Potassium release, a useful tool for studying antimicrobial peptides

Natural products chlorotonils exert a complex antibacterial mechanism and address multiple targets

Antimicrobial resistance is a threat to human health rendering current first-line antibiotics ineffective. New agents overcoming resistance mechanisms are urgently needed to guarantee successful treatment of human disease in the future. Chlorotonils, a natural product class with yet unknown mode of action, were shown to have broad-spectrum activity against multi-resistant Gram-positive bacteria and the malaria parasite Plasmodium falciparum, with promising activity and safety in murine infection models. Here, we report that chlorotonils can target the cell membrane, cell wall, and protein biosynthesis. They can be characterized by a rapid onset of action via interference with ion homeostasis leading to membrane depolarization, however, without inducing severe barrier failure or cellular lysis. Further characterization confirmed binding of chlorotonils to bacterial membrane lipids eventually leading to uncontrolled potassium transport. Additionally, we identified functional inhibition of the peptidoglycan biosynthesis protein YbjG and methionine aminopeptidase MetAP as secondary targets of chlorotonils.

A new antibiotic from an uncultured bacterium binds to an immutable target

Antimicrobial resistance is a leading mortality factor worldwide. Here we report the discovery of clovibactin, a new antibiotic, isolated from uncultured soil bacteria. Clovibactin efficiently kills drug-resistant bacterial pathogens without detectable resistance. Using biochemical assays, solid-state NMR, and atomic force microscopy, we dissect its mode of action. Clovibactin blocks cell wall synthesis by targeting pyrophosphate of multiple essential peptidoglycan precursors (C 55 PP, Lipid II, Lipid WTA ). Clovibactin uses an unusual hydrophobic interface to tightly wrap around pyrophosphate, but bypasses the variable structural elements of precursors, accounting for the lack of resistance. Selective and efficient target binding is achieved by the irreversible sequestration of precursors into supramolecular fibrils that only form on bacterial membranes that contain lipid-anchored pyrophosphate groups. Uncultured bacteria offer a rich reservoir of antibiotics with new mechanisms of action that could replenish the antimicrobial discovery pipeline.

Synergistic antibacterial and biofilm eradication activity of quaternary-ammonium compound with copper ion

Antibiotics overuse and misuse increase the emergence of multidrug-resistant bacterial strains, which often leads to the failure of conventional antibiotic therapies. Even worse, the tendency of bacteria to form biofilms further increases the therapeutic difficulty, because the extracellular matrix prevents the penetration of antibiotics and triggers bacterial tolerance. Therefore, developing novel antibacterial agents or therapeutic strategies with diverse antibacterial mechanisms and destruction of bacteria biofilm is a promising way to combat bacterial infections. In the present study, the combination of quaternary ammonium compound poly(diallyl dimethyl ammonium chloride) (PDDA) with Cu²⁺ was screened out to fight common pathogenic Staphylococcus aureus (S. aureus) through multi-mechanisms. This combination appeared strong synergistic antibacterial activity, and the fractional inhibitory concentration index was as low as 0.032. The synergistic antibacterial mechanism involved the destruction of the membrane function, generation of intracellular reactive oxygen, and promotion more Cu²⁺ into the cytoplasm. Further, the combination of PDDA and Cu²⁺ reduced the extracellular polysaccharide matrix, meanwhile killing the bacteria embedded in the biofilm. The biocompatibility study in vitro revealed this combination exhibited low cytotoxicity and hemolysis ratio even at 8 times of minimum bactericidal concentration. This work provides a novel antibacterial agents combination with higher efficiency to fight planktonic and biofilm conditions of S. aureus.

Determination of Bacterial Membrane Impairment by Antimicrobial Agents

The bacterial cytoplasmic membrane separates the cell from its environment and acts as a selective permeability barrier. In addition, it functions in energy conservation, transport, signaling, and biosynthesis processes. Antimicrobial agents disrupting these functions may lead to pleiotropic effects, including leakage of low molecular weight compounds such as ions, amino acids, and ATP and subsequent membrane depolarization. This updated chapter describes two techniques to assess antibiotic-induced membrane impairment in vivo.

Effect of naturally isolated hydroquinone in disturbing the cell membrane integrity of Pseudomonas aeruginosa MTCC 741 and Staphylococcus aureus MTCC 740

Nosocomial pathogens cause various health problems in human and many novel drugs are under investigation to combat the pathogens. The present study explains the naturally derived hydroquinone possible mode of action against Pseudomonas aeruginosa MTCC 741 and Staphylococcus aureus MTCC 740. Time kill studies, cell viability assays, membrane potential assays, and potassium release assays were carried out to study the mode of action. Time kill studies revealed the rapid death of bacterial pathogens exposed to 4X MIC (Minimum inhibitory concentration) of the hydroquinone. Cell viability assay results showed that nearly half of the cell destruction of test pathogens occurred within one hour. Transmission electron microscopic (TEM) observations revealed the disruption of the cell membrane, which caused severe ultrastructural changes in both test pathogens. Hydroquinone dissipated the membrane potential of test pathogens, as confirmed by the depolarization of membrane potential, increases in permeability and leakage of intracellular potassium ions. At the concentration of 2X MIC hydroquinone in 5 min, about 91.41% and 84.85% potassium ions were released from P. aeruginosa MTCC 741 and S. aureus MTCC 740, respectively. This is the first report on the mode of action of naturally derived hydroquinone against clinical pathogens.

