Mode of action of gramicidin S on Escherichia coli membrane

Klebsiella pneumoniae DedA family proteins have redundant roles in divalent cation homeostasis and resistance to phagocytosis

The DedA superfamily is a highly conserved family of membrane proteins. Deletion of Escherichia coli yqjA and yghB, encoding related DedA family proteins, results in sensitivity to elevated temperature, antibiotics, and alkaline pH. The human pathogen Klebsiella pneumoniae possesses genes encoding DedA family proteins with >90% amino acid identity to E. coli YqjA and YghB. We hypothesized that the deletion of K. pneumoniae yqjA and yghB will impact its physiology and may reduce its virulence. The K. pneumoniae ΔyqjA ΔyghB mutant (strain VT101) displayed a growth defect at 42°C and alkaline pH sensitivity, not unlike its E. coli counterpart. However, VT101 retained mostly wild-type resistance to antibiotics. We found VT101 was sensitive to the chelating agent EDTA, the anionic detergent SDS, and agents capable of alkalizing the bacterial cytoplasm such as bicarbonate or chloroquine. We could restore growth at alkaline pH and at elevated temperature by addition of 0.5-2 mM Ca2+ or Mg2+ to the culture media. VT101 displayed a slower uptake of calcium, which was dependent upon calcium channel activity. VT201, with similar deletions as VT101 but derived from a virulent K. pneumoniae strain, was highly susceptible to phagocytosis by alveolar macrophages and displayed a defect in the production of capsule. These findings suggest divalent cation homeostasis and virulence are interlinked by common functions of the DedA family.IMPORTANCEKlebsiella pneumoniae is a dangerous human pathogen. The DedA protein family is found in all bacteria and is a membrane transporter often required for virulence and antibiotic resistance. K. pneumoniae possesses homologs of E. coli YqjA and YghB, with 60% amino acid identity and redundant functions, which we have previously shown to be required for tolerance to biocides and alkaline pH. A K. pneumoniae strain lacking yqjA and yghB was found to be sensitive to alkaline pH, elevated temperature, and EDTA/SDS and displayed a defect in calcium uptake. Sensitivity to these conditions was reversed by addition of calcium or magnesium to the growth medium. Introduction of ΔyqjA and ΔyghB mutations into virulent K. pneumoniae resulted in the loss of capsule, increased phagocytosis by macrophages, and a partial loss of virulence. These results show that targeting the Klebsiella DedA family results in impaired divalent cation transport and, in turn, loss of virulence.

New nanostructured extracellular potassium ion probe for assay of cellular K+ transport

The concentration of potassium ion is an important indicator for human health, and its abnormality is often accompanied by various diseases. However, most tools currently used to study potassium ion transport are low throughput. Herein, we reported a new K⁺ fluorescent nanoprobe CP1-KS with high selectivity and sensitivity to K⁺ (fluorescence enhanced factor was up to 9.91 at 20 mM K⁺). The polymeric fluorescent probe CP1-KS was composed of the small-molecular K⁺ indicator KS and amphiphilic copolymer CP1. This sensor can be easily and uniformly dispersed in cell culture medium and is suitable for high throughput analysis. To assess the utility of the probe CP1-KS in biological field, this probe was employed as an extracellular fluorescent probe to monitor the efflux of K⁺ from cells (E coli, B. Subtilis 168, Hela and MCF-7 cells) under various stimulation including lysozyme, nigericin, digitonin, and ATP. Results demonstrated that CP1-KS is an effective analysis tool for extracellular K⁺ concentration. We believe that the nanoprobe has great potential in antibacterial drug screening, K⁺ ionophore function, K⁺ channel activity, cell membrane permeability analysis or other K⁺ related field in the future.

Synthesis and biological evaluation of backbone-aminated analogues of gramicidin S

We report the parallel synthesis of gramicidin S derivatives featuring backbone N-amino substituents. Analogues were prepared by incorporation of N-amino dipeptide subunits on solid support. Nine backbone-aminated macrocycles were evaluated for growth inhibitory activity against ESKAPE pathogens and hemolytic activity against human red blood cells. Diamination of the Orn residues in the β-strand region of gramicidin S was found to enhance broad-spectrum antimicrobial activity without a corresponding increase in hemolytic activity.

