Role of membranes in the activities of antimicrobial cationic peptides

Structural studies and model membrane interactions of two peptides derived from bovine lactoferricin

The powerful antimicrobial properties of bovine lactoferricin (LfcinB) make it attractive for the development of new antimicrobial agents. An 11-residue linear peptide portion of LfcinB has been reported to have similar antimicrobial activity to lactoferricin itself, but with lower hemolytic activity. The membrane-binding and membrane-perturbing properties of this peptide were studied together with an amidated synthetic version with an added disulfide bond, which was designed to confer increased stability and possibly activity. The antimicrobial and cytotoxic properties of the peptides were measured against Staphylococcus aureus and Escherichia coli and by hemolysis assays. The peptides were also tested in an anti-cancer assay against neuroblastoma cell lines. Vesicle disruption caused by these LfcinB derivatives was studied using the fluorescent reporter molecule calcein. The extent of burial of the two Trp residues in membrane mimetic environments were quantitated by fluorescence. Finally, the solution NMR structures of the peptides bound to SDS micelles were determined to provide insight into their membrane bound state. The cyclic peptide was found to have greater antimicrobial potency than its linear counterpart. Consistent with this property, the two Trp residues of the modified peptide were suggested to be embedded deeper into the membrane. Although both peptides adopt an amphipathic structure without any regular alpha-helical or beta-sheet conformation, the 3D-structures revealed a clearer partitioning of the cationic and hydrophobic faces for the cyclic peptide.

Antimicrobial activity of novel dendrimeric peptides obtained by phage display selection and rational modification

A large 10-mer phage peptide library was panned against whole Escherichia coli cells, and an antimicrobial peptide (QEKIRVRLSA) was selected. The peptide was synthesized in monomeric and dendrimeric tetrabranched form (multiple antigen peptide [MAP]), which generally allows a dramatic increase of peptide stability to peptidases and proteases. The antibacterial activity of the dendrimeric peptide against E. coli was much higher than that of the monomeric form. Modification of the original sequence, by residue substitution or sequence shortening, produced three different MAPs, M4 (QAKIRVRLSA), M5 (KIRVRLSA), and M6 (QKKIRVRLSA) with enhanced stability to natural degradation and antimicrobial activity against a large panel of gram-negative bacteria. The MICs of the most potent peptide, M6, were as low as 4 to 8 microg/ml against recent clinical isolates of multidrug-resistant Pseudomonas aeruginosa and members of the Enterobacteriaceae. The same dendrimeric peptides showed high stability to blood proteases, low hemolytic activity, and low cytotoxic effects on eukaryotic cells, making them promising candidates for the development of new antibacterial drugs.

High-throughput generation of small antibacterial peptides with improved activity

Cationic antimicrobial peptides are able to kill a broad variety of Gram-negative and Gram positive bacteria and thus are good candidates for a new generation of antibiotics to treat multidrug-resistant bacteria. Here we describe a high-throughput method to screen large numbers of peptides for improved antimicrobial activity. The method relies on peptide synthesis on a cellulose support and a Pseudomonas aeruginosa strain that constitutively expresses bacterial luciferase. A complete substitution library of 12-amino-acid peptides based on a linearized variant (RLARIVVIRVAR-NH(2)) of the bovine peptide bactenecin was screened and used to determine which substitutions at each position of the peptide chain improved activity. By combining the most favorable substitutions, we designed optimized 12-mer peptides showing broad spectrum activities with minimal inhibitory concentrations (MIC) as low as 0.5 microg/ml against Escherichia coli. Similarly, we generated an 8-mer substituted peptide that showed broad spectrum activity, with an MIC of 2 microg/ml, against E. coli and Staphylococcus aureus.

Mechanisms of action of newer antibiotics for Gram-positive pathogens

Certain Gram-positive bacteria, including meticillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and quinolone-resistant Streptococcus pneumoniae have achieved the status of "superbugs", in that there are few or no antibiotics available for therapy against these pathogens. Only a few classes of novel antibiotics have been introduced in the past 40 years, and all since 1999, including the streptogramin combination quinupristin/dalfopristin (Synercid), the oxazolidinone linezolid, and the lipopeptide daptomycin. This review discusses the mechanisms of antibiotic action against Gram-positive pathogens, and resistance counter-mechanisms developed by Gram-positive bacteria, with emphasis on the newer agents.

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.

A Molecular Mechanism for Lipopolysaccharide Protection of Gram-negative Bacteria from Antimicrobial Peptides

Cationic antimicrobial peptides serve as the first chemical barrier between all organisms and microbes. One of their main targets is the cytoplasmic membrane of the microorganisms. However, it is not yet clear why some peptides are active against one particular bacterial strain but not against others. Recent studies have suggested that the lipopolysaccharide (LPS) outer membrane is the first protective layer that actually controls peptide binding and insertion into Gram-negative bacteria. In order to shed light on these interactions, we synthesized and investigated a 12-mer amphipathic alpha-helical antimicrobial peptide (K(5)L(7)) and its diastereomer (4D-K(5)L(7)) (containing four d-amino acids). Interestingly, although both peptides strongly bind LPS bilayers and depolarize bacterial cytoplasmic membranes, only the diastereomer kills Gram-negative bacteria. Attenuated total reflectance Fourier transform infrared, CD, and surface plasmon resonance spectroscopies revealed that only the diastereomer penetrates the LPS layer. In contrast, K(5)L(7) binds cooperatively to the polysaccharide chain and the outer phosphate groups. As a result, the self-associated K(5)L(7) is unable to traverse through the tightly packed LPS molecules, revealed by epifluorescence studies with LPS giant unilamellar vesicles. The difference in the peptides' modes of binding is further demonstrated by the ability of the diastereomer to induce LPS miscellization, as shown by transmission electron microscopy. In addition to increasing our understanding of the molecular basis of the protection of bacteria by LPS, this study presents a potential strategy to overcome resistance by LPS, and it should help in the design of antimicrobial peptides for future therapeutic purposes.

