Cationic antimicrobial peptides – an update

As-CATH1-6, novel cathelicidins with potent antimicrobial and immunomodulatory properties from Alligator sinensis, play pivotal roles in host antimicrobial immune responses

Crocodilians are regarded as possessing a powerful immune system. However, the composition and action of the crocodilian immune system have remained unclear until now. Cathelicidins, the principal family of host defense peptides, play pivotal roles in vertebrate immune defense against microbial invasions. However, cathelicidins from crocodilians have not been extensively studied to date. In the present study, six novel cathelicidins (As-CATH1-6) were identified and characterized from the endangered Chinese alligator (Alligator sinensis). As-CATH1-6 exhibit no sequence similarity with any of the known cathelicidins. Structure analysis indicated that As-CATH1-3 adopt a random coil secondary conformation, whereas As-CATH4-6 were predicted to mainly adopt an amphipathic α-helix conformation. Among them, As-CATH4-6 exhibited potent, broad-spectrum and rapid antimicrobial activity by inducing the disruption of cell membrane integrity. They also exhibited strong ability to prevent the formation of bacterial biofilms and eradicate preformed biofilms. Furthermore, As-CATH4-6 exhibited potent anti-inflammatory activity by inhibiting the lipopolysaccharide (LPS)-induced production of nitric oxide (NO) and pro-inflammatory cytokines in mouse peritoneal macrophages. They directly neutralized LPS toxicity and therefore inhibited the binding of LPS to the TLR4 receptor and the subsequent activation of inflammatory response pathways. In a peritonitis mice model, As-CATH2-6 provided effective protection against bacterial infection through enhanced immune cell recruitment. In the host Chinese alligator, As-CATH1-6 are mainly expressed in immune organs and epithelial tissues. Bacterial infection significantly enhances their expression, which implies an important role in host anti-infective response. Taken together, the diversity and multiple functions of As-CATH1-6 partially reveal the powerful immune system of the Chinese alligator.

Effect of tacticity-derived topological constraints in bactericidal peptides

Topology is a key element in structure-activity relationship estimation while designing physiologically-active molecular constructs. Peptides may be a preferred choice for therapeutics, principally due to their biocompatibility, low toxicity and predictable metabolism. Peptide design only guarantees functional group constitution by opting specific amino acid sequence, and not their spatial orientation to bind and incite physiological response on chosen targets. This is principally because peptide conformation is subject to external flux, due to the isotactic stereochemistry of the peptide chain. Stereochemical engineering of the peptide main chain offers the possibility of multiplying the structural space of a typical sequence to many orders of magnitude, and limiting the otherwise fluxional non-specific functional group dispensation in space by offering greater conformational rigidity. We put to test, this conceptual possibility already established in theoretical models, by designing amphipathic peptide systems and experimenting with them on Gram-positive, Gram-negative and antibiotic-resistant bacteria. The unusual conformational rigidity and stability of syndiotactic peptides enable them to retain the designed electrostatic environment, while they encounter the membrane surface. All the six designed systems exhibited bactericidal activity, pointing to the utility and specificity of stereo-engineered peptide systems for therapeutic applications. Overall, we hope that this work provides important insights and useful directives in designing novel peptide systems with antimicrobial activity, by expanding the design space, incorporating D-amino acid as an additional design variable.

Role of amphipathicity and hydrophobicity in the balance between hemolysis and peptide-membrane interactions of three related antimicrobial peptides

Cationic antimicrobial peptides (CAMPs) represent important self defense molecules in many organisms, including humans. These peptides have a broad spectrum of activities, killing or neutralizing many Gram-negative and Gram-positive bacteria. The emergence of multidrug resistant microbes has stimulated research on the development of alternative antibiotics. In the search for new antibiotics, cationic antimicrobial peptides (CAMPs) offer a viable alternative to conventional antibiotics, as they physically disrupt the bacterial membranes, leading to lysis of microbial membranes and eventually cell death. In particular, the group of linear α-helical cationic peptides has attracted increasing interest from clinical as well as basic research during the last decade. In this work, we studied the biophysical and microbiological characteristics of three new designed CAMPs. We modified a previously studied CAMP sequence, in order to increase or diminish the hydrophobic face, changing the position of two lysines or replacing three leucines, respectively. These mutations modified the hydrophobic moment of the resulting peptides and allowed us to study the importance of this parameter in the membrane interactions of the peptides. The structural properties of the peptides were also correlated with their membrane-disruptive abilities, antimicrobial activities and hemolysis of human red blood cells.