Light-controllable dithienylethene-modified cyclic peptides: photoswitching the in vivo toxicity in zebrafish embryos

This study evaluates the embryotoxicity of dithienylethene-modified peptides upon photoswitching, using 19 analogues based on the β-hairpin scaffold of the natural membranolytic peptide gramicidin S. We established an in vivo assay in two variations (with ex vivo and in situ photoisomerization), using larvae of the model organism Danio rerio, and determined the toxicities of the peptides in terms of 50% lethal doses (LD₅₀). This study allowed us to: (i) demonstrate the feasibility of evaluating peptide toxicity with D. rerio larvae at 3–4 days post fertilization, (ii) determine the phototherapeutic safety windows for all peptides, (iii) demonstrate photoswitching of the whole-body toxicity for the dithienylethene-modified peptides in vivo, (iv) re-analyze previous structure–toxicity relationship data, and (v) select promising candidates for potential clinical development.

Natural compounds from plants controlling leishmanial growth via DNA damage and inhibiting trypanothione reductase and trypanothione synthetase: an in vitro and in silico approach

In the present study, four different natural compounds including quercetin, gallic acid, rutin, and lupeol were studied for their anti-leishmanial potentials with anticipated mechanism of action through in vitro and in silico approaches. Results showed that rutin was exceedingly active (IC₅₀; 91.2 µg/ml) against the promastigote form of Leishmania tropica compared to quercetin (IC₅₀; 182.3 µg/ml), gallic acid (IC₅₀; 198.00 µg/ml) and lupeol (IC₅₀; 200.77 µg/ml). Similarly, rutin was highly active against the amastigote form as well, followed by quercetin, gallic acid and lupeol with IC₅₀ values of 101.3 µg/ml, 137.4 µg/ml, 277.2 µg/ml, and 298.9 µg/ml, respectively. These compounds were found to be nontoxic to human blood erythrocytes even at the highest concentration (1000 µg/ml) tested. Rutin and lupeol showed promising DNA degradation/fragmentation activity against the DNA of treated promastigotes which increased with the increase in concentration of the compounds. The in silico investigation revealed that these ligands have high affinity with the important catalytic residues of trypanothione reductase (Try-R) where, rutin showed the lowest docking score (i.e., - 6.191) followed by lupeol (- 5.799), gallic acid and quercetin. In case of ligands' interaction with trypanothione synthetase (Try-S), rutin again showed highest interaction with docking score of - 6.601 followed by quercetin (- 4.996), lupeol and gallic acid. The ADMET prediction of these compounds showed that all the parameters were within the acceptable range as defined for human use while molecular dynamics simulation supported the good interaction of quercetin and rutin against both enzymes. These findings suggest that the studied compounds may control leishmanial growth via DNA damage and inhibiting Try-R and Try-S, the two unique but critical enzymes for leishmania growth.

Membrane-active antimicrobial poly(amino-modified alkyl) β-cyclodextrins synthesized via click reactions

The emergence of drug-resistant bacteria has led to the high demand for new antibiotics. In this report, we investigated membrane-active antimicrobial β-cyclodextrins. These contain seven amino-modified alkyl groups on a molecule, which act as functional moieties to permeabilize bacterial cell membranes. The polyfunctionalization of cyclodextrins was achieved through a click reaction assisted by microwave irradiation. A survey using derivatives with systematically varied functionalities clarified the unique correlation of the antimicrobial activity of these compounds with their molecular structure and hydrophobicity/hydrophilicity balances. The optimum hydrophobicity for the compounds being membrane-active was specific to bacterial strains and animal cells; this led to specific compounds having selective toxicity against bacteria including multidrug-resistant pathogens. The results demonstrate that cyclodextrin is a versatile molecular scaffold for rationally designed structures and can be used for the development of new antibiotics.

