward mammalian cells. A shortened variant with the best selectivity index toward bacteria demonstrated a pronounced synergy in combination with antibiotics against Gram-negative strains, albeit with a somewhat reduced ability to inhibit biofilm formation compared to native peptide. C-terminal amidation was examined for some analogues, which did not affect antimicrobial activity, but somewhat altered the cytotoxicity toward host cells. Interestingly, non-amidated peptides showed a slight delay in their impact on bacterial membrane integrity. Peptides with enhanced hydrophobicity showed increased toxicity, but in most cases their selectivity toward tumor cells also improved. While most analogues lacked hemolytic properties, a ChBac3.4 variant with two additional tryptophan residues demonstrated an appreciable activity toward human erythrocytes. The variant demonstrating the best tumor/nontumor cell selectivity was found to more actively initiate apoptosis in target cells, though its action was slower than that of the native ChBac3.4. Its antitumor effectiveness was successfully verified in vivo in a murine Ehrlich ascites carcinoma model. The obtained results demonstrate the potential of structural modification to manage caprine bactenecins’ selectivity and activity spectrum and confirm that they are promising prototypes for antimicrobial and anticancer drugs design.

Peptide Inhibitors of Bacterial Protein Synthesis with Broad Spectrum and SbmA-Independent Bactericidal Activity against Clinical Pathogens

Proline-rich antimicrobial peptides (PrAMPs) are promising lead compounds for developing new antimicrobials; however, their narrow spectrum of action is limiting. PrAMPs kill bacteria binding to their ribosomes and inhibiting protein synthesis. In this study, 133 derivatives of the PrAMP Bac7(1-16) were synthesized to identify the crucial residues for ribosome inactivation and antimicrobial activity. Then, five new Bac7(1-16) derivatives were conceived and characterized by antibacterial and membrane permeabilization assays, X-ray crystallography, and molecular dynamics simulations. Some derivatives displayed broad spectrum activity, encompassing Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa, and Staphylococcus aureus. Two peptides out of five acquired a weak membrane-perturbing activity while maintaining the ability to inhibit protein synthesis. These derivatives became independent of the SbmA transporter, commonly used by native PrAMPs, suggesting that they obtained a novel route to enter bacterial cells. PrAMP-derived compounds could become new-generation antimicrobials to combat antibiotic-resistant pathogens.

Antimicrobial Mechanism of pBD2 against Staphylococcus aureus

Antimicrobial peptides (AMPs) show high antibacterial activity against pathogens, which makes them potential new therapeutics to prevent and cure diseases. Porcine beta defensin 2 (pBD2) is a newly discovered AMP and has shown antibacterial activity against different bacterial species including multi-resistant bacteria. In this study, the functional mechanism of pBD2 antibacterial activity against Staphylococcus aureus was investigated. After S. aureus cells were incubated with different concentrations of pBD2, the morphological changes in S. aureus and locations of pBD2 were detected by electron microscopy. The differentially expressed genes (DEGs) were also analyzed. The results showed that the bacterial membranes were broken, bulging, and perforated after treatment with pBD2; pBD2 was mainly located on the membranes, and some entered the cytoplasm. Furthermore, 31 DEGs were detected and confirmed by quantitative real-time PCR (qRT-PCR). The known functional DEGs were associated with transmembrane transport, transport of inheritable information, and other metabolic processes. Our data suggest that pBD2 might have multiple modes of action, and the main mechanism by which pBD2 kills S. aureus is the destruction of the membrane and interaction with DNA. The results imply that pBD2 is an effective bactericide for S. aureus, and deserves further study as a new therapeutic substance against S. aureus.