Pharmacological correction of periodontitis using synthetic analogues of indolicidin

Introduction: The leading role of pathogenic microorganisms in the pathogenesis of periodontitis is beyond doubt. However, the use of antibiotics for periodontitis is associated with a number of problems. Indolicidins have a unique anti-microbial effect. The relevance of the search for new drugs for the treatment of acute periodontitis based on the natural indolicidin peptide becomes obvious.

Materials and methods: The investigation was performed on 320 Wistar male rats, using synthetic analogues of natural indolicidin: No. 7 and No. 8, which were administered intraperitoneally at a dose of 500 µg/kg in a volume of 0.2 ml once a day for 7 days. Periodontitis was simulated in animals according to the method proposed by Volozhin A.I. and Vinogradova S.I.

Results and discussion: The correcting effect of indolicidin analogues on the periodontitis course, was manifested by a decrease in edema and in the relative area of cell infiltrates, a significant increase in the relative area of normal tissue, and a correction of the periodontal composition. The use of indolicidin analogues led to an increase in the functional activity of neutrophils and macrophages, acute phase proteins concentration, a correction of pro-oxidant-antioxidant balance and production of vasoactive substances. The effect of indolicidin analogues was higher than that of lincomycin. The greater effectiveness of peptide No.8 compared to that of No.7 was established.

Conclusion: The investigation opens up the prospects of the synthesis of new antimicrobial drugs on the basis of the synthetic analogues of indolicidin.

Correction of Acute Parodontitis with Indolicidin Analogues

We studied the influence of synthetic indolicidin analogues on the development of acute periodontitis. The corrective effect was found in indolicidin analogues Nos. 7 and 8; it manifested in a decrease in the edema of gingival epithelium and lamina propria, a decrease in the relative area of inflammatory infiltrates, and a significant increase in the relative area of normal connective tissue. These changes were revealed as soon as on day 14 and were most pronounced in 21 days after the removal of the ligature. Indolicidin analogues Nos. 7 and 8 demonstrated similar effectiveness on the model of acute periodontitis.

Colistin causes profound morphological alteration but minimal cytoplasmic membrane perforation in populations of Escherichia coli and Pseudomonas aeruginosa

Whilst colistin (polymyxin E) represents the last mainstream treatment option for multidrug-resistant Gram-negative pathogens, details of its mechanism of action remain to be fully resolved. In this study, the effects of sub-inhibitory, inhibitory-bactericidal, and supra-bactericidal levels of colistin on the membrane integrity and morphology of Escherichia coli and Pseudomonas aeruginosa were investigated using potassium loss, flow cytometry, and scanning electron microscopy (SEM). Supra-bactericidal colistin concentrations induced just 4-12% intracellular potassium loss from bacteria after 24 h. Flow cytometry data suggested colistin might alter cell arrangement, and SEM confirmed the antibiotic causes bacterial aggregation. Filamentation was not detected in either species at any concentration or time-point up to 24 h. These results argue against the hypotheses that colistin kills bacteria by puncturing the cytoplasmic membrane or disrupting DNA synthesis. The colistin-induced bacterial aggregation detected has implications for the interpretation of MBC, time-kill, and other test results obtained with this antibiotic.

The antifungal activity of the peptide, periplanetasin-2, derived from American cockroach Periplaneta americana

The cockroach, which is a household insect, is an established model organism in research. Periplanetasin-2, derived from the American cockroach Periplaneta americana, exerted potent antifungal effect against pathogenic fungi without causing hemolysis. Periplanetasin-2 induced oxidative stress by generation of reactive oxygen species (ROS) and lipid peroxidation. Periplanetasin-2 also caused apoptosis by exposure of phosphatidylserine and fragmentation of DNA, exerted in a concentration-dependent manner. Hence, we investigated the mitochondrial apoptotic mechanism of periplanetasin-2 in Candida albicans. After treatment with periplanetasin-2, we observed mitochondrial depolarization and calcium accumulation. Moreover, we observed a decrease in cytosolic glutathione, and an increase in mitochondrial glutathione, indicating that periplanetasin-2 induced oxidative stress and high ROS production in the mitochondria. Because of this mitochondrial dysfunction, cytochrome c was released from the mitochondria into the cytosol, and caspase was activated in a time-dependent manner. In summary, the antifungal peptide periplanetasin-2 activates apoptotic signals in the mitochondria by induction of oxidative stress.

Chemical analysis and in vitro antimicrobial effects and mechanism of action of Trachyspermum copticum essential oil against Escherichia coli

OBJECTIVE

To find a natural plant essential oil (EO) with excellent antimicrobial effects on food-borne bacteria and to explore the mechanism of its antimicrobial function against Escherichia coli (E. coli).