Rhodomyrtone (Rom) is a membrane-active compound

Particularly in Asia medicinal plants with antimicrobial activity are used for therapeutic purpose. One such plant-derived antibiotic is rhodomyrtone (Rom) isolated from Rhodomyrtus tomentosa leaves. Rom shows high antibacterial activity against a wide range of Gram-positive bacteria, however, its mode of action is still unclear. Reporter gene assays and proteomic profiling experiments in Bacillus subtilis indicate that Rom does not address classical antibiotic targets like translation, transcription or DNA replication, but acts at the cytoplasmic membrane. In Staphylococcus aureus, Rom decreases the membrane potential within seconds and at low doses, causes release of ATP and even the excretion of cytoplasmic proteins (ECP), but does not induce pore-formation as for example nisin. Lipid staining revealed that Rom induces local membrane damage. Rom's antimicrobial activity can be antagonized in the presence of a very narrow spectrum of saturated fatty acids (C15:0, C16:0, or C18:0) that most likely contribute to counteract the membrane damage. Gram-negative bacteria are resistant to Rom, presumably due to reduced penetration through the outer membrane and its neutralization by LPS. Rom is cytotoxic for many eukaryotic cells and studies with human erythrocytes showed that Rom induces eryptosis accompanied by erythrocyte shrinkage, cell membrane blebbing, and membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Rom's distinctive interaction with the cytoplasmic membrane reminds on the amphipathic, alpha-helical peptides, the phenol-soluble modulins (PSMs), and renders Rom an important tool for the investigation of membrane physiology.

Structure–activity relationship of porphyrin-induced photoinactivation with membrane function in bacteria and erythrocytes

We analyzed the structure–activity relationship of natural porphyrins and the related analogs with the photoinactivation of membrane function in bacteria and erythrocytes.

Farnesol-Induced Disruption of the Staphylococcus aureus Cytoplasmic Membrane

Farnesol, a sesquiterpene alcohol with an aliphatic carbon chain, inhibited the growth of Staphylococcus aureus and induced the leakage of potassium ions. We investigated the action of farnesol on the cytoplasmic membrane of S. aureus. No ion channels that would account for the loss of potassium ions were detected. Electron paramagnetic resonance measurements suggested that farnesol proceeds into the cytoplasmic membrane of S. aureus cells, where it induces the disordering and eventual disruption of the cytoplasmic membrane. This was supported by the result that the effects of farnesol decreased by the addition of carotenoid which was the stabilizing reagent for the lipid bilayer.

Synergistic activity of the tyrocidines, antimicrobial cyclodecapeptides from Bacillus aneurinolyticus, with amphotericin B and caspofungin against Candida albicans biofilms

Tyrocidines are cationic cyclodecapeptides from Bacillus aneurinolyticus that are characterized by potent antibacterial and antimalarial activities. In this study, we show that various tyrocidines have significant activity against planktonic Candida albicans in the low-micromolar range. These tyrocidines also prevented C. albicans biofilm formation in vitro. Studies with the membrane-impermeable dye propidium iodide showed that the tyrocidines disrupt the membrane integrity of mature C. albicans biofilm cells. This membrane activity correlated with the permeabilization and rapid lysis of model fungal membranes containing phosphatidylcholine and ergosterol (70:30 ratio) induced by the tyrocidines. The tyrocidines exhibited pronounced synergistic biofilm-eradicating activity in combination with two key antifungal drugs, amphotericin B and caspofungin. Using a Caenorhabditis elegans infection model, we found that tyrocidine A potentiated the activity of caspofungin. Therefore, tyrocidines are promising candidates for further research as antifungal drugs and as agents for combinatorial treatment.