Rational design of alpha-helical antimicrobial peptides with enhanced activities and specificity/therapeutic index

In the present study, the 26-residue peptide sequence Ac-KWKSFLKTFKSAVKTVLHTALKAISS-amide (V681) was utilized as the framework to study the effects of peptide hydrophobicity/hydrophilicity, amphipathicity, and helicity (induced by single amino acid substitutions in the center of the polar and nonpolar faces of the amphipathic helix) on biological activities. The peptide analogs were also studied by temperature profiling in reversed-phase high performance liquid chromatography, from 5 to 80 degrees C, to evaluate the self-associating ability of the molecules in solution, another important parameter in understanding peptide antimicrobial and hemolytic activities. A higher ability to self-associate in solution was correlated with weaker antimicrobial activity and stronger hemolytic activity of the peptides. Biological studies showed that strong hemolytic activity of the peptides generally correlated with high hydrophobicity, high amphipathicity, and high helicity. In most cases, the D-amino acid substituted peptides possessed an enhanced average antimicrobial activity compared with L-diastereomers. The therapeutic index of V681 was improved 90- and 23-fold against Gram-negative and Gram-positive bacteria, respectively. By simply replacing the central hydrophobic or hydrophilic amino acid residue on the nonpolar or the polar face of these amphipathic derivatives of V681 with a series of selected D-/L-amino acids, we demonstrated that this method has excellent potential for the rational design of antimicrobial peptides with enhanced activities.

Synthesis and biological evaluation of gramicidin S dimers

The design and synthesis of analogues of the cyclic beta-sheet gramicidin S (GS), having additional functionalities in their turn regions, is reported. The monomeric GS analogues were transformed into dimers and their activities towards biological membranes, through antimicriobial and hemolytic assays, were evaluated. Finally, conductivity measurements have been performed to elucidate ion channel forming properties.

Mammalian defensins: structures and mechanism of antibiotic activity

Antibiotic peptides are important effector molecules in host-parasite interactions throughout the living world. In vertebrates, they function in first-line host defense by antagonizing a wide range of microbes including bacteria, fungi, and enveloped viruses. The antibiotic activity is thought to be based on their cationic, amphipathic nature, which enables the peptides to impair vital membrane functions. Molecular details for such activities have been elaborated with model membranes; however, there is increasing evidence that these models may not reflect the complex processes involved in the killing of microbes. For example, the overall killing activity of the bacterial peptide antibiotic nisin is composed of independent activities such as the formation of target-mediated pores, inhibition of cell-wall biosynthesis, formation of nontargeted pores, and induction of autolysis. We studied the molecular modes of action of human defense peptides and tried to determine whether they impair membrane functions primarily and whether additional antibiotic activities may be found. We compared killing kinetics, solute efflux kinetics, membrane-depolarization assays, and macromolecular biosynthesis assays and used several strains of Gram-positive cocci as test strains. We found that membrane depolarization contributes to rapid killing of a significant fraction of target cells within a bacterial culture. However, substantial subpopulations appear to survive the primary effects on the membrane. Depending on individual strains and species and peptide concentrations, such subpopulations may resume growth or be killed through additional activities of the peptides. Such activities can include the activation of cell-wall lytic enzymes, which appears of particular importance for killing of staphylococcal strains.

Effects of the antimicrobial peptide temporin L on cell morphology, membrane permeability and viability of Escherichia coli

Antimicrobial peptides are produced by all organisms in response to microbial invasion and are considered as promising candidates for future antibiotics. There is a wealth of evidence that many of them interact and increase the permeability of bacterial membranes as part of their killing mechanism. However, it is not clear whether this is the lethal step. To address this issue, we studied the interaction of the antimicrobial peptide temporin L with Escherichia coli by using fluorescence, confocal and electron microscopy. The peptide previously isolated from skin secretions of the frog Rana temporaria has the sequence FVQWFSKFLGRIL-NH2. With regard to fluorescence microscopy, we applied, for the first time, a triple-staining method based on the fluorochromes 5-cyano-2,3-ditolyl tetrazolium chloride, 4',6-diamidino-2-phenylindole and FITC. This technique enabled us to identify, in the same sample, both living and total cells, as well as bacteria with altered membrane permeability. These results reveal that temporin L increases the permeability of the bacterial inner membrane in a dose-dependent manner without destroying the cell's integrity. At low peptide concentrations, the inner membrane becomes permeable to small molecules but does not allow the killing of bacteria. However, at high peptide concentrations, larger molecules, but not DNA, leak out, which results in cell death. Very interestingly, in contrast with many antimicrobial peptides, temporin L does not lyse E. coli cells but rather forms ghost-like bacteria, as observed by scanning and transmission electron microscopy. Besides shedding light on the mode of action of temporin L and possibly that of other antimicrobial peptides, the present study demonstrates the advantage of using the triple-fluorescence approach combined with microscopical techniques to explore the mechanism of membrane-active peptides in general.

A novel carrier molecule for high-level expression of peptide antibiotics in Escherichia coli

Peptide antibiotics are often hard to express in engineered bacteria at high level. According to the properties of peptide antibiotics, a heterologous protein PaP3.30, encoded by ORF30 of Pseudomonas aeruginosa bacteriophage PaP3, was selected as a carrier molecule. The gene of the carrier molecule was constructed into the plasmid pQE-32 to give rise to the vector pQE-PaP30 for expression of peptide antibiotics in Escherichia coli. A his-tagged fusion protein was genetically constructed with a peptide antibiotic at its carboxy terminus. The novel carrier molecule was used for high-level expression of six peptide antibiotics with different sizes and isoelectric points in E. coli, which are hPAB-beta, MSI-78, Melletin, hBD-1, Cecropin A, and an ovine anion peptide. And further, one of six peptide antibiotics, hPAB-beta (an analog of a human peptide antibiotic), was taken as an example for studies of recovery of interesting products from the fusion partner, purification and antimicrobial activity evaluation. The results indicated that the expressed fusion protein existed as an inclusion body in the cytoplasm and the expression amounts of six peptide antibiotic fusions are all higher than 34% of the total cell protein. The expression products could be easily purified by Ni-NTA chromatography. Cyanogen bromide was used to cut at the methionine linker between the carrier and hPAB-beta peptide. hPAB-beta was recovered from the fusion partner and purified to homogeneity with High S cation-exchange and Bio-gel P6 gel chromatography. The bactericidal activities of the purified recombinant hPAB-beta against P. aeruginosa are 31-64 microg/ml, and against Staphylococcus aureus are > or = 128 microg/ml, being comparable to that of the chemical synthesis peptide. These results show that the carrier molecule can result in high-level expression of peptide antibiotics, and expression products can be easily recovered from their fusion partner and retain their bioactivity.