METHODS

The antimicrobial activity of seven EOs against Gram-negative E. coli ATCC 8739 and Gram-positive Staphylococcus aureus ATCC 6538 was investigated using agar disk diffusion method, and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of each EO was determined using the broth dilution method. The chemical composition of the Trachyspermum copticum (T. copticum) EO was analyzed using gas chromatography-mass spectrometry (GC/MS). In order to explore the mechanism of the antimicrobial action, 1 MIC and 2 MIC of T. copticum EO was added to a suspension of E. coli, the growth curve and the scanning electron microscopy (SEM) analysis of E. coli, and the release of cell constituents and protein and potassium ions from the bacterial cell were measured.

RESULTS

The T. copticum EO had the best antimicrobial activity against the test bacteria, and 10 compounds accounting for 94.57% of the total oil were identified, with the major components being thymol (46.22%), p-cymene (19.03%), and γ-terpinene (22.41%). The addition of 1 MIC that T. copticum EO significantly inhibited the growth of E. coli and increased the release of cell constituents and protein and potassium ions from the bacterial cells. Scanning electron micrographs showed that T. copticum EO caused most of the E. coli cell membranes to collapse and rupture, leading to cell death.

CONCLUSIONS

These results indicate that T. copticum EO is a good natural antimicrobial agent for food-borne pathogens.

Mechanism of Action of Isolated Caffeic Acid and Epicatechin 3-gallate from Euphorbia hirta against Pseudomonas aeruginosa

BACKGROUND

The escalating dominance of resistant Pseudomonas aeruginosa strains as infectious pathogen had urged the researchers to look for alternative and complementary drugs.

OBJECTIVE

The objective of this study is to address the biological targets and probable mechanisms of action underlying the potent antibacterial effect of the isolated compounds from Euphorbia hirta (L.) against P. aeruginosa.

MATERIALS AND METHODS

The action mechanisms of caffeic acid (CA) and epicatechin 3-gallate (ECG) on P. aeruginosa cells were investigated by several bacterial physiological manifestations involving outer membrane permeabilization, intracellular potassium ion efflux, and nucleotide leakage.

RESULTS

The findings revealed that ECG and CA targeted both cell wall and cytoplasmic membrane of P. aeruginosa. The cellular membrane destruction and ensuing membrane permeability perturbation of P. aeruginosa had led to the ascending access of hydrophobic antibiotics, release of potassium ions, and leakages of nucleotides.

CONCLUSION

The overall study concludes that ECG and CA isolated from E. hirta possess remarkable anti-infective potentials which can be exploited as drug template for the development of new antibacterial agent against resistant P. aeruginosa pathogen.

SUMMARY

Epicatechin 3-gallate (ECG) and caffeic acid (CA) exhibited remarkable bactericidal abilities by increasing the outer membrane and plasma membrane permeability of Pseudomonas aeruginosa pathogenECG and CA had facilitated the entry of hydrophobic antibiotics into P. aeruginosa by disintegrating the lipopolysaccharides layer of the outer membraneECG-induced potassium efflux with efficiency close to that obtained with cefepime suggesting mode of action through membrane disruptionBoth ECG and CA had caused consistent leakage of intracellular nucleotide content with the increase in time. Abbreviations used: ECG: Epicatechin 3-gallate; CA: Caffeic acid; E. hirta: Euphoria hirta.

Silymarin exerts antifungal effects via membrane-targeted mode of action by increasing permeability and inducing oxidative stress

Silymarin, which is derived from the seeds of Silybum marianum, has been widely used to prevent and treat liver disorders. It is also consumed as a dietary supplement to improve liver function, as it does not exhibit any toxic effects in humans. Recently, silymarin has been reported to show antimicrobial effects against various pathogenic microorganisms, but the mode of action remains unknown. Thus, we investigated the antifungal activity of silymarin and aimed to determine the underlying mechanism. Initially, a propidium iodide assay was carried out; the results indicated that silymarin induced injury to the fungal plasma membrane. Subsequently, large unilamellar vesicles encapsulating calcein and fluorescein isothiocyanate-labeled dextrans (FDs) 4, 10, and 20 were prepared to analyze whether silymarin affects an artificial membrane model. The results indicated that silymarin increased membrane permeability by disturbing the membrane structure, thereby allowing free access to molecules smaller than FD20 (approximately 3.3nm). The accumulation of reactive oxygen species (ROS) results in deleterious effects to various cellular components. In particular, ROS easily react with the membrane lipids and induce lipid peroxidation, which increases membrane permeability and disturbs hydrophobic phospholipids. Using 2',7'-dichlorodihydrofluorescein diacetate and thiobarbituric acid, we confirmed that silymarin induced harmful effects on the plasma membrane. Membrane depolarization and K+ leakage, which were associated with an increase in membrane permeability, were also observed in Candida albicans cells. An assay using 1,6-diphenyl-1,3,5-hexatriene showed that silymarin decreased membrane fluidity. Taken together, we suggest that silymarin exerts its antifungal activity by targeting the C. albicans plasma membrane.