Calcium homeostasis in Pseudomonas aeruginosa requires multiple transporters and modulates swarming motility

Pseudomonas aeruginosa is an opportunistic human pathogen causing severe acute and chronic infections. Earlier we have shown that calcium (Ca(2+)) induces P. aeruginosa biofilm formation and production of virulence factors. To enable further studies of the regulatory role of Ca(2+), we characterized Ca(2+) homeostasis in P. aeruginosa PAO1 cells. By using Ca(2+)-binding photoprotein aequorin, we determined that the concentration of free intracellular Ca(2+) ([Ca(2+)]in) is 0.14±0.05μM. In response to external Ca(2+), the [Ca(2+)]in quickly increased at least 13-fold followed by a multi-phase decline by up to 73%. Growth at elevated Ca(2+) modulated this response. Treatment with inhibitors known to affect Ca(2+) channels, monovalent cations gradient, or P-type and F-type ATPases impaired [Ca(2+)]in response, suggesting the importance of the corresponding mechanisms in Ca(2+) homeostasis. To identify Ca(2+) transporters maintaining this homeostasis, bioinformatic and LC-MS/MS-based membrane proteomic analyses were used. [Ca(2+)]in homeostasis was monitored for seven Ca(2+)-affected and eleven bioinformatically predicted transporters by using transposon insertion mutants. Disruption of P-type ATPases PA2435, PA3920, and ion exchanger PA2092 significantly impaired Ca(2+) homeostasis. The lack of PA3920 and vanadate treatment abolished Ca(2+)-induced swarming, suggesting the role of the P-type ATPase in regulating P. aeruginosa response to Ca(2+).

The Ktr potassium transport system in Staphylococcus aureus and its role in cell physiology, antimicrobial resistance and pathogenesis

Potassium (K(+) ) plays a vital role in bacterial physiology, including regulation of cytoplasmic pH, turgor pressure and transmembrane electrical potential. Here, we examine the Staphylococcus aureus Ktr system uniquely comprised of two ion-conducting proteins (KtrB and KtrD) and only one regulator (KtrA). Growth of Ktr system mutants was severely inhibited under K(+) limitation, yet detectable after an extended lag phase, indicating the presence of a secondary K(+) transporter. Disruption of both ktrA and the Kdp-ATPase system, important for K(+) uptake in other organisms, eliminated regrowth in 0.1 mM K(+) , demonstrating a compensatory role for Kdp to the Ktr system. Consistent with K(+) transport mutations, S. aureus devoid of the Ktr system became sensitive to hyperosmotic conditions, exhibited a hyperpolarized plasma membrane, and increased susceptibility to aminoglycoside antibiotics and cationic antimicrobials. In contrast to other organisms, the S. aureus Ktr system was shown to be important for low-K(+) growth under alkaline conditions, but played only a minor role in neutral and acidic conditions. In a mouse competitive index model of bacteraemia, the ktrA mutant was significantly outcompeted by the parental strain. Combined, these results demonstrate a primary mechanism of K(+) uptake in S. aureus and a role for this system in pathogenesis.

In situ potentiometric method to evaluate bacterial outer membrane-permeabilizing ability of drugs: example using antiprotozoal diamidines

We introduced a new assay system, combining tyrocidine A and a K(+)-selective electrode, to evaluate the bacterial outer membrane-permeabilizing ability of drugs. Tyrocidine A, in the presence of an outer membrane permeabilizer, increased the permeability to K(+) of the cytoplasmic membrane of Escherichia coli, because this antibiotic could markedly increase the permeability of phospholipid layers constituting the cytoplasmic membrane, while it acted weakly on the outer membrane. Hence, the novel function of agents increasing the permeability of the outer membrane could be examined directly by monitoring the tyrocidine A-induced leakage of K(+) from the bacterial cytoplasm using a K(+)-selective electrode. We found that antiprotozoal diamidines, such as diminazene, pentamidine, and 4',6-diamidino-2-phenylindole (DAPI), can increase the permeability of the bacterial outer membrane and appropriate lipophilicity is important for diamidines to permeabilize the outer membrane.

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.

Sequence inversion and phenylalanine surrogates at the beta-turn enhance the antibiotic activity of gramicidin S

A series of gramicidin S (GS) analogues have been synthesized where the Phe (i + 1) and Pro (i + 2) residues of the beta-turn have been swapped while the respective chiralities (D-, L-) at each position are preserved, and Phe is replaced by surrogates with aromatic side chains of diverse size, orientation, and flexibility. Although most analogues preserve the beta-sheet structure, as assessed by NMR, their antibiotic activities turn out to be highly dependent on the bulkiness and spatial arrangement of the aromatic side chain. Significant increases in microbicidal potency against both Gram-positive and Gram-negative pathogens are observed for several analogues, resulting in improved therapeutic profiles. Data indicate that seemingly minor replacements at the GS beta-turn can have significant impact on antibiotic activity, highlighting this region as a hot spot for modulating GS plasticity and activity.