Structural Transitions as Determinants of the Action of the Calcium-Dependent Antibiotic Daptomycin

Daptomycin is a cyclic anionic lipopeptide antibiotic recently approved for the treatment of complicated skin infections (Cubicin). Its function is dependent on calcium (as Ca2+). Circular dichroism spectroscopy indicated that daptomycin experienced two structural transitions: a transition upon interaction of daptomycin with Ca2+, and a further transition upon interaction with Ca2+ and the bacterial acidic phospholipid, phosphatidyl glycerol. The Ca2+-dependent insertion of daptomycin into model membranes promoted mild and more pronounced perturbations as assessed by the increase of lipid flip-flop and membrane leakage, respectively. The NMR structure of daptomycin indicated that Ca2+ induced a conformational change in daptomycin that increased its amphipathicity. These results are consistent with the hypothesis that the association of Ca2+ with daptomycin permits it to interact with bacterial membranes with effects that are similar to those of the cationic antimicrobial peptides.

Structure-activity relationships for the beta-hairpin cationic antimicrobial peptide polyphemusin I

The solution structure of polyphemusin I was determined using (1)H-NMR spectroscopy. Polyphemusin I was found to be an amphipathic, beta-hairpin connected by a type I' beta-turn. The 17 low-energy structures aligned very well over the beta-sheet region while both termini were poorly defined due in part to a hinge-like region centred in the molecule about arginine residues 6 and 16. Conversely, a linear analogue, PM1-S, with all cysteines simultaneously replaced with serine was found to be dynamic in nature, and a lack of medium and long-range NOEs indicated that this molecule displayed no favoured conformation. Circular dichroism (CD) spectroscopy confirmed that in solution, 50% trifluoroethanol (TFE) and in the presence of liposomes, PM1-S remained unstructured. The antimicrobial activity of PM1-S was found to be 4- to 16-fold less than that of polyphemusin I and corresponded with a 4-fold reduction in bacterial membrane depolarization. Both peptides were able to associate with lipid bilayers in a similar fashion; however, PM1-S was completely unable to translocate model membranes while polyphemusin I retained this activity. It was concluded that the disulfide-constrained, beta-sheet structure of polyphemusin I is required for maximum antimicrobial activity. Disruption of this structure results in reduced antimicrobial activity and completely abolishes membrane translocation indicating that the linear PM1-S acts through a different antimicrobial mechanism.

Cationic antimicrobial peptides : issues for potential clinical use

Many different types of organisms use antimicrobial peptides, typically 20-40 amino acids in length, for defence against infection. Most are capable of rapidly killing a wide range of microbial cells. They have been classified according to their active structures into six extensive groups. It is not yet clear how these peptides kill bacterial cells, but it is widely believed that some cationic antimicrobial peptides kill by disrupting bacterial membranes, allowing the free exchange of intra- and extra-cellular ions. The selectivity of these peptides appears to relate to differences between the external membranes of prokaryotic and eukaryotic cells. The action of the peptides may involve the formation of 'barrel-stave' or 'torroidal' pores, the introduction of packing defects in the membrane phospholipids, or large-scale disruption of the membrane by a very dense aggregation of parallel-oriented peptide, called the 'carpet mechanism'. Antimicrobial peptides are attractive candidates for clinical development because of their selectivity, their speed of action and because bacteria may not easily develop resistance against them. Some antimicrobial peptides are already in clinical and commercial use, including ambicin (nisin), polymixin B and gramicidin S. There have been several attempts at developing peptides to make them more suitable for clinical use. For those peptides that act against bacterial membranes, it is possible to differentiate between those structural features that contribute to the specificity of initial membrane binding and those that contribute to the subsequent breach of membrane integrity. The design of novel antimicrobial peptides would necessitate the optimisation of multiple parameters, a problem that has proved difficult to solve. Potential problems to be overcome include high production costs, toxicity against eukaryotic cells, susceptibility to proteolytic degradation and the development of allergies to the peptides. Biosynthesis, using recombinant DNA techniques, could make commercial-scale synthesis feasible but the peptides are usually lethal to the micro-organisms used to produce them. Proteolytic degradation can be reduced by modifying the peptides to contain nonstandard amino acids, or by restricting the use of peptides to topical applications. The problem of sensitisation could be overcome by the use of our own natural antibiotics to prevent or treat infections. Despite early hopes that bacteria would not easily develop resistance to antimicrobial peptides, it is clear that some strains of bacteria already have.

Effect of Drastic Sequence Alteration and d-Amino Acid Incorporation on the Membrane Binding Behavior of Lytic Peptides

The amphipathic alpha-helix is a common motif found in many cell lytic peptides including antimicrobial peptides. We have recently shown that significantly altering the amphipathic structure of a lytic peptide by reshuffling its sequence and/or replacing a few l-amino acids with their D-enantiomers did not significantly affect the antimicrobial activity of the peptides nor their ability to bind and permeate negatively charged (PE/PG) membranes. However, a pronounced effect was observed regarding their hemolytic activity and their ability to bind and permeate zwitterionic (PC/Cho) membranes. To shed light on these findings, here we used surface plasmon resonance (SPR) with mono- and bilayer membranes. We found that the L-amino acid (aa) peptides bound 10-25-fold stronger to PC/Cho bilayers compared with monolayers, whereas the diastereomers bound similarly to both membranes. A two-state reaction model analysis of the data indicated that this difference is due to the insertion of the L-aa peptides into the PC/Cho bilayers, whereas the diastereomers are surface-localized. In contrast, only an approximately 2-fold difference was found with negatively charged membranes. Changes in the amphipathicity markedly affected only the insertion of the L-aa peptides into PC/Cho bilayers. Furthermore, whereas the all-L-aa peptides bound similarly to the PC/Cho and PE/PG membranes, the diastereomers bound approximately 100-fold better to PE/PG compared with PC/Cho membranes, and selectivity was determined only in the first binding step. The effect of the peptides on the lipid order determined by using ATR-FTIR studies supported these findings. Besides shedding light on the mode of action of these peptides, the present study demonstrates SPR as a powerful tool to differentiate between non-cell-selective compared with bacteria-selective peptides, based on differences in their membrane binding behavior.