Characterization of the active fragments of Spodoptera litura Lebocin-1

Insects can produce various antimicrobial peptides (AMPs) upon immune stimulation. One class of AMPs are characterized by their high proline content in certain fragments. They are generally called proline-rich antimicrobial peptides (PrAMPs). We previously reported the characterization of Spodoptera litura lebocin-1 (SlLeb-1), a PrAMP proprotein. Preliminary studies with synthetic polypeptides showed that among the four deductive active fragments, the C-terminal fragment SlLeb-1 (124-158) showed strong antibacterial activities. Here, we further characterized the antibacterial and antifungal activities of 124-158 and its four subfragments: 124-155, 124-149, 127-158, and 135-158. Only 124-158 and 127-158 could agglutinate bacteria, while 124-158 and four subfragments all could agglutinate Beauveria bassiana spores. Confocal microscopy showed that fluorescent peptides were located on the microbial surface. Fragment 135-158 lost activity completely against Escherichia coli and Staphylococcus aureus, and partially against Bacillus subtilis. Only 124-149 showed low activity against Serratia marcescens. Negative staining, transmission, and scanning electron microscopy of 124-158 treated bacteria showed different morphologies. Flow cytometry analysis of S. aureus showed that 124-158 and four subfragments changed bacterial subpopulations and caused an increase of DNA content. These results indicate that active fragments of SlLeb-1 may have diverse antimicrobial effects against different microbes. This study may provide an insight into the development of novel antimicrobial agents.

Proline-Rich Peptides with Improved Antimicrobial Activity against E. coli, K. pneumoniae, and A. baumannii

Proline-rich antimicrobial peptides (PrAMPs) are promising agents to combat multi-drug resistant pathogens due to a high antimicrobial activity, yet low cytotoxicity. A library of derivatives of the PrAMP Bac5(1-17) was synthesized and screened to identify which residues are relevant for its activity. In this way, we discovered that two central motifs -PIRXP- cannot be modified, while residues at N- and C- termini tolerated some variations. We found five Bac5(1-17) derivatives bearing 1-5 substitutions, with an increased number of arginine and/or tryptophan residues, exhibiting improved antimicrobial activity and broader spectrum of activity while retaining low cytotoxicity toward eukaryotic cells. Transcription/translation and bacterial membrane permeabilization assays showed that these new derivatives still retained the ability to strongly inhibit bacterial protein synthesis, but also acquired permeabilizing activity to different degrees. These new Bac5(1-17) derivatives therefore show a dual mode of action which could hinder the selection of bacterial resistance against these molecules.

Non-Lytic Antibacterial Peptides That Translocate Through Bacterial Membranes to Act on Intracellular Targets

The advent of multidrug resistance among pathogenic bacteria has attracted great attention worldwide. As a response to this growing challenge, diverse studies have focused on the development of novel anti-infective therapies, including antimicrobial peptides (AMPs). The biological properties of this class of antimicrobials have been thoroughly investigated, and membranolytic activities are the most reported mechanisms by which AMPs kill bacteria. Nevertheless, an increasing number of works have pointed to a different direction, in which AMPs are seen to be capable of displaying non-lytic modes of action by internalizing bacterial cells. In this context, this review focused on the description of the in vitro and in vivo antibacterial and antibiofilm activities of non-lytic AMPs, including indolicidin, buforin II PR-39, bactenecins, apidaecin, and drosocin, also shedding light on how AMPs interact with and further translocate through bacterial membranes to act on intracellular targets, including DNA, RNA, cell wall and protein synthesis.

Sub-MIC effects of a proline-rich antibacterial peptide on clinical isolates of Acinetobacter baumannii

INTRODUCTION

Acinetobacter baumannii is one of the most important nosocomial pathogens, mainly due to its ability to accumulate antibiotic-resistances and to persist in the hospital environment - characteristics related to biofilm production. It is well-known that A. baumannii is inhibited by the proline-rich peptide Bac7(1-35), but its putative effects at sub-MICs were never considered.

AIMS

We examined the sub-MIC effect of Bac7(1-35) on the growth rate, resistance induction and some A. baumannii features linked to virulence.