What is the role of antimicrobial peptides (AMP) in acne vulgaris?

Acne vulgaris is the most common disorder of the pilosebaceous unit leading to inflamed skin characterized by the formation of comedones, papules, pustules and scarring. There is increasing evidence that the abundance of Propionibacterium acnes (P. acnes) in the inflamed acne lesions triggers inflammation. Therefore, in addition to treatment with retinoids, the use of antimicrobial agents has been established as a treatment option for acne. This indicates that antimicrobial mechanisms to control the growth of P. acnes may have an important influence on the severity of inflammatory acne. One import antimicrobial innate defense system comprises the production of antimicrobial peptides (AMP), small molecules with a broad spectrum of antimicrobial activity as well as immunomodulatory properties. Although the role of AMP in acne is still emerging, there is increasing evidence that AMP may be of importance in acne. The aim of this viewpoint is to provide some hypotheses about the potential function of AMP in the pathogenesis of acne and to discuss potential AMP-based therapies for the treatment of acne.

Biological activity of antibacterial peptides matches synergism between electrostatic and non electrostatic forces

Substitution of Ala 108 and Ala 111 in the 107-115 human lysozyme (hLz) fragment results in a 20-fold increased anti-staphylococcal activity while its hemolytic activity becomes significant (30%) at very high concentrations. This analog displays an additional positive charge near the N-terminus (108) and an extra Trp residue at the center of the molecule (111), indicating that this particular amino acid sequence improves its interaction with the bacterial plasma membrane. In order to understand the role of this arrangement in the membrane interaction, studies with model lipid membranes were carried out. The interactions of peptides, 107-115 hLz and the novel analog ([K(108)W(111)]107-115 hLz) with liposomes and lipid monolayers were evaluated by monitoring the changes in the fluorescence of the Trp residues and the variation of the monolayers surface pressure, respectively. Results obtained with both techniques revealed a significant affinity increase of [K(108)W(111)]107-115 hLz for lipids, especially when the membranes containing negatively charged lipids, such as phosphatidylglycerol. However, there is also a significant interaction with zwitterionic lipids, suggesting that other forces in addition to electrostatic interactions are involved in the binding. The analysis of adsorption isotherms and the insertion kinetics suggest that relaxation processes of the membrane structure are involved in the insertion process of novel peptide [K(108)W(111)]107-115 hLz but not in 107-115 hLz, probably by imposing a reorganization of water at the interphases. In this regard, the enhanced activity of peptide [K(108)W(111)]107-115 hLz may be explained by a synergistic effect between the increased electrostatic forces as well as the increased hydrophobic interactions.

High content analysis to determine cytotoxicity of the antimicrobial peptide, melittin and selected structural analogs

Antimicrobial peptides (AMPs) are naturally occurring entities with potential as pharmaceutical candidates and/or food additives. They are present in many organisms including bacteria, insects, fish and mammals. While their antimicrobial activity is equipotent with many commercial antibiotics, current limitations are poor pharmacokinetics, stability and potential toxicology issues. Most elicit antimicrobial action via perturbation of bacterial membranes. Consequently, associated cytotoxicity in human cells is reflected by their capacity to lyse erythrocytes. However, more rigorous toxicological assessment of AMPs is required in order to predict potential failure at a later stage of development. We describe a high-content analysis (HCA) screening protocol recently established for determination and prediction of safety in pharmaceutical drug discovery. HCA is a powerful, multi-parameter bioanalytical tool that amalgamates the actions of fluorescence microscopy with automated cell analysis software in order to understand multiple changes in cellular health. We describe the application of HCA in assessing cytotoxicity of the cytolytic α-helical peptide, melittin, and selected structural analogs. The data shows that structural modification of melittin reduces its cytotoxic action and that HCA is suitable for rapidly identifying cytotoxicity.

