Antimicrobial action of rabbit leukocyte CAP18(106-137)

Antimicrobial effect and proposed action mechanism of cordycepin against Escherichia coli and Bacillus subtilis

The detailed antibacterial mechanism of cordycepin efficacy against food-borne germs remains ambiguous. In this study, the antibacterial activity and action mechanism of cordycepin were assessed. The results showed that cordycepin effectively inhibited the growth of seven bacterial pathogens including both Gram-positive and Gram-negative bacterial pathogens; the minimum inhibitory concentrations (MIC) were 2.5 and 1.25 mg/ml against Escherichia coli and Bacillus subtilis, respectively. Scanning electron microscope and transmission electron microscope examination confirmed that cordycepin caused obvious damages in the cytoplasmatic membranes of both E. coli and B. subtilis. Outer membrane permeability assessment indicated the loss of barrier function and the leakage of cytoplasmic contents. Propidium iodide and carboxyfluorescein diacetate double staining approach coupled with flow cytometry analysis indicated that the integrity of cell membrane was severely damaged during a short time, while the intracellular enzyme system still remained active. This clearly suggested that membrane damage was one of the reasons for cordycepin efficacy against bacteria. Additionally, results from circular dichroism and fluorescence analysis indicated cordycepin could insert to genome DNA base and double strand, which disordered the structure of genomic DNA. Basis on these results, the mode of bactericidal action of cordycepin against E. coli and B. subtilis was found to be a dual mechanism, disrupting bacterial cell membranes and binding to bacterial genomic DNA to interfere in cellular functions, ultimately leading to cell death.

Antimicrobial peptide CAP18 and its effect on Yersinia ruckeri infections in rainbow trout Oncorhynchus mykiss (Walbaum): comparing administration by injection and oral routes

The antimicrobial peptide CAP18 has been demonstrated to have a strong in vitro bactericidal effect on Yersinia ruckeri, but its activity in vivo has not been described. In this work, we investigated whether CAP18 protects rainbow trout Oncorhynchus mykiss (Walbaum) against enteric red mouth disease caused by this pathogen either following i.p. injection or by oral administration (in feed). It was found that injection of CAP18 into juvenile rainbow trout before exposure to Y. ruckeri was associated with lowered mortality compared to non-medicated fish although it was less effective than the conventional antibiotic oxolinic acid. Oral administration of CAP18 to trout did not prevent infection. The proteolytic effect of secretions on the peptide CAP18 in the fish gastrointestinal tract is suggested to account for the inferior effect of oral administration.

Comparative Evaluation of the Antimicrobial Activity of Different Antimicrobial Peptides against a Range of Pathogenic Bacteria

ANALYSIS OF A SELECTED SET OF ANTIMICROBIAL PEPTIDES

The rapid emergence of resistance to classical antibiotics has increased the interest in novel antimicrobial compounds. Antimicrobial peptides (AMPs) represent an attractive alternative to classical antibiotics and a number of different studies have reported antimicrobial activity data of various AMPs, but there is only limited comparative data available. The mode of action for many AMPs is largely unknown even though several models have suggested that the lipopolysaccharides (LPS) play a crucial role in the attraction and attachment of the AMP to the bacterial membrane in Gram-negative bacteria. We compared the potency of Cap18, Cap11, Cap11-1-18m2, Cecropin P1, Cecropin B, Bac2A, Bac2A-NH2, Sub5-NH2, Indolicidin, Melittin, Myxinidin, Myxinidin-NH2, Pyrrhocoricin, Apidaecin and Metalnikowin I towards Staphylococcus aureus, Enterococcus faecalis, Pseudomonas aeruginosa, Escherichia coli, Aeromonas salmonicida, Listeria monocytogenes, Campylobacter jejuni, Flavobacterium psychrophilum, Salmonella typhimurium and Yersinia ruckeri by minimal inhibitory concentration (MIC) determinations. Additional characteristics such as cytotoxicity, thermo and protease stability were measured and compared among the different peptides. Further, the antimicrobial activity of a selection of cationic AMPs was investigated in various E. coli LPS mutants.