METHODOLOGY

Growth kinetics in the presence of sub-MICs of Bac7(1-35) were evaluated spectrophotometrically. Peptide uptake was quantified by cytometric analysis. The ability of Bac7(1-35) to interfere with biofilm production was investigated by the crystal violet method and confocal microscopy. Bacterial motility was observed at the interphase between a layer of a semi-solid medium and the polystyrene bottom of a Petri dish. The induction of resistance was evaluated after serial passages with sub-MICs of the peptide.

RESULTS

Although the MIC of Bac7(1-35) was between 2-4 µM for all tested strains, its effect on the growth rate at sub-MICs was strain-dependent and correlated with the amount of peptide internalized by each strain. Sub-MICs of Bac7(1-35) induced a strongly strain-dependent effect on biofilm formation and reduced motility in almost all strains, but interestingly the peptide did not induce resistance.

CONCLUSION

Bac7(1-35) is internalized into A. baumannii and is able to inhibit biofilm formation and bacterial motility, without inducing resistance. This study stresses the importance of considering possible effects that antimicrobials could have at sub-MICs, mimicking a common condition during antibiotic treatment.

Innate humoral immune defences in mammals and insects: The same, with differences ?

The insect immune response demonstrates many similarities to the innate immune response of mammals and a wide range of insects is now employed to assess the virulence of pathogens and produce results comparable to those obtained using mammals. Many of the humoral responses in insects and mammals are similar (e.g. insect transglutaminases and human clotting factor XIIIa) however a number show distinct differences. For example in mammals, melanization plays a role in protection from solar radiation and in skin and hair pigmentation. In contrast, insect melanization acts as a defence mechanism in which the proPO system is activated upon pathogen invasion. Human and insect antimicrobial peptides share distinct structural and functional similarities, insects produce the majority of their AMPs from the fat body while mammals rely on production locally at the site of infection by epithelial/mucosal cells. Understanding the structure and function of the insect immune system and the similarities with the innate immune response of mammals will increase the attractiveness of using insects as in vivo models for studying host – pathogen interactions.

Characterization, mechanism of action and optimization of activity of a novel peptide-peptoid hybrid against bacterial pathogens involved in canine skin infections

Integumentary infections like pyoderma represent the main reason for antimicrobial prescription in dogs. Staphylococcus pseudintermedius and Pseudomonas aeruginosa are frequently identified in these infections, and both bacteria are challenging to combat due to resistance. To avoid use of important human antibiotics for treatment of animal infections there is a pressing need for novel narrow-spectrum antimicrobial agents in veterinary medicine. Herein, we characterize the in vitro activity of the novel peptide-peptoid hybrid B1 against canine isolates of S. pseudintermedius and P. aeruginosa. B1 showed potent minimum inhibitory concentrations (MICs) against canine S. pseudintermedius and P. aeruginosa isolates as well rapid killing kinetics. B1 was found to disrupt the membrane integrity and affect cell-wall synthesis in methicillin-resistant S. pseudintermedius (MRSP). We generated 28 analogues of B1, showing comparable haemolysis and MICs against MRSP and P. aeruginosa. The most active analogues (23, 26) and B1 were tested against a collection of clinical isolates from canine, of which only B1 showed potent activity. Our best compound 26, displayed activity against P. aeruginosa and S. pseudintermedius, but not the closely related S. aureus. This work shows that design of target-specific veterinary antimicrobial agents is possible, even species within a genus, and deserves further exploration.

Combined Antibacterial Effects of Goat Cathelicidins With Different Mechanisms of Action