The binding of an amphipathic peptide to lipid monolayers at the air/water interface is modulated by the lipid headgroup structure

We used monolayer techniques combined with infrared reflection absorption spectroscopy (IRRAS) to study the behavior of the 18-mer cationic peptide KLA1 (KLAL KLAL KAW KAAL KLA-NH2) at the air/water interface as well as its interaction with lipid films of different composition. The adsorption of the peptide from the subphase to the air/water interface was observed measuring the increase in surface pressure (π) at constant surface area. The binding of the peptide to lipid monolayers was followed by recording the change in lipid area at a constant surface pressure (π = 30 mN m(-1)). At the air/water interface, the peptide initially adopted an α-helix at large surface area per molecule, that is, low surface pressure, but further accumulation of the peptide at the interface induced a conformational change from α-helix to intermolecular β-sheet, driven by intermolecular aggregation. When the peptide was injected into the subphase underneath lipid monolayers, it adsorbed pronouncedly to anionic monolayers containing phosphatidylglycerol forming an α-helix, but not to zwitterionic lipid monolayers. The large change in area observed upon peptide binding suggests that the peptide helix was incorporated into the apolar chain region of the lipids. An apparent partition coefficient of (0.3-1) × 10(6) M(-1) could be calculated for binding to pure POPG monolayers. Significant differences in binding affinity were observed comparing PG/PC with PG/PE monolayers, with the latter showing a higher binding constant. This shows that not only electrostatic and hydrophobic effects but also specific interactions between the headgroups of the lipids and the peptide side chains modulate the binding affinity.

Antibacterials from the sea

The ocean contains a host of macroscopic life in a great microbial soup. Unlike the terrestrial environment, an aqueous environment provides perpetual propinquity and blurs spatial distinctions. Marine organisms are under a persistent threat of infection by resident pathogenic microbes including bacteria, and in response they have engineered complex organic compounds with antibacterial activity from a diverse set of biological precursors. The diluting effect of the ocean drives the construction of potent molecules that are stable to harsh salty conditions. Members of each class of metabolite-ribosomal and non-ribosomal peptides, alkaloids, polyketides, and terpenes-have been shown to exhibit antibacterial activity. The sophistication and diversity of these metabolites points to the ingenuity and flexibility of biosynthetic processes in Nature. Compared with their terrestrial counterparts, antibacterial marine natural products have received much less attention. Thus, a concerted effort to discover new antibacterials from marine sources has the potential to contribute significantly to the treatment of the ever increasing drug-resistant infectious diseases.

Structural and antimicrobial properties of human pre-elafin/trappin-2 and derived peptides against Pseudomonas aeruginosa

Background

Pre-elafin/trappin-2 is a human innate defense molecule initially described as a potent inhibitor of neutrophil elastase. The full-length protein as well as the N-terminal "cementoin" and C-terminal "elafin" domains were also shown to possess broad antimicrobial activity, namely against the opportunistic pathogen P. aeruginosa. The mode of action of these peptides has, however, yet to be fully elucidated. Both domains of pre-elafin/trappin-2 are polycationic, but only the structure of the elafin domain is currently known. The aim of the present study was to determine the secondary structures of the cementoin domain and to characterize the antibacterial properties of these peptides against P. aeruginosa.

Results

We show here that the cementoin domain adopts an α-helical conformation both by circular dichroism and nuclear magnetic resonance analyses in the presence of membrane mimetics, a characteristic shared with a large number of linear polycationic antimicrobial peptides. However, pre-elafin/trappin-2 and its domains display only weak lytic properties, as assessed by scanning electron micrography, outer and inner membrane depolarization studies with P. aeruginosa and leakage of liposome-entrapped calcein. Confocal microscopy of fluorescein-labeled pre-elafin/trappin-2 suggests that this protein possesses the ability to translocate across membranes. This correlates with the finding that pre-elafin/trappin-2 and elafin bind to DNA in vitro and attenuate the expression of some P. aeruginosa virulence factors, namely the biofilm formation and the secretion of pyoverdine.

Conclusions

The N-terminal cementoin domain adopts α-helical secondary structures in a membrane mimetic environment, which is common in antimicrobial peptides. However, unlike numerous linear polycationic antimicrobial peptides, membrane disruption does not appear to be the main function of either cementoin, elafin or full-length pre-elafin/trappin-2 against P. aeruginosa. Our results rather suggest that pre-elafin/trappin-2 and elafin, but not cementoin, possess the ability to modulate the expression of some P.aeruginosa virulence factors, possibly through acting on intracellular targets.