CAP18 SHOWS A HIGH BROAD SPECTRUM ANTIMICROBIAL ACTIVITY

Of all the tested AMPs, Cap18 showed the most efficient antimicrobial activity, in particular against Gram-negative bacteria. In addition, Cap18 is highly thermostable and showed no cytotoxic effect in a hemolytic assay, measured at the concentration used. However, Cap18 is, as most of the tested AMPs, sensitive to proteolytic digestion in vitro. Thus, Cap18 is an excellent candidate for further development into practical use; however, modifications that should reduce the protease sensitivity would be needed. In addition, our findings from analyzing LPS mutant strains suggest that the core oligosaccharide of the LPS molecule is not essential for the antimicrobial activity of cationic AMPs, but in fact has a protective role against AMPs.

Estimation of microbial viability using flow cytometry

For microorganisms in particular, viability is a term that is difficult to define and a state consequently difficult to measure. The traditional (and gold-standard) usage equates viability and culturability (i.e., the ability to multiply), but the process of determining culturability is often too slow. Flow cytometry provides the opportunity to make rapid and quantitative measurements of dye uptake in large numbers of cells, and we can therefore exploit the flow cytometric approach to evaluate so-called viability stains and to develop protocols for more routine assessments of microbial viability. This unit is primarily commentary, but several basic protocols have been included to ensure that users have a firm basis for attempting these reasonably difficult assays on traditional flow cytometer instruments. What is clear is that each assay must be carefully validated with the particular microorganism of interest before being applied in any research, clinical, or service form.

Heterogeneous expression of human cathelicidin hCAP18/LL-37 in inflammatory bowel diseases

BACKGROUND

Inflammatory bowel diseases (IBDs) are characterized by a breakdown of colon epithelial barrier function. Antimicrobial peptides like cathelicidins are molecules of the innate immune system located at epithelial surfaces. Cathelicidins influence microbial growth and inflammation and may play a role in IBD. In this study, the expression of human cathelicidin hCAP18/LL-37 was investigated in the intestinal mucosa from patients suffering from ulcerative colitis or Crohn's disease.

METHODS

Biopsy material from colon and ileal mucosa of a total of 89 patients (34 with Crohn's disease, 27 with ulcerative colitis, 28 control patients) was evaluated for cathelicidin expression by real-time reverse-transcriptase polymerase chain reaction and immunohistochemistry. Colon epithelial cells were stimulated in vitro with various cytokines to evaluate mechanisms that influence cathelicidin production.

RESULTS

Cathelicidin expression was significantly increased in inflamed and non-inflamed colon mucosa from ulcerative colitis patients compared to non-inflamed control mucosa. In patients with Crohn's disease cathelicidin expression was not changed in inflamed or non-inflamed colon or ileal mucosa independent of NOD2 status. Biopsies evaluated by immunohistochemistry showed epithelial cathelicidin expression in the upper crypt that was diffuse in controls and only basal in IBD patients. Inflammation mediators, alone or in combination with the known cathelicidin inducer butyrate, had no effect on cathelicidin expression in cultured colon cells.

CONCLUSIONS

In IBD the colonic expression of human cathelicidin is altered: cathelicidin expression is increased in inflamed and non-inflamed mucosa in patients suffering from ulcerative colitis but not in Crohn's disease. This deficiency may further compromise the antimicrobial barrier in Crohn's disease.