Determination of Bacterial Membrane Impairment by Antimicrobial Agents

The bacterial cytoplasmic membrane separates the cell from its environment and acts as a selective permeability barrier. In addition, it functions in energy conservation, transport, and biosynthesis processes. Antimicrobial agents disrupting these functions may lead to pleiotropic effects, including leakage of low molecular weight compounds such as ions, amino acids and ATP, and subsequent membrane depolarization. This article describes two techniques to assess antibiotic-induced membrane impairment in vivo.

Silibinin triggers yeast apoptosis related to mitochondrial Ca2+ influx in Candida albicans

Candida albicans is a common yeast that resides in the human body, but can occasionally cause systemic fungal infection, namely candidiasis. As this infection rate is gradually increasing, it is becoming a major problem to public health. Accordingly, we for the first time investigated the antifungal activity and mode of action of silibinin, a natural product extracted from Silybum marianum (milk thistle), against C. albicans. On treatment with 100μM silibinin, generation of reactive oxygen species (ROS) from mitochondria, which can cause yeast apoptosis via oxidative stress, was increased by 24.17% compared to that in untreated cells. Subsequently, we found disturbances in ion homeostasis such as release of intracellular K+ and accumulation of cytoplasmic and mitochondrial Ca2+. Among these phenomena, mitochondrial Ca2+ overload particularly plays a crucial role in the process of apoptosis, promoting the activation of pro-apoptotic factors. Therefore, we investigated the significance of mitochondrial Ca2+ in apoptosis by employing 20mM ruthenium red (RR). Additional apoptosis hallmarks such as mitochondrial membrane depolarization, cytochrome c release, caspase activation, phosphatidylserine (PS) exposure, and DNA damage were observed in response to silibinin treatment, whereas RR pre-treatment seemed to block these responses. In summary, our results suggest that silibinin induces yeast apoptosis mediated by mitochondrial Ca2+ signaling in C. albicans.

Strain Dependent Genetic Networks for Antibiotic-Sensitivity in a Bacterial Pathogen with a Large Pan-Genome

The interaction between an antibiotic and bacterium is not merely restricted to the drug and its direct target, rather antibiotic induced stress seems to resonate through the bacterium, creating selective pressures that drive the emergence of adaptive mutations not only in the direct target, but in genes involved in many different fundamental processes as well. Surprisingly, it has been shown that adaptive mutations do not necessarily have the same effect in all species, indicating that the genetic background influences how phenotypes are manifested. However, to what extent the genetic background affects the manner in which a bacterium experiences antibiotic stress, and how this stress is processed is unclear. Here we employ the genome-wide tool Tn-Seq to construct daptomycin-sensitivity profiles for two strains of the bacterial pathogen Streptococcus pneumoniae. Remarkably, over half of the genes that are important for dealing with antibiotic-induced stress in one strain are dispensable in another. By confirming over 100 genotype-phenotype relationships, probing potassium-loss, employing genetic interaction mapping as well as temporal gene-expression experiments we reveal genome-wide conditionally important/essential genes, we discover roles for genes with unknown function, and uncover parts of the antibiotic's mode-of-action. Moreover, by mapping the underlying genomic network for two query genes we encounter little conservation in network connectivity between strains as well as profound differences in regulatory relationships. Our approach uniquely enables genome-wide fitness comparisons across strains, facilitating the discovery that antibiotic responses are complex events that can vary widely between strains, which suggests that in some cases the emergence of resistance could be strain specific and at least for species with a large pan-genome less predictable.

BEEP: An assay to detect bio-energetic and envelope permeability alterations in Pseudomonas aeruginosa

We developed an effective and rapid assay to detect both bio-energetic and envelope permeability (BEEP) alterations of Pseudomonas aeruginosa. The assay is based on quantification of extracellular ATP in bacterial cultures using luciferase as a reporter. To demonstrate the validity of our assay we conducted a biased screen of a transposon insertion library in P. aeruginosa strain PAO1 in order to expedite the isolation of mutants with defects in bioenergetic pathways. We successfully isolated insertion mutants that were reduced for extracellular ATP accumulation and identified the corresponding mutations that caused the phenotype. Most of the genes identified from this analysis were associated with energy metabolism and several appeared to be potentially novel bioenergetic targets. In addition, we show that treatment of P. aeruginosa strain PAO1 with antibiotics that disrupt the bacterial cell envelope leads to greater extracellular ATP accumulation. In summary, increases in extracellular ATP accumulation above wild type levels indicated a perturbation of membrane permeability while decreases in extracellular ATP accumulation indicated defects in bioenergetics.