On the antibacterial action of cyclic peptides: insights from coarse-grained MD simulations

[RRKWLWLW] cyclic peptides have been shown to exhibit remarkable in vitro and in vivo antibacterial activity. Peptides alike seem to be promising for the development of new compounds to combat microbial pathogens, yet the molecular level understanding of their mechanism of action remains unclear. Here, we use coarse-grained (CG) molecular dynamics (MD) simulations of these cyclic peptides interacting with antibacterial cytoplasmic membrane models composed of a mixture of palmitoyl-oleoyl-phosphatidyl-ethanolamine (POPE) and palmitoyl-oleoyl-phosphatidylglycerol (POPG) lipid bilayers to provide a better understanding of their mode of action. In particular, the MD simulations performed at various concentrations of membrane-bound cyclic peptides reveal a novel type of mechanism in which the peptides first self-assemble at the membrane interface into amphipathic nanotubes. At high enough concentrations, coating of the membrane causes extrusion of lipids from the exposed bilayer leaflet, leading ultimately to a release of phospholipid micellar aggregates. Furthermore, the cyclic peptides also induce a drastic change in the lateral pressure profiles of the exposed leaflet, indicating a direct effect on the mechanical properties of the bilayer.

Therapeutic index of gramicidin S is strongly modulated by D-phenylalanine analogues at the beta-turn

Analogues of the cationic antimicrobial peptide gramicidin S (GS), cyclo(Val-Orn-Leu-D-Phe-Pro)2, with d-Phe residues replaced by different (restricted mobility, mostly) surrogates have been synthesized and used in SAR studies against several pathogenic bacteria. While all D-Phe substitutions are shown by NMR to preserve the overall beta-sheet conformation, they entail subtle structural alterations that lead to significant modifications in biological activity. In particular, the analogue incorporating D-Tic (1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) shows a modest but significant increase in therapeutic index, mostly due to a sharp decrease in hemolytic effect. The fact that NMR data show a shortened distance between the D-Tic aromatic ring and the Orn delta-amino group may help explain the improved antibiotic profile of this analogue.

Structurally diverse natural products that cause potassium leakage trigger multicellularity in Bacillus subtilis

We report a previously undescribed quorum-sensing mechanism for triggering multicellularity in Bacillus subtilis. B. subtilis forms communities of cells known as biofilms in response to an unknown signal. We discovered that biofilm formation is stimulated by a variety of small molecules produced by bacteria--including the B. subtilis nonribosomal peptide surfactin--that share the ability to induce potassium leakage. Natural products that do not cause potassium leakage failed to induce multicellularity. Small-molecule-induced multicellularity was prevented by the addition of potassium, but not sodium or lithium. Evidence is presented that potassium leakage stimulates the activity of a membrane protein kinase, KinC, which governs the expression of genes involved in biofilm formation. We propose that KinC responds to lowered intracellular potassium concentration and that this is a quorum-sensing mechanism that enables B. subtilis to respond to related and unrelated bacteria.

Effects of two terpene alcohols on the antibacterial activity and the mode of action of farnesol against Staphylococcus aureus

We have studied changes in the antibacterial activity and the mode of action of farnesol against Staphylococcus aureus when two terpene alcohols with an aliphatic carbon chain were added, individually, to a bacterial suspension that contained farnesol. Geraniol increased the growth-inhibitory activity of farnesol, but suppressed its ability to damage cell membranes, which is one of the predominant features of the growth-inhibitory activity of farnesol. Geranylgeraniol decreased the growth-inhibitory activity of farnesol and also suppressed its cell-damaging activity. It is possible that the presence of a terpene alcohol can both enhance and suppress the antibacterial activity of farnesol, and even change its mode of action. Thus, it is important to study not only the antibacterial activity of each constituent of an essential oil but also the interactions between them in efforts to characterize the antibacterial activity of the essential oil.