Thermodynamics of RTA3 peptide binding to membranes and consequences for antimicrobial activity

RTA3 is an α-helical, amphipathic peptide with broad-spectrum activity against Gram-negative bacteria and low mammalian cell toxicity. RTA3 contains a cysteine residue, replacement of which with an alanine or serine (RTA3-C15S) virtually abolishes antimicrobial activity. Much of the activity of RTA3 can be recovered in RTA3-C15L, indicating that the C15 residue functions largely as a bulky hydrophobic side chain promoting target cell membrane interactions. The poorly active RTA3-C15S is a useful variant for assessing the mechanistic aspects of RTA3 activity. Binding and membrane perturbation in vesicles containing different proportions of negative surface charge are analyzed in terms of amino acid-specific free energy contributions to interfacial binding, which likely underlie variations in antimicrobial activity amongst RTA3 variants. Comparison with published free energy scales indicates that the reduced electrostatic contribution to binding to membranes having reduced negative surface charge can be compensated in RTA3 (but not RTA3-C15S) by a slightly deeper insertion of the C-terminus of the peptide to maximize hydrophobic contributions to binding. Analysis of inner membrane (IM)- and outer membrane (OM)-selective permeabilization of Escherichiacoli demonstrates a broad similarity between peptide effects on vesicles with low negative surface charge (20% negatively charged lipids), E.coli membrane perturbation, and antimicrobial activity, supporting a role for membrane perturbation in the killing mechanism of RTA3. The results demonstrate that large variations in antimicrobial activity on subtle changes in amino acid sequence in helical amphipathic peptides can be rationalized in terms of the thermodynamics of peptide binding to membranes, allowing a more systematic understanding of antimicrobial activity in these peptides.

Peptide induced demixing in PG/PE lipid mixtures: a mechanism for the specificity of antimicrobial peptides towards bacterial membranes?

Antimicrobial peptides attract a lot of interest as potential candidates to overcome bacterial resistance. So far, nearly all the proposed scenarios for their mechanism of action are associated with perforating and breaking down bacterial membranes after a binding process. In this study we obtained additional information on peptide induced demixing of bacterial membranes as a possible mechanism of specificity of antimicrobial peptides. We used DSC and FT-IR to study the influence of a linear and cyclic arginine- and tryptophan-rich antimicrobial peptide having the same sequence (RRWWRF) on the thermotropic phase transitions of lipid membranes. The cyclization of the peptide was found to enhance its antimicrobial activity and selectivity ( Dathe, M. Nikolenko, H. Klose, J. Bienert, M. Biochemistry 43 (2004) 9140-9150). A particular preference of the binding of the peptides to DPPG headgroups compared to other headgroups of negatively charged phospholipids, namely DMPA, DPPS and cardiolipin was observed. The main transition temperature of DPPG bilayers was considerably decreased by the bound peptides. The peptides caused a substantial down-shift of the transition of DPPG/DMPC. In contrast, they induced a demixing in DPPG/DPPE bilayers and led to the appearance of two peaks in the DSC curves indicating a DPPG-peptide-enriched domain and a DPPE-enriched domain. These results could be confirmed by FT-IR-spectroscopic measurements. We therefore propose that the observed peptide-induced lipid demixing in PG/PE-membranes could be a further specific effect of the antimicrobial peptides operating only on bacterial membranes, which contain appreciable amounts of PE and PG, and which could in principle also occur in liquid-crystalline membranes.

Review of surfactin chemical properties and the potential biomedical applications

Surfactin, a highly powerful biosurfactant produced by various strains of the genus Bacillus, exhibits antibacterial, antiviral, antitumor and hemolytic action. This anionic cyclic lipopeptide is constituted by a heptapeptide interlinked with a β-hydroxy fatty acid. Due to its amhipathic nature surfactin incorporates into the phospholipid bilayer and induces permeabilization and perturbation of target cells. The rising antibiotic resistance as well as a number of remarkable surfactin activities shows that it deserves special interest and is considered as a candidate compound for combating several health related issues. In this review, the current state of knowledge of surfactin properties, biomedical potential and limitations for its application is presented.