Bestowing antifungal and antibacterial activities by lipophilic acid conjugation to D,L-amino acid-containing antimicrobial peptides: a plausible mode of action

The dramatically increased frequency of opportunistic fungal infections has prompted research to diversify the arsenal of antifungal agents. Antimicrobial peptides constitute a promising family for future antibiotics with a new mode of action. However, only a few are effective against fungal pathogens because of their ability to self-assemble. Recently, we showed that the conjugation of fatty acids to the potent antibacterial peptide magainin endowed it with antifungal activity concomitant with an increase in its oligomeric state in solution. To investigate whether a high potency of the parental peptide is prerequisite for antifungal activity, we conjugated undecanoic acid (UA) and palmitic acid (PA) to inactive diastereomers of magainin containing four d-amino acids ([D]-4-magainin), as well as to a weakly active diastereomeric lytic peptide containing Lys and Leu ([D]-K(5)L(7)). All lipopeptides gained potent activity toward Cryptococcus neoformans. Most importantly, [D]-K(5)L(7)-UA was highly potent against all microorganisms tested, including bacteria, yeast, and opportunistic fungi. All lipopeptides increased the permeability of Escherichia coli spheroplasts and intact C. neoformans, as well as their corresponding membranes, phosphatidylethanol (PE)/phosphatidylglycerol (PG) and phosphatidylcholine (PC)/PE/phosphatidylinositol (PI)/ergosterol, respectively. The extent of membrane-permeating activity correlated with their biological function, suggesting that the plasma membrane was one of their major targets. Circular dichroism (CD) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed that their mode of oligomerization in solution, structure, and organization in membranes have important roles regarding their antibacterial and antifungal activities. Together with the advantage of using diastereomers versus all l-amino acid peptides, this study paves the way to the design of a new group of potent antifungal peptides urgently needed to combat opportunistic fungal infection.

Genetic analysis and complete primary structure of microcin L

Escherichia coli LR05, in addition to producing MccB17, J25, and D93, secretes microcin L, a newly discovered microcin that exhibits strong antibacterial activity against related Enterobacteriaceae, including Salmonella enterica serovars Typhimurium and Enteritidis. Microcin L was purified using a two-step procedure including solid-phase extraction and reverse-phase C(18) high-performance liquid chromatography. A 4,901-bp region of the DNA plasmid of E. coli LR05 was sequenced revealing that the microcin L cluster consists of four genes, mclC, mclI, mclA, and mclB. The structural gene mclC encoded a 105-amino-acid precursor with a 15-amino-acid N-terminal extension ending with a Gly-Ala motif upstream of the cleavage site. This motif is typical of the class II microcins and other gram-positive bacteriocins exported by ABC transporters. The mclI immunity gene was identified upstream of the mclC gene and encodes a 51-amino-acid protein with two potential transmembrane domains. Located on the reverse strand, two genes, mclA and mclB, encoded the proteins MclA and MclB, respectively. They bear strong relatedness with the ABC transporter proteins and accessory factors involved in the secretion of microcins H47, V, E492, and 24. The microcin L genetic system resembles the genetic organization of MccV. Furthermore the MccL primary structure has been determined. It is a 90-amino-acid peptide of 8,884 Da with two disulfide bridges. The N-terminal region has significant homologies with several gram-positive bacteriocins. The C-terminal 32-amino-acid sequence is 87.5% identical to that of MccV. Together, these results strongly indicate that microcin L is a gram-negative class II microcin.

Antimicrobial activity and bacterial-membrane interaction of ovine-derived cathelicidins

Three ovine-derived cathelicidins, SMAP29, OaBac5mini, and OaBac7.5mini, were compared with respect to their antibacterial activities and interactions with membranes. SMAP29 was confirmed to be alpha-helical, broad spectrum, and able to disrupt both the outer and the cytoplasmic membranes at relatively low concentrations. In contrast, the two proline- and arginine-rich OaBac peptides had more-modest antibacterial activities, reduced levels of lipopolysaccharide binding, and a lesser ability to depolarize the cytoplasmic membrane, consistent with a cytoplasmic target.

Identification and functional characterization of novel scorpion venom peptides with no disulfide bridge from Buthus martensii Karsch

The scorpion venom peptides with no disulfide bridge are rarely identified and poorly characterized so far. Here, we report the identification and characterization of four novel disulfide-bridge-free venom peptides (BmKa1, BmKa2, BmKb1 and BmKn2) from Buthus martensii Kasch. BmKa1 and BmKa2 are very acidic and hydrophilic, showing no any similarity to other proteins, whereas BmKb1 and BmKn2 both are basic, alpha-helical peptide with an amidated C-terminus, showing a little homology with other peptides. Functional tests with synthetic peptide showed that BmKn2 has strong antimicrobial activity against both Gram-positive and Gram-negative bacteria, whereas BmKb1 has weak activity in inhibiting the growth of these bacteria.

Effects of l- or d-Pro incorporation into hydrophobic or hydrophilic helix face of amphipathic α-helical model peptide on structure and cell selectivity

A synthetic amphipathic alpha-helical model peptide, KLW, displays non-cell selective cytotoxicity. To investigate the effects of L- or D-Pro kink incorporation into hydrophobic or hydrophilic helix face of KLW on structure, cell selectivity, and membrane-binding affinity, we designed a series of four peptides, in which Leu(9) and Lys(11) in the hydrophobic and hydrophilic helix face of KLW, respectively, are substituted with L- or D-Pro. A L- or D-Pro substitution (KLW-L9P or KLW-L9p) of Leu(9) at the hydrophobic helix face of KLW induced a more significant reduction in hemolytic activity with improved antibacterial activity than that (KLW-K11P or KLW-K11p) of Lys(11) in the hydrophilic helix face. In addition, D-Pro-containing peptides (KLW-L9p and KLW-K11p) displayed less hemolytic activity than L-Pro-containing peptides (KLW-L9P and KLW-K11P). Tryptophan fluorescence studies revealed that bacterial cell selectivity of KLW-L9P, KLW-L9p, and KLW-K11p is closely related to selective interactions with negatively charged phospholipids. CD analysis revealed that L- or D-Pro incorporation into KLW reduces the alpha-helicity of the peptide and D-Pro incorporation induces more significant disruption in alpha-helical structure than L-Pro incorporation. Our results collectively suggest that D-Pro incorporation into the hydrophobic helix face of non-cell selective amphipathic alpha-helical peptides may be useful for the design of novel antimicrobial peptides possessing high bacterial cell selectivity without hemolytic activity.

Antibacterial activities in various tissues of the horse mussel, Modiolus modiolus

A search for antibacterial activity in different organs/tissues of the horse mussel, Modiolus modiolus, was conducted. Dried samples were extracted with 60% (v/v) acetonitrile, containing 0.1% (v/v) trifluoroacetic acid. Due to high salt content, two liquid phases were obtained; an acetonitrile-rich phase (ACN extract) and an aqueous phase. The aqueous phase was further subjected to solid phase extraction (SPE). Eluates from SPE and ACN extracts were tested for antibacterial, lysozyme, and toxic activity. Antibacterial activity was demonstrated in extracts from several tissues, including plasma, haemocytes, labial palps, byssus, mantle, and gills. Some of the extracts were sensitive to proteinase K treatment, indicating antibacterial peptides and/or proteins. Lysozyme-like activity and toxic activity against Artemia salina nauplii was detected in fractions from the gills, mantle, muscle, and haemocytes. Results from this study indicate that M. modiolus is a promising source for identifying novel drug lead compounds.