Killing of staphylococci by θ-defensins involves membrane impairment and activation of autolytic enzymes

θ-Defensins are cyclic antimicrobial peptides expressed in leukocytes of Old world monkeys. To get insight into their antibacterial mode of action, we studied the activity of RTDs (rhesus macaque θ-defensins) against staphylococci. We found that in contrast to other defensins, RTDs do not interfere with peptidoglycan biosynthesis, but rather induce bacterial lysis in staphylococci by interaction with the bacterial membrane and/or release of cell wall lytic enzymes. Potassium efflux experiments and membrane potential measurements revealed that the membrane impairment by RTDs strongly depends on the energization of the membrane. In addition, RTD treatment caused the release of Atl-derived cell wall lytic enzymes probably by interaction with membrane-bound lipoteichoic acid. Thus, the premature and uncontrolled activity of these enzymes contributes strongly to the overall killing by θ-defensins. Interestingly, a similar mode of action has been described for Pep5, an antimicrobial peptide of bacterial origin.

Antibacterial Mode of Action of Ib-AMP1 Against Escherichia coli O157:H7

Continual occurrence of foodborne outbreaks, along with the increase in antibiotic resistance which burdens clinical treatments, has urged scientists to search for other potential promising antimicrobial agents. Antimicrobial peptides are emerging as one of the potential alternatives. The mode of action of a given AMP is critical and essential for future application; however, it is still not completely known for many of these compounds. Ib-AMP1 is a plant-derived AMP, purified from seeds of Impatiens balsamina and has been shown to exert antibacterial and antifungal activity at the micromolar level. A study had shown that the therapeutic index of Ib-AMP1 against eight human pathogens is 23.5. The objective of the present study was to determine the in vivo mode of action of Ib-AMP1 against Escherichia coli O157:H7. A concentration-dependent effect of Ib-AMP1 on the E. coli O157:H7 cell membrane occurred. Ib-AMP1 treatments resulted in efflux of K(+) and ATP, suggesting pores of sufficient size to allow efflux of large molecules. Ib-AMP1 at sublethal concentrations exerts a greater effect at the intracellular level. In contrast, Ib-AMP1 at a lethal concentration permeabilizes cell membranes and may directly or indirectly inhibit intracellular macromolecule synthesis. Collectively, results of this study suggest Ib-AMP1 is bactericidal interfering within outer and inner membrane integrity permitting efflux of ATP and interfering with intracellular biosynthesis of DNA, RNA and protein.

Lights, Camera, Action! Antimicrobial Peptide Mechanisms Imaged in Space and Time

Deeper understanding of the bacteriostatic and bactericidal mechanisms of antimicrobial peptides (AMPs) should help in the design of new antibacterial agents. Over several decades, a variety of biochemical assays have been applied to bulk bacterial cultures. While some of these bulk assays provide time resolution of the order of 1min, they do not capture faster mechanistic events. Nor can they provide subcellular spatial information or discern cell-to-cell heterogeneity within the bacterial population. Single-cell, time-resolved imaging assays bring a completely new spatiotemporal dimension to AMP mechanistic studies. We review recent work that provides new insights into the timing, sequence, and spatial distribution of AMP-induced effects on bacterial cells.

Glochidioboside Kills Pathogenic Bacteria by Membrane Perturbation

The aim of this study was to evaluate the antibacterial effects of glochidioboside and determine its mechanism of action. Glochidioboside has been reported to be isolated from some plants but the underlying biological properties have remained largely obscure until now. To identify the antibacterial activity of all biological properties, pathogenic bacteria susceptibility test was performed, and the result shows that the compound displays remarkable antibacterial activity against antibiotic-resistant bacteria not to mention general pathogen. To demonstrate membrane disruption and depolarization, SYTOX green and bis-(1,3-dibutylbarbituric acid) trimethine oxonol were used with Escherichia coli O157, and indicated that glochidioboside affected cytoplasmic membranes by permeabilization and depolarization, respectively. Calcein efflux was evident in a membrane model that encapsulated fluorescent dye, and supported the hypothesis of a membrane-active mechanism. To confirm the release of intracellular matrix owing to membrane damage, the movements of potassium ion were observed; the results indicated that the cells treated with glochidioboside leaked potassium ion, thus the damage induced by the compounds lead to leaking intracellular components. We propose that glochidioboside kills pathogenic bacteria via perturbation of integrity of the membrane.

Antimicrobial Peptides from the Aurein Family Form Ion-Selective Pores in Bacillus subtilis

The mechanism of action of aurein 2.2 and aurein 2.3, antimicrobial peptides from the frog Litoria aurea, was investigated. Proteomic profiling of the Bacillus subtilis stress response indicates that the cell envelope is the main target for both aureins. Upon treatment, the cytoplasmic membrane depolarizes and cellular ATP levels decrease. Global element analysis shows that intracellular concentrations of certain metal ions (potassium, magnesium, iron, and manganese) strongly decrease. Selective translocation of some ions over others was demonstrated in vitro. The same set of ions also leaks from B. subtilis cells treated with sublethal concentrations of gramicidin S, MP196, and nisin. Aureins do not permeabilize the cell membrane for propidium iodide thus excluding formation of large, unspecific pores. Our data suggest that the aureins acts by forming small pores thereby causing membrane depolarization, and by triggering the release of certain metal ions thus disturbing cellular ion homeostasis.