Being essential components of innate immune system, animal antimicrobial peptides (AMPs) also known as host-defense peptides came into sharp focus as possible alternatives to conventional antibiotics due to their high efficacy against a broad range of MDR pathogens and low rate of resistance development. Mammalian species can produce a set of co-localized AMPs with different structures and mechanisms of actions. Here we examined the combined antibacterial effects of cathelicidins, structurally diverse family of host-defense peptides found in vertebrate species. As a model we have used structurally distinct cathelicidins expressed in the leukocytes of goat Capra hircus. The recombinant analogs of natural peptides were obtained by heterologous expression in bacterial system and biological activities as well as the major mechanisms of antibacterial action of the peptides were investigated. As the result, the marked synergistic effect against wide panel of bacterial strains including extensively drug-resistant ones was observed for the pair of membranolytic α-helical amphipathic peptide ChMAP-28 and Pro-rich peptide mini-ChBac7.5Nα targeting a bacterial ribosome. ChMAP-28 was shown to damage the outer bacterial membrane at sub-inhibitory concentrations that could facilitate Pro-rich peptide translocation into the cell. Finally, resistance changes under a long-term continuous selective pressure of each individual peptide and the synergistic combination of both peptides were tested against Escherichia coli strains. The combination was shown to keep a high activity after the 26-days selection experiment in contrast to mini-ChBac7.5Nα used alone and the reference antibiotic polymyxin B. We identified the point mutation leading to amino acid substitution V102E in the membrane transport protein SbmA of the mini-ChBac7.5Nα-resistant strain obtained by selection. The experiments revealed that the presence of sub-inhibitory concentrations of ChMAP-28 restored the activity of mini-ChBac7.5Nα against this strain and clinical isolate with a weak sensitivity to mini-ChBac7.5Nα. The obtained results suggest a potential medical application of synergistic combinations of natural cathelicidins, which allows using a lower therapeutic dose and minimizes the risk of resistance development.

Antimicrobial activities of a proline-rich proprotein from Spodoptera litura

Antimicrobial peptides (AMPs) are produced by the stimulated humoral immune system. Most mature AMPs contain less than 50 amino acid residues. Some of them are generated from proproteins upon microbial challenges. Here, we report the antimicrobial activities of a proline-rich proprotein, named SlLebocin1 (SlLeb1), from the tobacco cutworm Spodoptera litura. SlLebocin1 cDNA contains a 477-bp open reading frame (ORF). It is mainly expressed in hemocytes and the midgut in naïve larvae. The transcript level was significantly induced in hemocytes but repressed in the midgut and fat body by bacterial challenges. The proprotein contains 158 amino acids with 3 RXXR motifs that are characteristic of some Lepidopteral lebocin proproteins. Four peptides corresponding to the predicted processed fragments were synthesized chemically, and their antimicrobial activities against two Gram-negative and two Gram-positive bacterial strains were analyzed. The peptides showed differential antimicrobial activities. For Escherichia coli and Bacillus subtilis, only the C-terminal fragment (124-158) showed strong inhibitory effects. For Staphylococcus aureus, all peptides showed partial inhibitions. None of them inhibited Serratia marcescens. Bacterial morphologies were examined by the scanning electron microscopy and confocal laser scanning microscopy. The antimicrobial peptides either disrupted cellular membrane or inhibited cell division and caused elongated/enlarged morphologies. The results may provide ideas for designing novel antibiotics.

Fragments of the Nonlytic Proline-Rich Antimicrobial Peptide Bac5 Kill Escherichia coli Cells by Inhibiting Protein Synthesis

Unlike most antimicrobial peptides (AMPs), the main mode of action of the subclass of proline-rich antimicrobial peptides (PrAMPs) is not based on disruption of the bacterial membrane. Instead, PrAMPs exploit the inner membrane transporters SbmA and YjiL/MdtM to pass through the bacterial membrane and enter the cytosol of specific Gram-negative bacteria, where they exert an inhibitory effect on protein synthesis. Despite sharing a high proline and arginine content with other characterized PrAMPs, the PrAMP Bac5 has a low sequence identity with them. Here we investigated the mode of action of three N-terminal Bac5 fragments, Bac5(1-15), Bac5(1-25), and Bac5(1-31). We show that Bac5(1-25) and Bac5(1-31) retained excellent antimicrobial activity toward Escherichia coli and low toxicity toward eukaryotic cells, whereas Bac5(1-15) was inactive. Bac5(1-25) and Bac5(1-31) inhibited bacterial protein synthesis in vitro and in vivo Competition assays suggested that the binding site of Bac5 is within the ribosomal tunnel, where it prevents the transition from the initiation to the elongation phase of translation, as reported for other PrAMPs, such as the bovine PrAMP Bac7. Surprisingly, unlike Bac7, Bac5(1-25) exhibited species-specific inhibition, being an excellent inhibitor of protein synthesis on E. coli ribosomes but a poor inhibitor on Thermus thermophilus ribosomes. This indicates that while Bac5 most likely has an overlapping binding site with Bac7, the mode of interaction is distinct, suggesting that Bac5 fragments may be interesting alternative lead compounds for the development of new antimicrobial agents.