Beta-sheet pore-forming peptides selected from a rational combinatorial library: mechanism of pore formation in lipid vesicles and activity in biological membranes

In a previous report we described the selection of potent, beta-sheet pore-forming peptides from a combinatorial library designed to mimic membrane-spanning beta-hairpins (Rausch, J. M., Marks, J. R., and Wimley, W. C. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 10511-10515). Here, we characterize their mechanism of action and compare the structure-function relationships in lipid vesicles to their activity in biological membranes. The pore-forming peptides bind to membrane interfaces and self-assemble into beta-sheets that cause a transient burst of graded leakage across the bilayers. Despite the continued presence of the structured peptides in the bilayer, at most peptide concentrations leakage is incomplete and ceases quickly after peptide addition with a deactivation half-time of several minutes. Molecules up to 3,000 Da escape from the transient pores, but much larger molecules do not. Fluorescence spectroscopy and quenching showed that the peptides reside mainly on the bilayer surface and are partially exposed to water, rather than in a membrane-spanning state. The "carpet" or "sinking raft" model of peptide pore formation offers a viable explanation for our observations and suggests that the selected pore-formers function with a mechanism that is similar to the natural pore-forming antimicrobial peptides. We therefore also characterized the antimicrobial and cytotoxic activity of these peptides. All peptides studied, including non-pore-formers, had sterilizing antimicrobial activity against at least some microbes, and most have low activity against mammalian cell membranes. Thus, the structure-function relationships that were apparent in the vesicle systems are similar to, but do not correlate completely with, the activity of the same peptides in biological membranes. However, of the peptides tested, only the pore-formers selected in the high-throughput screen have potent, broad-spectrum sterilizing activity against Gram-positive and Gram-negative bacteria as well as against fungi, while having only small lytic effects on human cells.

β-Lactoglobulin as source of bioactive peptides

Beta-lactoglobulin (beta-Lg) is currently an important source of biologically active peptides. These peptides are inactive within the sequence of the precursor protein, but they can be released by in vivo or in vitro enzymatic proteolysis. Once released, these peptides play important roles in the human health, including antihypertensive, antioxidant and antimicrobial activities as well as opioid-like features and ability to decrease the body-cholesterol levels. Bioactive peptides derived from beta-Lg are currently a point of intensive research. Their structure, biological significance and mechanism of action are briefly presented and discussed in this review.

Effects of topology, length, and charge on the activity of a kininogen-derived peptide on lipid membranes and bacteria

Effects of topology, length, and charge on peptide interactions with lipid bilayers was investigated for variants of the human kininogen-derived peptide HKH20 (HKHGHGHGKHKNKGKKNGKH) by ellipsometry, CD, fluorescence spectroscopy, and z-potential measurements. The peptides display primarily random coil conformation in buffer and at lipid bilayers, and their lipid interaction is dominated by electrostatics, the latter evidenced by higher peptide adsorption and resulting membrane rupture for an anionic than for a zwitterionic membrane, as well as by strongly reduced adsorption and membrane rupture at high ionic strength. At sufficiently high peptide charge density, however, electrostatic interactions contribute to reducing the peptide adsorption and membrane defect formation. Truncating HKH20 into overlapping 10 amino acid peptides resulted in essentially eliminated membrane rupture and in a reduced amount peptide charges pinned at the lipid bilayer. Finally, cyclic HKH20 was found to be less efficient than the linear peptide in causing liposome rupture, partly due to a lower adsorption. Analogous results were found regarding bactericidal effects.

Interaction of the gelsolin-derived antibacterial PBP 10 peptide with lipid bilayers and cell membranes

PBP 10, an antibacterial, cell membrane-permeant rhodamine B-conjugated peptide derived from the polyphosphoinositide binding site of gelsolin, interacts selectively with both lipopolysaccharides (LPS) and lipoteichoic acid (LTA), the distinct components of gram-negative and gram-positive bacteria, respectively. Isolated LPS and LTA decrease the antimicrobial activities of PBP 10, as well as other antimicrobial peptides, such as cathelicidin-LL37 (LL37) and mellitin. In an effort to elucidate the mechanism of bacterial killing by PBP 10, we compared its effects on artificial lipid bilayers and eukaryotic cell membranes with the actions of the mellitin, magainin II, and LL37 peptides. This study reveals that pore formation is unlikely to be involved in PBP 10-mediated membrane destabilization. We also investigated the effects of these peptides on platelets and red blood cells (RBCs). Comparison of these antimicrobial peptides shows that only mellitin has a toxic effect on platelets and RBCs in a concentration range concomitant with its bactericidal activity. The hemolytic activities of the PBP 10 and LL37 peptides significantly increase when RBCs are osmotically swollen in hypotonic solution, indicating that these antibacterial peptides may take advantage of the more extended form of bacterial membranes in exerting their killing activities. Additionally, we found that LL37 hemolytic activity was much higher when RBCs were induced to expose phosphatidylserine to the external leaflet of their plasma membranes. This finding suggests that asymmetrical distribution of phospholipids in the external membranes of eukaryotic cells may represent an important factor in determining the specificity of antibacterial peptides for targeting bacteria rather than eukaryotic cells.