Fungicidal mechanisms of the antimicrobial peptide Bac8c

Bac8c (RIWVIWRR-NH2) is an analogue peptide derived through complete substitution analysis of the linear bovine host defense peptide variant Bac2A. In the present study, the antifungal mechanism of Bac8c against pathogenic fungi was investigated, with a particular focus on the effects of Bac8c on the cytoplasmic membrane. We used bis-(1,3-dibutylbarbituric acid) trimethine oxonol [DiBAC4(3)] staining and 3,3'-dipropylthiacarbocyanine iodide [DiSC3(5)] assays to show that Bac8c induced disturbances in the membrane potential of Candida albicans. An increase in membrane permeability and suppression of cell wall regeneration were also observed in Bac8c-treated C. albicans. We studied the effects of Bac8c treatment on model membranes to elucidate its antifungal mechanism. Using calcein and FITC-labeled dextran leakage assays from Bac8c-treated large unilamellar vesicles (LUVs) and giant unilamellar vesicles (GUVs), we found that Bac8c has a pore-forming action on fungal membranes, with an estimated pore radius of between 2.3 and 3.3 nm. A membrane-targeted mechanism of action was also supported by the observation of potassium release from the cytosol of Bac8c-treated C. albicans. These results indicate that Bac8c is considered as a potential candidate to develop a novel antimicrobial agent because of its low-cost production characteristics and high antimicrobial activity via its ability to induce membrane perturbations in fungi.

Scolopendin 2, a cationic antimicrobial peptide from centipede, and its membrane-active mechanism

Scolopendin 2 is a 16-mer peptide (AGLQFPVGRIGRLLRK) derived from the centipede Scolopendra subspinipes mutilans. We observed that this peptide exhibited antimicrobial activity in a salt-dependent manner against various fungal and bacterial pathogens and showed no hemolytic effect in the range of 1.6 μM to 100 μM. Circular dichroism analysis showed that the peptide has an α-helical properties. Furthermore, we determined the mechanism(s) of action using flow cytometry and by investigating the release of intracellular potassium. The results showed that the peptide permeabilized the membranes of Escherichia coli O157 and Candida albicans, resulting in loss of intracellular potassium ions. Additionally, bis-(1,3-dibutylbarbituric acid) trimethine oxonol and 3,3'-dipropylthiacarbocyanine iodide assays showed that the peptide caused membrane depolarization. Using giant unilamellar vesicles encapsulating calcein and large unilamellar vesicles containing fluorescein isothiocyanate-dextran, which were similar in composition to typical E. coli O157 and C. albicans membranes, we demonstrated that scolopendin 2 disrupts membranes, resulting in a pore size between 4.8 nm and 5.0 nm. Thus, we have demonstrated that a cationic antimicrobial peptide, scolopendin 2, exerts its broad-spectrum antimicrobial effects by forming pores in the cell membrane.

The lantibiotic NAI-107 binds to bactoprenol-bound cell wall precursors and impairs membrane functions

The lantibiotic NAI-107 is active against Gram-positive bacteria including vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus. To identify the molecular basis of its potency, we studied the mode of action in a series of whole cell and in vitro assays and analyzed structural features by nuclear magnetic resonance (NMR). The lantibiotic efficiently interfered with late stages of cell wall biosynthesis and induced accumulation of the soluble peptidoglycan precursor UDP-N-acetylmuramic acid-pentapeptide (UDP-MurNAc-pentapeptide) in the cytoplasm. Using membrane preparations and a complete cascade of purified, recombinant late stage peptidoglycan biosynthetic enzymes (MraY, MurG, FemX, PBP2) and their respective purified substrates, we showed that NAI-107 forms complexes with bactoprenol-pyrophosphate-coupled precursors of the bacterial cell wall. Titration experiments indicate that first a 1:1 stoichiometric complex occurs, which then transforms into a 2:1 (peptide: lipid II) complex, when excess peptide is added. Furthermore, lipid II and related molecules obviously could not serve as anchor molecules for the formation of defined and stable nisin-like pores, however, slow membrane depolarization was observed after NAI-107 treatment, which could contribute to killing of the bacterial cell.

Dry-heating of lysozyme increases its activity against Escherichia coli membranes

For food as well as for medical applications, there is a growing interest in novel and natural antimicrobial molecules. Lysozyme is a promising candidate for the development of such molecules. This protein is largely studied and known for its muramidase activity against Gram-positive bacteria, but it also shows antimicrobial activity against Gram-negative bacteria, especially when previously modified. In this study, the activity of dry-heated lysozyme (DH-L) against Escherichia coli has been investigated and compared to that of native lysozyme (N-L). Whereas N-L only delays bacterial growth, DH-L causes an early-stage population decrease. The accompanying membrane permeabilization suggests that DH-L induces either larger pores or more pores in the outer membrane as compared to N-L, as well as more ion channels in the inner membrane. The strong morphological modifications observed by optical microscopy and atomic force microscopy when E. coli cells are treated with DH-L are consistent with the suggested disturbances of membrane integrity. The higher hydrophobicity, surface activity, and positive charge induced by dry-heating could be responsible for the increased activity of DH-L on the E. coli membranes.