Antibacterial activity of long-chain fatty alcohols against Staphylococcus aureus

The antibacterial activity against Staphylococcus aureus of long-chain fatty alcohols was investigated, with a focus on normal alcohols. The antibacterial activity varied with the length of the aliphatic carbon chain and not with the water/octanol partition coefficient. 1-Nonanol, 1-decanol and 1-undecanol had bactericidal activity and membrane-damaging activity. 1-Dodecanol and 1-tridecanol had the highest antibacterial activity among the long-chain fatty alcohols tested, but had no membrane-damaging activity. Consequently, it appears that not only the antibacterial activity but also the mode of action of long-chain fatty alcohols might be determined by the length of the aliphatic carbon chain.

S,S,S-Tris(2-ethylhexyl) phosphorotrithioate as an effective solvent mediator for a mexiletine-sensitive membrane electrode

S,S,S-tris(2-ethylhexyl) phosphorotrithioate proved to be an effective solvent mediator for constructing a mexiletine-sensitive membrane electrode in combination with an ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate. Among a series of phosphorus compounds containing phosphoryl (P=O) groups, this solvent mediator showed the highest sensitivity to mexiletine in phosphate-buffered physiological saline containing 0.15 mol L-1 NaCl and 0.01 mol L-1 NaH2PO4/Na2HPO4 (pH 7.4), giving a detection limit of 2x10(-6) mol L-1 with a slope of 58.8 mV decade-1. This is the best reported detection limit of any mexiletine-sensitive electrode developed to date. Owing to its high selectivity toward inorganic cations, the electrode was used to determine the level of mexiletine in saliva, the monitoring of which is quite effective for controlling the dose of this drug noninvasively. The mexiletine concentrations determined with the mexiletine-sensitive electrode compared favorably with those determined by high-performance liquid chromatography.

Biphasic effects of geranylgeraniol, teprenone, and phytol on the growth of Staphylococcus aureus

We examined the antibacterial activities against Staphylococcus aureus of three diterpenes, namely, geranylgeraniol, teprenone, and phytol, by using a broth dilution with shaking method to identify the effects of diterpenes with long aliphatic carbon chains. We also performed time-kill assays and measured the leakage of K(+) ions from bacterial cells in response to these diterpenes. The diterpenes used inhibited the growth of S. aureus at concentrations of 0.15 microg/ml, as determined by damage to the cell membranes, and had clear bactericidal activity. However, the inhibitory effects of the diterpenes decreased when the concentration of each was raised above a certain level. The diterpenes tested in this study appeared to have both growth-inhibitory and growth-accelerating effects, and the net effect of each depended on its concentration.

Potassium-selective membrane electrodes based on macrocyclic metacyclophanes analogous to calixarenes

The tetrabutyl ester derived from 9,16,25,32-tetrahydroxy[3.1.3.1]metacyclophane was an excellent ionophore for constructing a K+-selective membrane electrode. This ionophore exhibited a much higher selectivity toward K+ than the structurally similar potassium ionophore IV commercially available from Fluka. In particular, the interference from organic ammonium ions decreased remarkably. Potassium ionophore IV possessed oxygen atoms in the ring structure, while the present ionophore changed the oxygen atoms to carbon atoms. Thus, the removal of oxygen atoms in ring constituents of the metacyclophane acted to reduce the interaction with the NH3+ group of organic ammonium ions. The size of the cavity of the present ionophore was between those of calix[4]arene and calix[6]arene derivatives, which act as Na+ and Cs+ ionophores, respectively, demonstrating that the ability to recognize alkali metal cations was strongly cavity size-dependent. The present K+-selective electrode had less interference from Rb+ and Cs+ than an electrode constructed using valinomycin, but suffered greater interference from Na+.