Treatment and prevention of Staphylococcus epidermidis experimental biomaterial-associated infection by bactericidal peptide 2

Biomaterial-associated infections (BAI) are the major cause of failure of indwelling medical devices and are predominantly caused by staphylococci, especially Staphylococcus epidermidis. We investigated the in vitro microbicidal activity of the synthetic antimicrobial peptide bactericidal peptide 2 (BP2) and its efficacy in a murine model of S. epidermidis BAI. BP2 showed potent microbicidal activity at micromolar concentrations against a broad spectrum of microorganisms, including antibiotic-resistant bacteria. The staphylocidal activity of BP2 was not affected by physiological salt concentrations and was only slightly affected by the presence of human plasma. In the BAI model, injection of BP2 (5 mg/kg of body weight) 1 h after challenge with S. epidermidis resulted in an 80% reduction in the number of culture-positive implants and a 100-fold reduction in survival of S. epidermidis in peri-implant tissue at 24 h postchallenge. When BP2 was injected along implants 3 h prior to bacterial challenge, the median numbers of CFU cultured from biomaterial implants and peri-implant tissue were reduced by 85% and 90%, respectively. In conclusion, BP2 has potent, broad-spectrum in vitro microbicidal activity and showed potent in vivo activity in a murine model of S. epidermidis biomaterial-associated infection.

Bacterial killing by heparin-binding peptides from PRELP and thrombospondin

Antimicrobial peptides (AMP) are effector molecules of the innate immune system. A cross-functionality exists between AMPs and heparin-binding peptides. Here, we show that the peptides QPTRRPRPGTGPGRRPRPRPRP (QPT22), derived from proline arginine-rich end leucine-rich repeat protein (PRELP) and KRFKQDGGWSHWSPWSS (KRF17) from thrombospondin exert antimicrobial activities against the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, as well as against the Gram-positive Bacillus subtilis. Analysis by fluorescence microscopy demonstrated that QPT22 and KRF17 bind to bacterial membranes. Particularly QPT22 was found to induce membrane leakage of liposomes. The peptides displayed no hemolytic activities, nor did they exert membrane permeabilising effects on human epithelial cells. Additional peptides derived from heparin-binding regions of laminin, vitronectin, and fibronectin exerted similar antibacterial effects. Several peptides also showed activity against Staphylococcus aureus. Thus, the data disclose a novel antimicrobial activity of heparin-binding regions of matrix glycoproteins. The findings can be utilized in the development of novel AMPs for therapeutic use.

Antimicrobial peptides: therapeutic potential

A significant component of the innate immune system of a wide variety of animals and plants is arbitrated by cationic host defence peptides. In man, these peptides, in addition to exhibiting a direct antimicrobial activity, seems to provide a range of non-antimicrobial bioactivities related to defence, inflammation and wound healing. Despite the fact that such peptides have so far failed to reach the market, there are continued initiatives to advance such potential therapeutics to, and through, the clinic. The reasons behind such initiatives include: reduced manufacturing costs for peptides; allowing entry into therapeutic areas previously inaccessible due to cost; the continued identification of previously unknown bioactivities of such peptides; and the resurgence of interest in peptide therapeutics. As a result, clinical programmes based on cationic host defence peptides exist in the areas of infection, dermatology, cancer and inflammation. The probability of clinical success for host defence peptide-based therapeutics is on the rise as options for a wider range of clinical indications emerge.