APD: the Antimicrobial Peptide Database

An antimicrobial peptide database (APD) has been established based on an extensive literature search. It contains detailed information for 525 peptides (498 antibacterial, 155 antifungal, 28 antiviral and 18 antitumor). APD provides interactive interfaces for peptide query, prediction and design. It also provides statistical data for a select group of or all the peptides in the database. Peptide information can be searched using keywords such as peptide name, ID, length, net charge, hydrophobic percentage, key residue, unique sequence motif, structure and activity. APD is a useful tool for studying the structure-function relationship of antimicrobial peptides. The database can be accessed via a web-based browser at the URL: http://aps.unmc.edu/AP/main.html.

A new group of antifungal and antibacterial lipopeptides derived from non-membrane active peptides conjugated to palmitic acid

We report on the synthesis, biological function, and a plausible mode of action of a new group of lipopeptides with potent antifungal and antibacterial activities. These lipopeptides are derived from positively charged peptides containing d- and l-amino acids (diastereomers) that are palmitoylated (PA) at their N terminus. The peptides investigated have the sequence K(4)X(7)W, where X designates Gly, Ala, Val, or Leu (designated d-X peptides). The data revealed that PA-d-G and PA-d-A gained potent antibacterial and antifungal activity despite the fact that both parental peptides were completely devoid of any activity toward microorganisms and model phospholipid membranes. In contrast, PA-d-L lost the potent antibacterial activity of the parental peptide but gained and preserved partial antifungal activity. Interestingly, both d-V and its palmitoylated analog were inactive toward bacteria, and only the palmitoylated peptide was highly potent toward yeast. Both PA-d-L and PA-d-V lipopeptides were also endowed with hemolytic activity. Mode of action studies were performed by using tryptophan fluorescence and attenuated total reflectance Fourier transform infrared and circular dichroism spectroscopy as well as transmembrane depolarization assays with bacteria and fungi. The data suggest that the lipopeptides act by increasing the permeability of the cell membrane and that differences in their potency and target specificity are the result of differences in their oligomeric state and ability to dissociate and insert into the cytoplasmic membrane. These results provide insight regarding a new approach of modulating hydrophobicity and the self-assembly of non-membrane interacting peptides in order to endow them with both antibacterial and antifungal activities urgently needed to combat bacterial and fungal infections.

Transmission electron microscopic observations of membrane effects of antibiotic cecropin B on Escherichia coli

The pathway of cell membrane lysis by the peptide antibiotic cecropin B (CB), which contains both a hydrophobic and an amphipathic alpha-helix, was analysed by assessing the morphological changes of Escherichia coli following treatment with the peptide. Exposure of green fluorescent protein (GFP)-expressing E. coli to CB does not lead to an efflux of GFP. Moreover, transmission electron microscopic (TEM) examination of cecropin B-treated cells showed that severe swelling precedes cell death and that the outer membrane becomes distended away from the plasma membrane. Using immuno-gold staining and TEM of E. coli expressing the maltose-binding protein in the cytoplasm, it was apparent that the protein remains restricted to the cytoplasmic compartment. These observations suggest that CB causes gross disruption of the outer membrane of Gram-negative bacteria. Circular dichroism measurements of CB in the presence of cell membrane-mimicking liposomes showed that CB forms secondary structure dependent on the ratio of [lipid]/[peptide]. These observations from this study are important for the future design of custom antimicrobial peptides.

Antibacterial activity and mechanism of action of tick defensin against Gram-positive bacteria

Defensins are a major group of antimicrobial peptides and are found widely in vertebrates, invertebrates and plants. Invertebrate defensins have been identified from insects, scorpions, mussels and ticks. In this study, chemically synthesized tick defensin was used to further investigate the activity spectrum and mode of action of natural tick defensin. Synthetic tick defensin showed antibacterial activity against many Gram-positive bacteria but not Gram-negative bacteria and low hemolytic activity, characteristic of invertebrate defensins. Furthermore, bactericidal activity against pathogenic Gram-positive bacteria including Bacillus cereus, Enterococcus faecalis and methicillin-resistant Staphylococcus aureus was observed. However, more than 30 min was necessary for tick defensin to completely kill bacteria. The interaction of tick defensin with the bacterial cytoplasmic membrane and its ability to disrupt the membrane potential was analyzed. Tick defensin was able to disrupt the membrane potential over a period of 30-60 min consistent with its relatively slow killing. Transmission electron microscopy of Micrococcus luteus treated with tick defensin showed lysis of the cytoplasmic membrane and leakage of cellular cytoplasmic contents. These findings suggest that the primary mechanism of action of tick defensin is bacterial cytoplasmic membrane lysis. In addition, incomplete cell division with multiple cross-wall formation was occasionally seen in tick defensin-treated bacteria showing pleiotropic secondary effects of tick defensin.

Susceptibility of oral bacteria to an antimicrobial decapeptide

Naturally occurring antimicrobial peptides have emerged as alternative classes of antimicrobials. In general, these antimicrobial peptides exhibit selectivity for prokaryotes and minimize the problems of engendering microbial resistance. As an alternative method to search for more effective broad-spectrum peptide antimicrobials, investigators have developed peptide libraries by using synthetic combinatorial technology. A novel decapeptide, KKVVFKVKFK (KSL), has been identified that shows a broad range of antibacterial activity. The purpose of this study was to test the efficacy of this antimicrobial peptide in killing selected strains of oral pathogens and resident saliva bacteria collected from human subjects. Cytotoxic activity of KSL against mammalian cells and the structural features of this decapeptide were also investigated, the latter by using two-dimensional NMR in aqueous and DMSO solutions. MICs of KSL for the majority of oral bacteria tested in vitro ranged from 3 to 100 microg ml(-1). Minimal bactericidal concentrations of KSL were, in general, within one to two dilutions of the MICs. KSL exhibited an ED(99) (the dose at which 99 % killing was observed after 15 min at 37 degrees C) of 6.25 microg ml(-1) against selected strains of Lactobacillus salivarius, Streptococcus mutans, Streptococcus gordonii and Actinobacillus actinomycetemcomitans. In addition, KSL damaged bacterial cell membranes and caused 1.05 log units reduction of viability counts of saliva bacteria. In vitro toxicity studies showed that KSL, at concentrations up to 1 mg ml(-1), did not induce cell death or compromise the membrane integrity of human gingival fibroblasts. NMR studies suggest that KSL adopts an alpha-helical structure in DMSO solution, which mimics the polar aprotic membrane environment, whereas it remains unstructured in aqueous medium. This study shows that KSL may be a useful antimicrobial agent for inhibiting the growth of oral bacteria that are associated with caries development and early plaque formation.