Bacterial cytoplasmic membrane permeability assay using ion-selective electrodes

We used K(+) and tetraphenylphosphonium (TPP(+)) electrodes simultaneously to evaluate the ability of antimicrobial peptides to form channels (or more generally to increase permeability) and to abolish membrane potential in bacterial cytoplasmic membranes in situ. Such evaluations are usually made independently by colorimetric monitoring of the hydrolysis of a chromogenic substrate by a cytoplasmic enzyme or by fluorimetric determination of membrane depolarization using a membrane potential-sensitive dye. In the present study, the K(+) electrode was used to evaluate channel-forming ability by monitoring the efflux of K(+) originally present in the cytoplasm of bacteria, while the TPP(+) electrode was used to examine membrane depolarization causing the efflux of TPP(+) accumulated in the cytoplasm of bacteria dependent on membrane potential. Thus, the combination of these two electrodes enabled us to clarify how the peptide-induced formation of ion channels is involved in disrupting the energy-generating system in situ.

Effects of single D-amino acid substitutions on disruption of beta-sheet structure and hydrophobicity in cyclic 14-residue antimicrobial peptide analogs related to gramicidin S

Gramicidin S (GS) is a 10-residue cyclic beta-sheet peptide with lytic activity against the membranes of both microbial and human cells, i.e. it possesses little to no biologic specificity for either cell type. Structure-activity studies of de novo-designed 14-residue cyclic peptides based on GS have previously shown that higher specificity against microbial membranes, i.e. a high therapeutic index (TI), can be achieved by the replacement of a single L-amino acid with its corresponding D-enantiomer [Kondejewski, L.H. et al. (1999) J. Biol. Chem. 274, 13181]. The diastereomer with a D-Lys substituted at position 4 caused the greatest improvement in specificity vs. other L to D substitutions within the cyclic 14-residue peptide GS14, through a combination of decreased peptide amphipathicity and disrupted beta-sheet structure in aqueous conditions [McInnes, C. et al. (2000) J. Biol. Chem. 275, 14287]. Based on this information, we have created a series of peptide diastereomers substituted only at position 4 by a D- or L-amino acid (Leu, Phe, Tyr, Asn, Lys, and achiral Gly). The amino acids chosen in this study represent a range of hydrophobicities/hydrophilicities as a subset of the 20 naturally occurring amino acids. While the D- and L-substitutions of Leu, Phe, and Tyr all resulted in strong hemolytic activity, the substitutions of hydrophilic D-amino acids D-Lys and D-Asn in GS14 at position 4 resulted in weaker hemolytic activity than in the L-diastereomers, which demonstrated strong hemolysis. All of the L-substitutions also resulted in poor antimicrobial activity and an extremely low TI, while the antimicrobial activity of the D-substituted peptides tended to improve based on the hydrophilicity of the residue. D-Lys was the most polar and most efficacious substitution, resulting in the highest TI. Interestingly, the hydrophobic D-amino acid substitutions had superior antimicrobial activity vs. the L-enantiomers although substitution of a hydrophobic D-amino acid increases the nonpolar face hydrophobicity. These results further support the role of hydrophobicity of the nonpolar face as a major influence on microbial specificity, but also highlights the importance of a disrupted beta-sheet structure on antimicrobial activity.

Potassium and tetraphenylphosphonium ion-selective electrodes for monitoring changes in the permeability of bacterial outer and cytoplasmic membranes

A tetraphenylphosphonium ion (TPP(+))-selective electrode, originally developed as a membrane potential indicator, is useful for measuring increases in the permeability of bacterial outer membranes induced by antimicrobial agents. The combination of this electrode with a potassium ion-selective electrode enabled us to determine changes in the permeability of bacterial outer and cytoplasmic membranes simultaneously. Outer membrane permeabilization induced by antimicrobial agents, chlorhexidine and polyhexamethylene biguanide (PHMB), as monitored with the TPP(+) electrode, correlated closely with the ability of the agents to release lipopolysaccharide (LPS) from the outer membrane.