Investigation of the cytotoxicity of eukaryotic and prokaryotic antimicrobial peptides in intestinal epithelial cells in vitro

Antimicrobial peptides (AMPs) are a diverse group of proteinaceous compounds ranging in size, complexity and antimicrobial spectrum. The activity of AMPs against gut pathogens warrants the study of the interaction of AMPs with the mammalian gastrointestinal tract. In particular, the investigation of the in vitro cytotoxicity of these peptides is critical before they can be considered in clinical infections. The cytotoxicity of gallidermin, nisin A, natural magainin peptides, and melittin was investigated in two gastrointestinal cell models (HT29 and Caco-2) with the MTT conversion assay, neutral red dye uptake assay and compared with that of vancomycin. The hemolytic activities were also investigated in sheep erythrocytes and the effect of AMPs on paracellular permeability was examined by transepithelial resistance (TEER) and TEM. Gallidermin was the least cytotoxic AMP followed by nisin A, magainin I, magainin II and melittin. Melittin and nisin were the only peptides to result in significant hemolysis. However, while nisin caused hemolysis at concentrations which were 1000-fold higher than those required for antimicrobial activity, melittin was hemolytic at concentrations in the same order of magnitude as its antimicrobial activity. Melittin was the only AMP to affect paracellular permeability. Long term melittin treatment also resulted in loss of microvilli, an increase in cell debris and destruction of intestinal tight junctions and cell-cell adhesion. Gallidermin shows most promise as a therapeutic agent, with relatively low cytotoxicity and potent antimicrobial activities. Melittin, while showing little potential as an antimicrobial agent, may have potential in delivery of poorly bioavailable drugs.

Mode of action of the new antibiotic for Gram-positive pathogens daptomycin: comparison with cationic antimicrobial peptides and lipopeptides

With the steady rise in the number of antibiotic-resistant Gram-positive pathogens, it has become increasingly important to find new antibacterial agents which are highly active and have novel and diversified mechanisms of action. Two classes will be discussed here: the cationic antimicrobial peptides, which are amphiphilic in nature, targeting membranes and increasing their permeability; and lipopeptides, which consist of linear or cyclic peptides with an N-terminus that is acylated with a fatty acid side chain. One member of the cyclic lipopeptide family, the anionic molecule daptomycin, has been extensively studied and is the major focus of this review. Models will be presented on its mode of action and comparisons will be made to the known modes of action of cationic antimicrobial peptides and other lipopeptides.

The historical delivery of antibiotics from microbial natural products—Can history repeat?

Microbial natural products are the origin of most of the antibiotics on the market today. However, research in antibiotics and natural products has declined significantly during the last decade as a consequence of diverse factors, among which the lack of interest of industry in the field and the strong competition from collections of synthetic compounds as source of drug leads. As a consequence, there is an alarming scarcity of new antibiotic classes in the pipelines of the pharmaceutical industry. Still, microbial natural products remain the most promising source of novel antibiotics, although new approaches are required to improve the efficiency of the discovery process. The impact of microbial biodiversity, the influence of growth conditions on the production of secondary metabolites, the choice of the best approach at the screening step and the challenges faced during the isolation and identification of the active compounds are examined in this review as the critical factors contributing to success in the effort of antibiotic discovery from microbial natural products.

Cationic host defense (antimicrobial) peptides

Members of the cationic host defense (antimicrobial) peptide family are widely distributed in nature, existing in organisms from insects to plants to mammals and non-mammalian vertebrates. Although many demonstrate direct antimicrobial activity against bacteria, fungi, eukaryotic parasites and/or viruses, it has been established that cationic peptides have a key modulatory role in the innate immune response. More recent evidence suggests that host defense peptides are effective adjuvants, are synergistic with other immune effectors, polarize the adaptive response, and support wound healing. In addition, the mechanisms of action are being unraveled, which support more effective implementation of derivatives of these endogenous peptides as therapeutic agents.

Oral tryptic casein hydrolysate enhances phagocytosis by mouse peritoneal and blood phagocytic cells but fails to prevent induced inflammation

Mouse experiments were conducted in order to find whether oral application of tryptic casein hydrolysate (TCH) results in enhancement of phagocytosing capacity of murine phagocytic cells as well as whether such application might be of use for prevention of inflammatory processes. Phagocytosing capacity of phagocytic cells of mice that received oral TCH once daily in a dose of 1.0 mg/g body weight dissolved in 0.5 ml of distilled water for five successive days was significantly higher (p < 0.05) than that of mice given equivalent volumes of distilled water, with a phagocytosing capacity enhancement index being 1.39 and 1.34 regarding peritoneal macrophages and blood phagocytic cells, respectively. Taken on the other hand, the immunostimulatory effects of oral TCH were found to be not enough to prevent mice from inflammation that was induced experimentally using acute (paw edema) and contact hypersensitivity models. A possibility for development of food protein enzymatic hydrolysates as antimicrobial immunostimulants acting through improvement of phagocytic cell functioning is discussed.