Can we predict biological activity of antimicrobial peptides from their interactions with model phospholipid membranes?

Cationic antibacterial peptides are produced in all living organisms and possess either selective activity toward a certain type of cell or microorganism, or a broad spectrum of activity toward several types of cells including prokaryotic and mammalian cells. In order to exert their activity, peptides first interact with and traverse an outer barrier, e.g., mainly LPS and peptidoglycan in bacteria or a glycocalix layer and matrix proteins in mammalian cells. Only then, can the peptides bind and insert into the cytoplasmic membrane. The mode of action of many antibacterial peptides is believed to be the disruption of the lipidic plasma membrane. Therefore, model phospholipid membranes have been used to study the mode of action of antimicrobial peptides. These studies have demonstrated that peptides that act preferentially on bacteria are also able to interact with and permeate efficiently anionic phospholipids, whereas peptides that lyse mammalian cells bind and permeate efficiently both acidic and zwitterionic phospholipids membranes, mimicking the plasma membranes of these cells. It is now becoming increasingly clear that selective activity of these peptides against different cells depends also on other parameters that characterize both the peptide and the target cell. With respect to the peptide's properties, these include the volume of the molecule, its structure, and its oligomeric state in solution and in membranes. Regarding the target membrane, these include the structure, length, and complexity of the hydrophilic polysaccharide found in its outer layer. These parameters affect the ability of the peptides to diffuse through the cell's outer barrier and to reach its cytoplasmic plasma membrane.

The relationship between peptide structure and antibacterial activity

Cationic antimicrobial peptides are a class of small, positively charged peptides known for their broad-spectrum antimicrobial activity. These peptides have also been shown to possess anti-viral and anti-cancer activity and, most recently, the ability to modulate the innate immune response. To date, a large number of antimicrobial peptides have been chemically characterized, however, few high-resolution structures are available. Structure-activity studies of these peptides reveal two main requirements for antimicrobial activity, (1) a cationic charge and (2) an induced amphipathic conformation. In addition to peptide conformation, the role of membrane lipid composition, specifically non-bilayer lipids, on peptide activity will also be discussed.

Human salivary MUC7 mucin peptides: effect of size, charge and cysteine residues on antifungal activity

We have previously shown that MUC7 (human salivary low-molecular-mass mucin) 20-mer: LAHQKPFIRKSYKCLHKRCR (residues 32-51 of the parent MUC7, with a net positive charge of 7) possesses a broad-spectrum antimicrobial activity [Bobek and Situ (2003) Antimicrob. Agents Chemother. 47, 645-652]. The aims of the present study were to determine the minimum peptide chain length and its location within the 20-mer region that retains potent antifungal activity against Candida albicans and Cryptococcus neoformans and to examine the effect of net charge of the peptide as well as the role of cysteine residues on the fungicidal activity. First, several C-terminal truncated MUC7 20-mer fragments (16-mer, 12-mer, 11-mer, 10-mer and 8-mer) and one N-terminal fragment (8-mer-N) were synthesized and tested. The results showed that MUC7 12-mer, located at the C-terminal region of MUC7 20-mer, having a net charge of +6 and exhibiting an amphipathic helical conformation, not only retained but exceeded the antifungal activity of that of 20-mer. Secondly, several variants of the 12-mer peptide containing a lower or no net positive charge, or no cysteine residues were synthesized and tested. A clear correlation between the net positive charge of the 12-mer, its potency and initial interaction of peptide with fungal cells was found by killing assays, fluorescence microscopy and fungal cell-membrane potential measurements. Furthermore, cysteine residues, which play a critical role in bacterial binding, were found to be not important for the fungicidal activity of these peptides. These results identified MUC7 12-mer as a potential candidate for development into a novel antifungal therapeutic agent.

Microcin E492 antibacterial activity: evidence for a TonB-dependent inner membrane permeabilization on Escherichia coli

The mechanism of action of microcin E492 (MccE492) was investigated for the first time in live bacteria. MccE492 was expressed and purified to homogeneity through an optimized large-scale procedure. Highly purified MccE492 showed potent antibacterial activity at minimal inhibitory concentrations in the range of 0.02-1.2 microM. The microcin bactericidal spectrum of activity was found to be restricted to Enterobacteriaceae and specifically directed against Escherichia and Salmonella species. Isogenic bacteria that possessed mutations in membrane proteins, particularly of the TonB-ExbB-ExbD complex, were assayed. The microcin bactericidal activity was shown to be TonB- and energy-dependent, supporting the hypothesis that the mechanism of action is receptor mediated. In addition, MccE492 depolarized and permeabilized the E. coli cytoplasmic membrane. The membrane depolarization was TonB dependent. From this study, we propose that MccE492 is recognized by iron-siderophore receptors, including FepA, which promote its import across the outer membrane via a TonB- and energy-dependent pathway. MccE492 then inserts into the inner membrane, whereupon the potential becomes destabilized by pore formation. Because cytoplasmic membrane permeabilization of MccE492 occurs beneath the threshold of the bactericidal concentration and does not result in cell lysis, the cytoplasmic membrane is not hypothesized to be the sole target of MccE492.

Gone gene fishing: how to catch novel marine antimicrobials

Medical or health-promoting products of marine origin are often regarded with skepticism--some, such as shark fins and cod liver oil, are frequently perceived as low-tech "alternative treatments" largely because they have not been exploited to their full potential. The marine environment is an enormous source of biodiversity--80% of all life is found under the oceans' surfaces--yet very little of this rich resource has been utilized. Furthermore, most marine organisms rely heavily on antimicrobial components of their innate immune defenses to combat pathogens. The past three years has seen a revolution in the methods used to identify novel antimicrobials from marine sources; among the most promising are marine cationic antimicrobial peptides (CAPs).