SMAP-29 has two LPS-binding sites and a central hinge

The CD spectra of SMAP-29, an antimicrobial peptide from sheep, showed disordered structure in aqueous buffers, and significant helicity in membrane-like environments, including SDS micelles, lipopolysaccharide (LPS) dispersions, and trifluoroethanol buffer systems. A structure determined by NMR in 40% perdeuterated trifluoroethanol indicated that residues 8-17 were helical, residues 18-19 formed a hinge, and residues 20-28 formed an ordered, hydrophobic segment. SMAP-29 was flexible in 40% trifluoroethanol, forming two sets of conformers that differed in the relative orientation of the N-terminal domain. We used a chromogenic Limulus assay to determine the EC50 of the peptide (the concentration that bound 50% of the added LPS). Studies with full-length and truncated SMAP-29 molecules revealed that each end of the holopeptide contained an LPS-binding domain. The higher affinity LPS-binding domain was situated in the flexible N-terminal portion. LPS binding to full-length SMAP-29 showed positive cooperativity, so the EC50 of the peptide (2.6 microm) was considerably lower than that of the individual LPS-binding domains. LPS-binding studies with a mixture of truncated peptides revealed that this cooperativity was primarily intramolecular (i.e. involving the N- and C-terminal LPS-binding sites of the same peptide molecule). CAP-18[106 -142], an antimicrobial cathelicidin peptide of rabbits, resembled SMAP-29 in that it contained N- and C-terminal LPS-binding domains, had an EC50 of 2.5 microm, and bound LPS with positive cooperativity. We conclude that the presence of multiple binding sites that function cooperatively allow peptides such as SMAP-29 and CAP-18 to bind LPS with high affinity.

Human lactoferrin and peptides derived from its N terminus are highly effective against infections with antibiotic-resistant bacteria

Since human lactoferrin (hLF) binds to bacterial products through its highly positively charged N terminus, we investigated which of the two cationic domains is involved in its bactericidal activity. The results revealed that hLF lacking the first three residues (hLF(-3N)) was less efficient than hLF in killing of antibiotic-resistant Staphylococcus aureus, Listeria monocytogenes, and Klebsiella pneumoniae. Both hLF preparations failed to kill Escherichia coli O54. In addition, hLF(-3N) was less effective than hLF in reducing the number of viable bacteria in mice infected with antibiotic-resistant S. aureus and K. pneumoniae. Studies with synthetic peptides corresponding to the first 11 N-terminal amino acids, designated hLF(1-11), and fragments thereof demonstrated that peptides lacking the first three N-terminal residues are less effective than hLF(1-11) in killing of bacteria. Furthermore, a peptide corresponding to residues 21 to 31, which comprises the second cationic domain, was less effective than hLF(1-11) in killing of bacteria in vitro and in mice having an infection with antibiotic-resistant S. aureus or K. pneumoniae. Using fluorescent probes, we found that bactericidal hLF peptides, but not nonbactericidal peptides, caused an increase of the membrane permeability. In addition, hLF killed the various bacteria, most probably by inducing intracellular changes in these bacteria without affecting the membrane permeability. Together, hLF and peptides derived from its N terminus are highly effective against infections with antibiotic-resistant S. aureus and K. pneumoniae, and the first two arginines play an essential role in this activity.

Determining the Antimicrobial Actions of Tea Tree Oil

Research into the mode of action of the essential oil of Melaleuca alternifolia (Tea tree oil) is briefly reviewed. Its mode of action is interpreted in terms of the membrane-toxicity of its monoterpenoid components and different approaches for determining cell membrane damage are discussed.

Candidacidal activities of human lactoferrin peptides derived from the N terminus

In light of the need for new antifungal agents, the candidacidal activities of human lactoferrin (hLF) and synthetic peptides representing the first, hLF(1-11), and second, hLF(21-31), cationic domains of its N terminus were compared. The results revealed that hLF(1-11) was more effective in killing fluconazole-resistant Candida albicans than hLF(21-31) and much more effective than lactoferrin, as determined microbiologically and by propidium iodide (PI) staining. By using hLF(1-11) and various derivatives, it was found that the second and third residues of the N terminus of hLF(1-11) were critical for its candidacidal activity. Detailed investigation to elucidate the mechanism of action of hLF(1-11) revealed a dose-dependent release of ATP by Candida upon exposure to hLF(1-11). Our observations that sodium azide reduced the PI uptake and candidacidal activity of hLF(1-11) and that, upon exposure to hLF(1-11), the fluorescent dye rhodamine 123 first accumulated inside the mitochondria and later was released into the cytoplasm indicate that the peptide triggers the energized mitochondrion. Furthermore, oxidized ATP, which interferes with the interaction of ATP with its extracellular receptors, blocked the candidacidal action of hLF(1-11), as measured microbiologically and by PI staining. Addition of ATP (or analogues) was not a sufficient stimulus to kill C. albicans or to act synergistically with suboptimal concentrations of the peptide. The main conclusions are that the first two arginines at the N terminus of hLF are critical in the candidacidal activity of hLF(1-11) and that extracellular ATP is essential but not sufficient for the peptide to exert its candidacidal activity.