The activity of a mammalian proline-rich peptide against Gram-negative bacteria, including drug-resistant strains, relies on a nonmembranolytic mode of action

Background

A peptide of 2,733 Da named SP-E, previously isolated from pig saliva and already described for its antifungal activity and absence of toxicity against mammalian cells, is characterized by a high content of proline residues (70% of entire sequence), that confer structural features probably related to peptide activity.

Purpose

The aim of this study was to evaluate the activity of SP-E against Gram-negative bacteria, including drug-resistant clinical isolates.

Methods

SP-E and shorter fragments of the same peptide were tested in vitro against the selected bacteria by colony forming unit assays. Scanning electron microscopy and confocal microscopy were also applied. SP-E potential therapeutic activity was evaluated in vivo in a Galleria mellonella model of bacterial infection.

Results

SP-E proved to be active against the tested bacteria with EC₅₀ values in the micro-molar range. Though maintaining antibacterial properties, the shorter peptides showed lower activity in respect to the parental molecule. Kinetics of killing action and nonmembranolytic internalization within Escherichia coli and Pseudomonas aeruginosa cells strongly suggested a cytosolic mechanism of action involving one or more intracellular molecular targets. A single injection of SP-E exerted a therapeutic effect in G. mellonella larvae infected with P. aeruginosa.

Conclusion

The biological properties of SP-E strongly back this peptide as a new promising multitasking antimicrobial molecule.

Tryptophan-Rich and Proline-Rich Antimicrobial Peptides

Due to the increasing emergence of drug-resistant pathogenic microorganisms, there is a world-wide quest to develop new-generation antibiotics. Antimicrobial peptides (AMPs) are small peptides with a broad spectrum of antibiotic activities against bacteria, fungi, protozoa, viruses and sometimes exhibit cytotoxic activity toward cancer cells. As a part of the native host defense system, most AMPs target the membrane integrity of the microorganism, leading to cell death by lysis. These membrane lytic effects are often toxic to mammalian cells and restrict their systemic application. However, AMPs containing predominantly either tryptophan or proline can kill microorganisms by targeting intracellular pathways and are therefore a promising source of next-generation antibiotics. A minimum length of six amino acids is required for high antimicrobial activity in tryptophan-rich AMPs and the position of these residues also affects their antimicrobial activity. The aromatic side chain of tryptophan is able to rapidly form hydrogen bonds with membrane bilayer components. Proline-rich AMPs interact with the 70S ribosome and disrupt protein synthesis. In addition, they can also target the heat shock protein in target pathogens, and consequently lead to protein misfolding. In this review, we will focus on describing the structures, sources, and mechanisms of action of the aforementioned AMPs.

Short Proline-Rich Lipopeptide Potentiates Minocycline and Rifampin against Multidrug- and Extensively Drug-Resistant Pseudomonas aeruginosa