In vitro and in vivo assessment of 99mTc-UBI specificity for bacteria

Technetium-99m labeled ubiquicidin peptide 29-41 ((99m)Tc-UBI) is a cationic human antimicrobial peptide fragment that has been shown to bind bacteria in vitro and accumulates at sites of infection in experimental animals. To help determine if (99m)Tc-UBI is bound to the bacterial cell envelope by a simple nonspecific electrostatic interaction, a comparative study of the in vitro binding of (99m)Tc-UBI and two different (99m)Tc labeled cationic peptides ((99m)Tc-Tat-1-Scr and (99m)Tc-Tat-2-Scr) to bacteria and to two tumor cell line (LS174T and ACHN) was performed. The in vivo specificity of (99m)Tc-UBI for infection in mice was also evaluated using dual labels in the same animal and comparing the target/non-target ratio for (67)Ga-citrate and (99m)Tc-UBI at sites of induced infection and sterile inflammation. Under conditions of this study, the in vitro binding of (99m)Tc-UBI, (99m)Tc-Tat-1-Scr and (99m)Tc-Tat-2-Scr to S. aureus was 35, 78 and 87% respectively. While the binding of (99m)Tc-Tat-1-Scr and (99m)Tc-Tat-2-Scr was 37 and 33% to colon tumor cells (LS174T) and 39 and 41% to renal tumor cells (ACHN) respectively, the binding of (99m)Tc-UBI to both cell types was much lower at less than 4%. In vivo studies revealed that there is a significant difference (p < 0.05) in the radioactive accumulation of (99m)Tc-UBI between the sites of infection and inflammation compared to (67)Ga-citrate. Thus, (99m)Tc-UBI showed an average infection/inflammation ratio of 2.08 +/- 0.49 compared to 1.14 +/- 0.45 for (67)Ga-citrate. In conclusion, the in vitro and in vivo results provide evidence that a specific mechanism is responsible of the (99m)Tc-UBI bacterial intracellular accumulation.

Structure-activity relationships of de novo designed cyclic antimicrobial peptides based on gramicidin S

The cyclic beta-sheet structure possessed by the 10-residue antibiotic peptide gramicidin S was taken as the structural framework for the de novo design of biologically active peptides with membrane-active properties. We have shown from previous studies that gramicidin S is a broad-spectrum antibiotic effective against Gram-positive bacteria, Gram-negative bacteria, and fungi, but is toxic to human red blood cells. We tested the effect of ring size on antimicrobial activity and hemolytic activity on peptides varying from 4 to 16 residues. Interestingly, we were able to dissociate hemolytic activity and antimicrobial activity by increasing the ring size of the peptide to 14 residues (peptide GS14). Furthermore, we increased specificity for microbial membranes while decreasing toxicity to red blood cells by substituting enantiomers (D-amino acids for L-amino acids and vice versa) into the GS14 sequence. The enantiomeric substitutions all disrupted beta-sheet structure in benign medium and decreased peptide amphipathicity. The least amphipathic peptide, produced by substituting a D-Lys at position 4 of GS14 (peptide GS14K4), also had the highest therapeutic index, i.e., highest degree of specificity for microbial cells over human cells. Solution structures of GS14 analogs solved by NMR spectroscopy showed that the D-amino acid side chain was located on the nonpolar face of GS14K4. Another analog, a beta-sheet peptide with reduced amphipathicity (peptide GS14 K3L4), also had a lysine (lysine 3) on the nonpolar face as determined by the NMR structure. Both GS14K4 and GS14 K3L4 had reduced amphipathicity relative to GS14 and much higher therapeutic indices. Finally, the alteration of the nonpolar face hydrophobicity of GS14K4 analogs provided a range of activities and specificities, where the peptides with the intermediate hydrophobicities among the series had the highest therapeutic indices. The optimal peptide hydrophobicities varied depending on the microorganism being tested, with higher hydrophobicity requirements against Gram-positive bacteria and yeast compared with Gram-negative microorganisms. The net result of these studies suggests that it is possible to rationally design a cyclic membrane-active antimicrobial peptide with high specificity towards prokaryotic (bacterial and fungal) membranes and minimal toxicity to eukaryotic (e.g., mammalian) membranes.

Mode of action of membrane active antimicrobial peptides

Water-membrane soluble protein and peptide toxins are used in the defense and offense systems of all organisms, including plants and humans. A major group includes antimicrobial peptides, which serve as a nonspecific defense system that complements the highly specific cell-mediated immune response. The increasing resistance of bacteria to conventional antibiotics stimulated the isolation and characterization of many antimicrobial peptides for potential use as new target antibiotics. The finding of thousands of antimicrobial peptides with variable lengths and sequences, all of which are active at similar concentrations, suggests a general mechanism for killing bacteria rather than a specific mechanism that requires preferred active structures. Such a mechanism is in agreement with the "carpet model" that does not require any specific structure or sequence. It seems that when there is an appropriate balance between hydrophobicity and a net positive charge the peptides are active on bacteria. However, selective activity depends also on other parameters, such as the volume of the molecule, its structure, and its oligomeric state in solution and membranes. Further, although many studies support that bacterial membrane damage is a lethal event for bacteria, other studies point to a multihit mechanism in which the peptide binds to several targets in the cytoplasmic region of the bacteria.

Burkholderia is highly resistant to human Beta-defensin 3

The bactericidal activity of the novel beta-defensin hBD-3 against 28 species and 55 strains of gram-positive cocci and gram-negative fermentative and nonfermentative rods was tested. All strains proved to be highly or intermediately susceptible to hBD-3 (minimal bactericidal concentration [MBC], </=50 micro g/ml), except for Burkholderia cepacia, for all 23 tested strains of which MBCs were >100 micro g/ml.

Alternatives to antibiotics: bacteriocins, antimicrobial peptides and bacteriophages

Bacteriocins, antimicrobial peptides, and bacteriophage have attracted attention as potential substitutes for, or as additions to, currently used antimicrobial compounds. This publication will review research on the potential application of these alternative antimicrobial agents to poultry production and processing. Bacteriocins are proteinaceous compounds of bacterial origin that are lethal to bacteria other than the producing strain. It is assumed that some of the bacteria in the intestinal tract produce bacteriocins as a means to achieve a competitive advantage, and bacteriocin-producing bacteria might be a desirable part of competitive exclusion preparations. Purified or partially purified bacteriocins could be used as preservatives or for the reduction or elimination of certain pathogens. Currently only nisin, produced by certain strains of Lactococcus lactis subsp. lactis, has regulatory approval for use in certain foods, and its use for poultry products has been studied extensively. Exploration of the application of antimicrobial peptides from sources other than bacteria to poultry has not yet commenced to a significant extent. Evidence for the ability of chickens to produce such antimicrobial peptides has been provided, and it is likely that these peptides play an important role in the defense against various pathogens. Bacteriophages have received renewed attention as possible agents against infecting bacteria. Evidence from several trials indicates that phage therapy can be effective under certain circumstances. Numerous obstacles for the use of phage as antimicrobials for poultry or poultry products remain. Chiefly among them are the narrow host range of many phages, the issue of phage resistance, and the possibility of phage-mediated transfer of genetic material to bacterial hosts. Regulatory issues and the high cost of producing such alternative antimicrobial agents are also factors that might prevent application of these agents in the near future.