Friend or foe? Antimicrobial peptides trigger pathogen virulence

In an age of antibiotic-resistant pathogens, antimicrobial peptides have emerged as novel therapeutics hailed for their bactericidal and immunomodulatory properties. However, a recent paper by Bader et al. demonstrates that these molecules also trigger bacteria to arm themselves against host immune responses. The authors show that the two-component regulatory system PhoP-PhoQ of Salmonella is activated not only in cation-deficient environments as previously thought, but also by binding to antimicrobial peptides, thus promoting gene transcription necessary for Salmonella survival within the host. Thus, the antimicrobial peptide might be a double-edged sword, promoting antibacterial immunity while simultaneously triggering pathogen virulence.

Systemic antibacterial activity of novel synthetic cyclic peptides

Cyclic peptides with an even number of alternating d,l-alpha-amino acid residues are known to self-assemble into organic nanotubes. Such peptides previously have been shown to be stable upon protease treatment, membrane active, and bactericidal and to exert antimicrobial activity against Staphylococcus aureus and other gram-positive bacteria. The present report describes the in vitro and in vivo pharmacology of selected members of this cyclic peptide family. The intravenous (i.v.) efficacy of six compounds with MICs of less than 12 microg/ml was tested in peritonitis and neutropenic-mouse thigh infection models. Four of the six peptides were efficacious in vivo, with 50% effective doses in the peritonitis model ranging between 4.0 and 6.7 mg/kg against methicillin-sensitive S. aureus (MSSA). In the thigh infection model, the four peptides reduced the bacterial load 2.1 to 3.0 log units following administration of an 8-mg/kg i.v. dose. Activity against methicillin-resistant S. aureus was similar to MSSA. The murine pharmacokinetic profile of each compound was determined following i.v. bolus injection. Interestingly, those compounds with poor efficacy in vivo displayed a significantly lower maximum concentration of the drug in serum and a higher volume of distribution at steady state than compounds with good therapeutic properties. S. aureus was unable to easily develop spontaneous resistance upon prolonged exposure to the peptides at sublethal concentrations, in agreement with the proposed interaction with multiple components of the bacterial membrane canopy. Although additional structure-activity relationship studies are required to improve the therapeutic window of this class of antimicrobial peptides, our results suggest that these amphipathic cyclic d,l-alpha-peptides have potential for systemic administration and treatment of otherwise antibiotic-resistant infections.

Properties and structure–activity studies of cyclic β-hairpin peptidomimetics based on the cationic antimicrobial peptide protegrin I

The properties and structure-activity relationships (SAR) of a macrocyclic analogue of porcine protegrin I (PG-I) have been investigated. The lead compound, having the sequence cyclo-(-Leu-Arg-Leu-Lys-Lys-Arg-Arg-Trp-Lys-Tyr-Arg-Val-d-Pro-Pro-), shows antimicrobial activity against Gram-positive and -negative bacteria, but a much lower haemolytic activity and a much reduced ability to induce dye release from phosphatidylcholine/phosphatidylglycerol liposomes, when compared to PG-I. The enantiomeric form of the lead peptide shows comparable antimicrobial activity, a property shared with other cationic antimicrobial peptides acting on cell membranes. SAR studies involving the synthesis and biological profiling of over 100 single site substituted analogues, showed that the antimicrobial activity was tolerant to a large number of the substitutions tested. Some analogues showed slightly improved antimicrobial activities (2-4-fold lowering of MICs), whereas other substitutions caused large increases in haemolytic activity on human red blood cells.

Avian antimicrobial peptides: the defense role of β-defensins

Avian antimicrobial peptides, classified as beta-defensins, have been identified from bloods of chicken, turkey, and ostrich; epithelial cells of chicken and turkey; and king penguin stomach contents. Beta-defensins are a family of antimicrobial peptides characterized by six cysteine residues forming beta-defensin motifs that are also found in bovine, ovine, pig, and human. These peptides are active against a wide range of microorganisms including Gram-positive and Gram-negative bacteria, fungi, and yeast. Analysis of evolutionary relationships of vertebrate beta-defensins showed that there might be a common ancestral gene between avian and other mammalian peptides. This ancient gene may have been passed down and evolved from species older than the oldest living birds, forming a beta-defensin-like precursor molecule. This review describes potential applications of these peptides in health care products.