Microbicidal effects of α- and θ-defensins against antibiotic-resistant Staphylococcus aureus and Pseudomonas aeruginosa

Antibiotic-resistant bacterial pathogens threaten public health. Because many antibiotics target specific bacterial enzymes or reactions, corresponding genes may mutate under selection and lead to antibiotic resistance. Accordingly, antimicrobials that selectively target overall microbial cell integrity may offer alternative approaches to therapeutic design. Naturally occurring mammalian α- and θ-defensins are potent, non-toxic microbicides that may be useful for treating infections by antibiotic-resistant pathogens because certain defensin peptides disrupt bacterial, but not mammalian, cell membranes. To test this concept, clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA), including vancomycin heteroresistant strains, and ciprofloxacin-resistant Pseudomonas aeruginosa (Cip(R)-PA) were tested for sensitivity to α-defensins Crp-4, RMAD-4 and HNPs 1-3, and to RTD-1, macaque θ-defensin-1. In vitro, 3 μM Crp-4, RMAD-4 and RTD-1 reduced MRSA cell survival by 99%, regardless of vancomycin susceptibility. For PA clinical isolates that differ in fluoroquinolone resistance and virulence phenotype, peptide efficacy was independent of strain ciprofloxacin resistance, site of isolation or virulence factor expression. Thus, Crp-4, RMAD-4 and RTD-1 are effective in vitro antimicrobials against clinical isolates of MRSA and Cip(R)-PA, perhaps providing templates for development of α- and θ-defensin-based microbicides against antibiotic resistant or virulent infectious agents.

Hen egg white lysozyme permeabilizes Escherichia coli outer and inner membranes

Natural preservatives answer the consumer demand for long shelf life foods, synthetic molecules being perceived as a health risk. Lysozyme is already used because of its muramidase activity against Gram-positive bacteria. It is also described as active against some Gram-negative bacteria; membrane disruption would be involved, but the mechanism remains unknown. In this study, a spectrophotometric method using the mutant Escherichia coli ML-35p has been adapted to investigate membrane disruption by lysozyme for long durations. Lysozyme rapidly increases the permeability of the outer membrane of E. coli due to large size pore formation. A direct delayed activity of lysozyme against the inner membrane is also demonstrated, but without evidence of perforations.

Proteomic response of Bacillus subtilis to lantibiotics reflects differences in interaction with the cytoplasmic membrane

Mersacidin, gallidermin, and nisin are lantibiotics, antimicrobial peptides containing lanthionine. They show potent antibacterial activity. All three interfere with cell wall biosynthesis by binding lipid II, but they display different levels of interaction with the cytoplasmic membrane. On one end of the spectrum, mersacidin interferes with cell wall biosynthesis by binding lipid II without integrating into bacterial membranes. On the other end of the spectrum, nisin readily integrates into membranes, where it forms large pores. It destroys the membrane potential and causes leakage of nutrients and ions. Gallidermin, in an intermediate position, also readily integrates into membranes. However, pore formation occurs only in some bacteria and depends on membrane composition. In this study, we investigated the impact of nisin, gallidermin, and mersacidin on cell wall integrity, membrane pore formation, and membrane depolarization in Bacillus subtilis. The impact of the lantibiotics on the cell envelope was correlated to the proteomic response they elicit in B. subtilis. By drawing on a proteomic response library, including other envelope-targeting antibiotics such as bacitracin, vancomycin, gramicidin S, or valinomycin, YtrE could be identified as the most reliable marker protein for interfering with membrane-bound steps of cell wall biosynthesis. NadE and PspA were identified as markers for antibiotics interacting with the cytoplasmic membrane.

Mode of action and safety of lactosporin, a novel antimicrobial protein produced by Bacillus coagulans ATCC 7050

AIMS

To determine the mechanism of action of antimicrobial protein, lactosporin, against Gardnerella vaginalis and to evaluate its safety in vitro.

METHODS AND RESULTS

Bacillus coagulans ATCC 7050 was grown at 37°C for 18 h. The cell-free supernatant was concentrated 10-fold and screened for antimicrobial activity against indicator strain Micrococcus luteus. The mode of action of lactosporin was determined by measuring the potassium release and monitoring the changes in transmembrane potential (Δψ) and transmembrane pH (ΔpH) of the sensitive cells. Lactosporin caused the efflux of potassium ions from M. luteus cells and dissipation of ΔpH in G. vaginalis, while it had no effect on the Δψ. The safety of lactosporin was evaluated by using EpiVaginal(™) ectocervical (VEC-100) tissue model. Over 80% of the cells in the vaginal tissue remained viable after exposure to lactosporin for 24 h.

CONCLUSIONS

Lactosporin potentially exerts its antimicrobial activity by selective dissipation of ΔpH and/or by causing leakage of ions from the sensitive cells. Safety studies suggest that lactosporin is a noncytotoxic antimicrobial for vaginal application.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study revealed that lactosporin is an effective and safe antimicrobial preparation with potential application for the control of bacterial vaginosis.