Exploring peptide membrane interaction using surface plasmon resonance: differentiation between pore formation versus membrane disruption by lytic peptides

Lytic peptides comprise a large group of membrane-active peptides used in the defensive and offensive systems of all organisms. Differentiating between their modes of interaction with membranes is crucial for understanding how these peptides select their target cells. Here we utilized SPR to study the interaction between lytic peptides and lipid bilayers (L1 sensor chip). Using studies also on hybrid monolayers (HPA sensor chip) revealed that SPR is a powerful tool for obtaining a real-time monitoring of the steps involved in the mode of action of membrane-active peptides, some of which previously could not be detected directly by other techniques and reported here for the first time. We investigated the mode of action of peptides that represent two major families: (i) the bee venom, melittin, as a model of a non-cell-selective peptide that forms transmembrane pores and (ii) magainin and a diastereomer of melittin (four amino acids were replaced by their D enantiomers), as models of bacteria-selective non-pore-forming peptides. Fitting the SPR data to different interaction models allows differentiating between two major steps: membrane binding and membrane insertion. Melittin binds to PC/cholesterol approximately 450-fold better than its diastereomer and magainin, mainly because it is inserted into the inner leaflet (2/3 of the binding energy), whereas the other two are not. In contrast, there is only a slight difference in the binding of all the peptides to negatively charged PE/PG mono- and bilayer membranes (in the first and second steps), indicating that the inner leaflet contributes only slightly to their binding to PE/PG bilayers. Furthermore, the 100-fold stronger binding of the cell-selective peptides to PE/PG as compared with PC/cholesterol resulted only from electrostatic attraction to the negatively charged headgroups of the outer leaflet. These results clearly differentiate between the two general mechanisms: pore formation by melittin only in zwitterionic membranes and a detergent-like effect (carpet mechanism) for all the peptides in negatively charged membranes, in agreement with their biological function.

Translocation of analogues of the antimicrobial peptides magainin and buforin across human cell membranes

Cationic antimicrobial peptides play important roles in innate immunity. Compared with extensive studies on peptide-bacteria interactions, little is known about peptide-human cell interactions. Using human cervical carcinoma HeLa and fibroblastic TM12 cells, we investigated the cellular uptake of fluorescent analogues of the two representative antimicrobial peptides magainin 2 and buforin 2 in comparison with the representative Arg-rich cell-penetrating Tat-(47-57) peptide (YGRKKRRQRRR). The dose, time, temperature, and energy dependence of translocation suggested that the three peptides cross cell membranes through different mechanisms. The magainin peptide was internalized within a time scale of tens of minutes. The cooperative concentration dependence of uptake suggested that the peptide forms a pore as an intermediate similar to the observations in model membranes. Furthermore, the translocation was coupled with cytotoxicity, which was larger for tumor HeLa cells. In contrast, the buforin peptide translocated within 10 min by a temperature-independent, less concentration-dependent passive mechanism without showing any significant cytotoxicity at the highest concentration investigated (100 microm). The uptake of the Tat peptide was proportional to the peptide concentration, and the concentration dependence was lost upon ATP depletion. The peptide exhibited a moderate cytotoxicity at higher concentrations. The time course did not show saturation even after 120 min. The buforin peptide, covalently attached to the 28-kDa green fluorescent protein, also entered cells, suggesting a potency of the peptide as a vector for macromolecular delivery into cells. However, the mechanism appeared to be different from that of the parent peptide.

Preassembly of Membrane-Active Peptides Is an Important Factor in Their Selectivity toward Target Cells†

Membrane-active peptides comprise a large group of toxins used in the defense and offense systems of all organisms including plants and humans. They act on diverse targets including microorganisms and mammalian cells, but the factors that determine their target cell selectivity are not yet clear. Here, we tested the role of peptide length and preassembly on the ability of diastereomeric cationic antimicrobial peptides to discriminate among bacteria, erythrocytes, and fungal cells, by using peptides with variable lengths (13, 16, and 19 amino acids long) and their covalently linked pentameric bundles. All the bundles expressed similar potent antifungal activity (minimal inhibitory concentration of 0.2-0.3 microM) and high antimicrobial activity. Hemolytic activity was also observed at concentrations higher than those required for antifungal activity. In contrast, all the monomers showed length-dependent antimicrobial activity, were less active toward bacteria and fungi, and were devoid of hemolytic activity. BIAcore biosensor experiments revealed a approximately 300-fold increase in peptide-membrane binding affinity between the 13- and 19-residue monomers toward zwitterionic (egg phosphatidylcholine (PC)/egg spingomyelin (SM)/cholesterol) vesicles. All the monomeric peptides display a similar high affinity to negatively charged (E. coli phosphatidylethanolamine (PE)/egg phosphatidylglycerol (PG)) vesicles regardless of their length. In contrast, irrespective of the size of the monomeric subunit, all the bundles bind irreversibly and strongly disrupt both PC/SM/cholesterol and PE/PG membranes. Attenuated total reflectance Fourier-transform infrared spectroscopy revealed that peptide assembly also affects structure as observed by an increased alpha-helical and beta-sheet content in membranes and enhances acyl chain disruption of PC/cholesterol. The correlation between the antibacterial activity and ability to depolarize the transmembrane potential of E. coli spheroplasts, as well as the ability to induce calcein release from vesicles, suggests that the bacterial membrane is their target. The data demonstrate that preassembly of cationic diastereomeric antimicrobial peptides is an essential factor in their membrane targeting.

Antibacterial activity in Strongylocentrotus droebachiensis (Echinoidea), Cucumaria frondosa (Holothuroidea), and Asterias rubens (Asteroidea)

A search for antibacterial activity in different body parts of the green sea urchin Strongylocentrotus droebachiensis, the common starfish Asterias rubens, and the sea cucumber Cucumaria frondosa was conducted. Antibacterial activity was detected in extracts from several tissues in all species tested, but mainly in the coelomocyte and body wall extracts. Relatively high antibacterial activity could also be detected in gastrointestinal organs and eggs from A. rubens and in eggs from C. frondosa. Differences between active extracts regarding hydrophobicity and sensitivity to heat and proteinase K treatment indicated that several different compounds were responsible for the antibacterial activities detected. Lysozyme-like activity could be detected in several tissues from A. rubens. Haemolytic activity could be detected in all species tested, especially in the body wall extracts. Results from the current study suggest that marine echinoderms are a potential source for the discovery of novel antibiotics.