A series of 16 short proline-rich lipopeptides (SPRLPs) were constructed to mimic longer naturally existing proline-rich antimicrobial peptides. Antibacterial assessment revealed that lipopeptides containing hexadecanoic acid (C16) possess optimal antibacterial activity relative to others with shorter lipid components. SPRLPs were further evaluated for their potential to serve as adjuvants in combination with existing antibiotics to enhance antibacterial activity against drug-resistant Pseudomonas aeruginosa Out of 16 prepared SPRLPs, C12-PRP was found to significantly potentiate the antibiotics minocycline and rifampin against multidrug- and extensively drug-resistant (MDR/XDR) P. aeruginosa clinical isolates. This nonhemolytic C12-PRP is comprised of the heptapeptide sequence PRPRPRP-NH2 acylated to dodecanoic acid (C12) at the N terminus. The adjuvant potency of C12-PRP was apparent by its ability to reduce the MIC of minocycline and rifampin below their interpretative susceptibility breakpoints against MDR/XDR P. aeruginosa An attempt to optimize C12-PRP through peptidomimetic modification was performed by replacing all l- to d-amino acids. C12-PRP demonstrated that it was amenable to optimization, since synergism with minocycline and rifampin were retained. Moreover, C12-PRP displayed no cytotoxicity against human liver carcinoma HepG2 and human embryonic kidney HEK-293 cell lines. Thus, the SPRLP C12-PRP is a lead adjuvant candidate that warrants further optimization. The discovery of agents that are able to resuscitate the activity of existing antibiotics against drug-resistant Gram-negative pathogens, especially P. aeruginosa, is of great clinical interest.

Myticalins: A Novel Multigenic Family of Linear, Cationic Antimicrobial Peptides from Marine Mussels (Mytilus spp.)

The application of high-throughput sequencing technologies to non-model organisms has brought new opportunities for the identification of bioactive peptides from genomes and transcriptomes. From this point of view, marine invertebrates represent a potentially rich, yet largely unexplored resource for de novo discovery due to their adaptation to diverse challenging habitats. Bioinformatics analyses of available genomic and transcriptomic data allowed us to identify myticalins, a novel family of antimicrobial peptides (AMPs) from the mussel Mytilus galloprovincialis, and a similar family of AMPs from Modiolus spp., named modiocalins. Their coding sequence encompasses two conserved N-terminal (signal peptide) and C-terminal (propeptide) regions and a hypervariable central cationic region corresponding to the mature peptide. Myticalins are taxonomically restricted to Mytiloida and they can be classified into four subfamilies. These AMPs are subject to considerable interindividual sequence variability and possibly to presence/absence variation. Functional assays performed on selected members of this family indicate a remarkable tissue-specific expression (in gills) and broad spectrum of activity against both Gram-positive and Gram-negative bacteria. Overall, we present the first linear AMPs ever described in marine mussels and confirm the great potential of bioinformatics tools for the de novo discovery of bioactive peptides in non-model organisms.

Myticalins: A Novel Multigenic Family of Linear, Cationic Antimicrobial Peptides from Marine Mussels (Mytilus spp.)

The application of high-throughput sequencing technologies to non-model organisms has brought new opportunities for the identification of bioactive peptides from genomes and transcriptomes. From this point of view, marine invertebrates represent a potentially rich, yet largely unexplored resource for de novo discovery due to their adaptation to diverse challenging habitats. Bioinformatics analyses of available genomic and transcriptomic data allowed us to identify myticalins, a novel family of antimicrobial peptides (AMPs) from the mussel Mytilus galloprovincialis, and a similar family of AMPs from Modiolus spp., named modiocalins. Their coding sequence encompasses two conserved N-terminal (signal peptide) and C-terminal (propeptide) regions and a hypervariable central cationic region corresponding to the mature peptide. Myticalins are taxonomically restricted to Mytiloida and they can be classified into four subfamilies. These AMPs are subject to considerable interindividual sequence variability and possibly to presence/absence variation. Functional assays performed on selected members of this family indicate a remarkable tissue-specific expression (in gills) and broad spectrum of activity against both Gram-positive and Gram-negative bacteria. Overall, we present the first linear AMPs ever described in marine mussels and confirm the great potential of bioinformatics tools for the de novo discovery of bioactive peptides in non-model organisms.

Proline-rich antimicrobial peptides targeting protein synthesis

Proline-rich antimicrobial peptides (PrAMPs) bind within the exit tunnel of the ribosome and inhibit translation elongation. Structures of ribosome-bound PrAMPs reveal the interactions with ribosomal components and could pave the way for the development of novel peptide-based antimicrobial agents.