Amphipathic, alpha-helical antimicrobial peptides

Gene-encoded antimicrobial peptides are an important component of host defense in animals ranging from insects to mammals. They do not target specific molecular receptors on the microbial surface, but rather assume amphipathic structures that allow them to interact directly with microbial membranes, which they can rapidly permeabilize. They are thus perceived to be one promising solution to the growing problem of microbial resistance to conventional antibiotics. A particularly abundant and widespread class of antimicrobial peptides are those with amphipathic, alpha-helical domains. Due to their relatively small size and synthetic accessibility, these peptides have been extensively studied and have generated a substantial amount of structure-activity relationship (SAR) data. In this review, alpha-helical antimicrobial peptides are considered from the point of view of six interrelated structural and physicochemical parameters that modulate their activity and specificity: sequence, size, structuring, charge, amphipathicity, and hydrophobicity. It begins by providing an overview of how these vary in peptides from different natural sources. It then analyzes how they relate to the currently accepted model for the mode of action of alpha-helical peptides, and discusses what the numerous SAR studies that have been carried out on these compounds and their analogues can tell us. A comparative analysis of the many alpha-helical, antimicrobial peptide sequences that are now available then provides further information on how these parameters are distributed and interrelated. Finally, the systematic variation of parameters in short model peptides is used to throw light on their role in antimicrobial potency and specificity. The review concludes with some considerations on the potentials and limitations for the development of alpha-helical, antimicrobial peptides as antiinfective agents.

The mode of antimicrobial action of the essential oil of Melaleuca alternifolia (tea tree oil)

The essential oil of Melaleuca alternifolia (tea tree) exhibits broad-spectrum antimicrobial activity. Its mode of action against the Gram-negative bacterium Escherichia coli AG100, the Gram-positive bacterium Staphylococcus aureus NCTC 8325, and the yeast Candida albicans has been investigated using a range of methods. We report that exposing these organisms to minimum inhibitory and minimum bactericidal/fungicidal concentrations of tea tree oil inhibited respiration and increased the permeability of bacterial cytoplasmic and yeast plasma membranes as indicated by uptake of propidium iodide. In the case of E. coli and Staph. aureus, tea tree oil also caused potassium ion leakage. Differences in the susceptibility of the test organisms to tea tree oil were also observed and these are interpreted in terms of variations in the rate of monoterpene penetration through cell wall and cell membrane structures. The ability of tea tree oil to disrupt the permeability barrier of cell membrane structures and the accompanying loss of chemiosmotic control is the most likely source of its lethal action at minimum inhibitory levels.

Flow cytometric analysis of chlorhexidine action

The mechanism by which chlorhexidine kills bacteria is still ill defined. We have investigated the action of chlorhexidine on Escherichia coli JM101/psb311 using a combination of flow cytometry and traditional methods. Chlorhexidine-induced uptake by E. coli cells of bis-(1,3-dibutylbarturic acid) trimethine oxonol and propidium iodide, which monitor membrane potential and membrane integrity respectively, was shown to be concentration dependent for the range 0.003-0.3 mmol-1. In addition, cells in log phase growth were more susceptible to 0.03 mmol-1 chlorhexidine than those in stationary phase. There was, however, no direct correlation between dye uptake and decline in colony forming units.

Antimicrobial agents

Antimicrobial agents active against multi-resistant Gram-positive bacteria are considered to be of major commercial potential. Commercially viable agents that have been included in recent successful trials include the streptogramins, novel glycopeptides, oxazolidinones and potent quinolones. Cationic peptides have generated much interest, but their utility as successful drug candidates remains questionable. Novel compound classes for possible exploitation include non-beta-lactam beta-lactamase inhibitors, inhibitors of lipid A biosynthesis and tRNA synthetase inhibitors.