Triazacryptand-based fluorescent sensors for extracellular and intracellular K+ sensing

A 4-amino-naphthalimide derived fluorophore with a triazacryptand moiety ligand was synthesized as a potassium ion (K(+)) sensor (KS1). This sensor is a monomer possessing a polymerizable vinyl group. By taking advantage of the polymerizable characteristics of the vinyl group, KS1 was polymerized with 2-hydroxyethyl methacrylate (HEMA) and acrylamide (AM) to form K(+) sensing films for extracellular sensing. The sensitivity of the films to potassium ions can be further tuned through the adjustment of the HEMA and AM weight ratios as well as introduction of positive or negative charge-containing segments. KS1 and its poly(2-hydroxyethyl methacrylate)-co-poly(acrylamide) (PHEMA-co-PAM) thin films show high selectivity for K(+) over competing sodium ions (Na(+)) at physiological concentrations. Extracellular sensing was demonstrated using a KS1-conjugated PHEMA-co-PAM thin film to measure the K(+) efflux of Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) stimulated by lysozyme. Meanwhile, KS1 itself permeates human glioblastoma U87MG and human esophagus premalignant CP-A cell lines. KS1 was used to monitor K(+) efflux stimulated by adenosine-5'-triphosphate (ATP), amphotericin, and a mixture of nigericin, bumetanide and ouabain, demonstrating application of this material as an intracellular potassium ion sensor.

Paneth cell α-defensins in enteric innate immunity

Paneth cells at the base of small intestinal crypts of Lieberkühn secrete high levels of α-defensins in response to cholinergic and microbial stimuli. Paneth cell α-defensins are broad spectrum microbicides that function in the extracellular environment of the intestinal lumen, and they are responsible for the majority of secreted bactericidal peptide activity. Paneth cell α-defensins confer immunity to oral infection by Salmonella enterica serovar Typhimurium, and they are major determinants of the composition of the small intestinal microbiome. In addition to host defense molecules such as α-defensins, lysozyme, and Pla2g2a, Paneth cells also produce and release proinflammatory mediators as components of secretory granules. Disruption of Paneth cell homeostasis, with subsequent induction of endoplasmic reticulum stress, autophagy, or apoptosis, contributes to inflammation in diverse genetic and experimental mouse models.

Functional and ultrastructural changes in Pseudomonas aeruginosa and Staphylococcus aureus cells induced by Cinnamomum verum essential oil

AIMS

To study cellular damage induced by Cinnamomum verum essential oil in Pseudomonas aeruginosa ATCC 27853 and Staphylococcus aureus ATCC 29213.

METHODS AND RESULTS

The effect of cinnamon bark essential oil on these two strains was evaluated by plate counts, potassium leakage, flow cytometry and transmission electron microscopy (TEM). Exposure to this oil induced alterations in the bacterial membrane of Ps. aeruginosa, which led to the collapse of membrane potential, as demonstrated by bis-oxonol staining, and loss of membrane-selective permeability, as indicated by efflux of K(+) and propidium iodide accumulation. Thus, respiratory activity was inhibited, leading to cell death. In Staph. aureus, cells treated with the oil entered a viable but noncultivable (VNC) state. The oil initially caused a considerable decrease in the metabolic activity and in the replication capacity of these bacterial cells. The loss of membrane integrity appeared later, as indicated by bis-oxonol and Propidium iodide (PI) staining. Data provided by TEM showed various structural effects in response to cinnamon essential oil. In Ps. aeruginosa cells, coagulated cytoplasmic material was observed, and intracellular material was seen in the surrounding environment, while oil-treated Staph. aureus showed fibres extending from the cell surface.

CONCLUSIONS

Cinnamon essential oil damages the cellular membrane of Ps. aeruginosa, which leads to cell death. There is evidence of VNC Staph. aureus after exposure to the oil.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cinnamon essential oil shows effective antimicrobial activity and health benefits and is therefore considered a potential food additive. To use this oil as a natural food preservative, especially in combination with other preservation methods, a thorough understanding of the mechanism through which this oil exerts its antibacterial action is required.

Potassium sensing histidine kinase in Bacillus subtilis

The soil-dwelling organism Bacillus subtilis is able to form multicellular aggregates known as biofilms. It was recently reported that the process of biofilm formation is activated in response to the presence of various, structurally diverse small-molecule natural products. All of these small-molecule natural products made pores in the membrane of the bacterium, causing the leakage of potassium cations from the cytoplasm of the cell. The potassium cation leakage was sensed by the membrane histidine kinase KinC, triggering the genetic pathway to the production of the extracellular matrix that holds cells within the biofilm. This chapter presents the methodology used to characterize the leakage of cytoplasmic potassium as the signal that induces biofilm formation in B. subtilis via activation of KinC. Development of novel techniques to monitor activation of gene expression in microbial populations led us to discover the differentiation of a subpopulation of cells specialized to produce the matrix that holds all cells together within the biofilm. This phenomenon of cell differentiation was previously missed by conventional techniques used to monitor transcriptional gene expression.