Leakage of K+ ions from Staphylococcus aureus in response to tea tree oil

The leakage of K(+) ions from Staphylococcus aureus in response to tea tree oil (TTO) was investigated with an ion-selective electrode. The amount of leaked K(+) ions and the rate of leakage of K(+) ions induced by TTO were dependent on the concentration of TTO. Measurements of initial rates required less time than measurements of total amounts and provided an index of the interaction between TTO and the cell membrane. Thus, the initial rate of leakage might be a more useful measure of the antibacterial activity of TTO than the total amount.

Comparative study of the response of membrane electrodes based on calix[6]arene and calix[8]arene derivatives to organic ammonium ions

p-tert-Butylcalix[8]arene-octaacetic acid octaethyl ester and calix[8]arene-octaacetic acid octaethyl ester well recognized 2-phenylethylamine and phenylalanine methyl ester compared with the corresponding calix[6]arene derivatives. Moreover, the calix[8]arene derivatives, especially one having tert-butyl groups, gave better selectivity against biologically active amines having a complicated structure, such as norephedrine. We considered the interaction between calixarenes and organic ammonium ions from the viewpoint of molecular symmetries.

Assessment of Escherichia coli B with enhanced permeability to fluorochromes for flow cytometric assays of bacterial cell function

BACKGROUND

Flow cytometry has become a choice methodology for microbiological research. However, functional cytometric assays in live bacteria are still limited. This is due, in part, to the cell wall impairing penetration of vital dyes in bacteria, thus imposing permeabilization procedures. These manipulations may affect cell physiology, provoke cell aggregation or lysis, and they are time-consuming. Escherichia coli B strains have been used for mutagenic assays because of an altered lipopolysaccharide that provokes increased membrane permeability. We assessed the use of these strains as possible alternatives for flow cytometric assays to avoid the permeabilization steps.

METHODS

Suspensions of E. coli K-12 (strain AB1157) and E. coli B (strain WP2 uvrA/pKM101, denoted as strain IC188) were stained with several fluorochromes, including fluorescein isothiocyanate, propidium iodide, Nile Red, bis-(1,3-dibutylbarbituric acid) trimethine oxonol, hydroethidine, and dihydro-dichlorofluorescein diacetate, under basal conditions and following permeabilization, impairment of membrane potential, inhibition of dye efflux pump, and oxidative stress. Fluorescent staining of both strains was compared by epifluorescence microscopy and flow cytometry.

RESULTS

The E. coli B strain IC188 exhibited more efficient staining with vital fluorochromes than the E. coli K-12 strain AB1157 and maintained a similar membrane potential. In addition, IC188 showed higher sensitivity than AB1157 to reveal oxidative stress when challenged with prooxidants.

CONCLUSIONS

E. coli B strains may be useful for biochemical and toxicological studies based on flow cytometry and fluorescence microscopy.

Acidic phospholipids inhibit the DNA-binding activity of DnaA protein, the initiator of chromosomal DNA replication in Escherichia coli

In order to initiate chromosomal DNA replication in Escherichia coli, the DnaA protein must bind to both ATP and the origin of replication (oriC). Acidic phospholipids are known to inhibit DnaA binding to ATP, and here we examine the effects of various phospholipids on DnaA binding to oriC. Among the phospholipids in E. coli membrane, cardiolipin showed the strongest inhibition of DnaA binding to oriC. Synthetic phosphatidylglycerol containing unsaturated fatty acids inhibited binding more potently than did synthetic phosphatidylglycerol containing saturated fatty acids, suggesting that membrane fluidity is important. Thus, acidic phospholipids seem to inhibit DnaA binding to both oriC and adenine nucleotides in the same manner. Adenine nucleotides bound to DnaA did not affect the inhibitory effect of cardiolipin on DnaA binding to oriC. A mobility-shift assay re-vealed that acidic phospholipids inhibited formation of a DnaA-oriC complex containing several DnaA molecules. DNase I footprinting of DnaA binding to oriC showed that two DnaA binding sites (R2 and R3) were more sensitive to cardiolipin than other DnaA binding sites. Based on these in vitro data, the physiological relevance of this inhibitory effect of acidic phospholipids on DnaA binding to oriC is discussed.