Effective Permeability Barrier Research Articles

Enhancing frozen fish quality through polysaccharide-ice glazing: Insights from physical properties and preservation effects

Enhancing frozen fish quality through polysaccharide-ice glazing: Insights from physical properties and preservation effects

Understanding Drug Skin Permeation Enhancers Using Molecular Dynamics Simulations.

Our skin constitutes an effective permeability barrier that protects the body from exogenous substances but concomitantly severely limits the number of pharmaceutical drugs that can be delivered transdermally. In topical formulation design, chemical permeation enhancers (PEs) are used to increase drug skin permeability. In vitro skin permeability experiments can measure net effects of PEs on transdermal drug transport, but they cannot explain the molecular mechanisms of interactions between drugs, permeation enhancers, and skin structure, which limits the possibility to rationally design better new drug formulations. Here we investigate the effect of the PEs water, lauric acid, geraniol, stearic acid, thymol, ethanol, oleic acid, and eucalyptol on the transdermal transport of metronidazole, caffeine, and naproxen. We use atomistic molecular dynamics (MD) simulations in combination with developed molecular models to calculate the free energy difference between 11 PE-containing formulations and the skin's barrier structure. We then utilize the results to calculate the final concentration of PEs in skin. We obtain an RMSE of 0.58 log units for calculated partition coefficients from water into the barrier structure. We then use the modified PE-containing barrier structure to calculate the PEs' permeability enhancement ratios (ERs) on transdermal metronidazole, caffeine, and naproxen transport and compare with the results obtained from in vitro experiments. We show that MD simulations are able to reproduce rankings based on ERs. However, strict quantitative correlation with experimental data needs further refinement, which is complicated by significant deviations between different measurements. Finally, we propose a model for how to use calculations of the potential of mean force of drugs across the skin's barrier structure in a topical formulation design.

Lipid A heterogeneity and its role in the host interactions with pathogenic and commensal bacteria.

Lipopolysaccharide (LPS) is for most but not all Gram-negative bacteria an essential component of the outer leaflet of the outer membrane. LPS contributes to the integrity of the outer membrane, which acts as an effective permeability barrier to antimicrobial agents and protects against complement-mediated lysis. In commensal and pathogenic bacteria LPS interacts with pattern recognition receptors (e.g LBP, CD14, TLRs) of the innate immune system and thereby plays an important role in determining the immune response of the host. LPS molecules consist of a membrane-anchoring lipid A moiety and the surface-exposed core oligosaccharide and O-antigen polysaccharide. While the basic lipid A structure is conserved among different bacterial species, there is still a huge variation in its details, such as the number, position and chain length of the fatty acids and the decoration of the glucosamine disaccharide with phosphate, phosphoethanolamine or amino sugars. New evidence has emerged over the last few decades on how this lipid A heterogeneity confers distinct benefits to some bacteria because it allows them to modulate host responses in response to changing host environmental factors. Here we give an overview of what is known about the functional consequences of this lipid A structural heterogeneity. In addition, we also summarize new approaches for lipid A extraction, purification and analysis which have enabled analysis of its heterogeneity.

EcDBS1R4, an Antimicrobial Peptide Effective against Escherichia coli with In Vitro Fusogenic Ability.

Discovering antibiotic molecules able to hold the growing spread of antimicrobial resistance is one of the most urgent endeavors that public health must tackle. The case of Gram-negative bacterial pathogens is of special concern, as they are intrinsically resistant to many antibiotics, due to an outer membrane that constitutes an effective permeability barrier. Antimicrobial peptides (AMPs) have been pointed out as potential alternatives to conventional antibiotics, as their main mechanism of action is membrane disruption, arguably less prone to elicit resistance in pathogens. Here, we investigate the in vitro activity and selectivity of EcDBS1R4, a bioinspired AMP. To this purpose, we have used bacterial cells and model membrane systems mimicking both the inner and the outer membranes of Escherichia coli, and a variety of optical spectroscopic methodologies. EcDBS1R4 is effective against the Gram-negative E. coli, ineffective against the Gram-positive Staphylococcus aureus and noncytotoxic for human cells. EcDBS1R4 does not form stable pores in E. coli, as the peptide does not dissipate its membrane potential, suggesting an unusual mechanism of action. Interestingly, EcDBS1R4 promotes a hemi-fusion of vesicles mimicking the inner membrane of E. coli. This fusogenic ability of EcDBS1R4 requires the presence of phospholipids with a negative curvature and a negative charge. This finding suggests that EcDBS1R4 promotes a large lipid spatial reorganization able to reshape membrane curvature, with interesting biological implications herein discussed.

Bacterial lipid trafficking and outer membrane homeostasis

Gram‐negative bacteria can survive in harsh environments in part because of the presence of the outer membrane (OM), which comprises lipopolysaccharides (LPS) and phospholipids (PLs) in the outer and inner leaflets, respectively. This asymmetric distribution of lipids renders the OM a very effective permeability barrier against toxic compounds, including bile salts and hydrophobic antibiotics. To build a stable OM with the requisite lipid asymmetry, the transport and assembly of LPS, PLs and OM proteins into the OM must be coordinated. While assembly pathways for LPS and OM proteins have been well‐characterized, the processes by which PLs are transported to and from the OM, and their coordination with these other systems, are much less understood. In this talk, I will describe our work in understanding and characterizing PL transport systems in Escherichia coli, and discuss how these systems function in maintaining lipid homeostasis in the OM.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Towards a better understanding of the highly overpressured Lower Triassic Bunter reservoir rocks in the Terschelling Basin

Abstract Large lateral variations in pore-fluid pressures and associated overpressures occur in the compartmentalized Lower Triassic Bunter reservoir rocks in the Terschelling Basin (northern Dutch offshore). This study describes the main controlling factors on the distribution of these overpressures by combining seismically derived structural interpretations with pressure data. The most important controlling factor is considered to be the presence and type of both lateral and vertical permeability barriers. Lateral permeability barriers are formed by Zechstein salt and faults. The most effective vertical permeability barrier for maintaining high overpressures is the Upper Triassic Röt evaporite. Overpressures of more than 30 MPa are observed in the Bunter compartments where the Röt is present and laterally continuous. Overpressures are lower than 15 MPa in compartments where the Röt was affected by Mid-Cimmerian erosion. Based on structural observations, it is concluded that the highly overpressured compartments became fully hydraulically restricted during the Late Jurassic and Early Cretaceous. Calculations show that subsequent sedimentary loading, during the Cretaceous and Cenozoic, was able to generate the observed overpressures.

Uroplakin traffic through the Golgi apparatus induces its fragmentation: new insights from novel in vitro models

Uroplakins (UPs) play an essential role in maintaining an effective urothelial permeability barrier at the level of superficial urothelial cell (UC) layer. Although the organization of UPs in the apical plasma membrane (PM) of UCs is well known, their transport in UCs is only partially understood. Here, we dissected trafficking of UPs and its differentiation-dependent impact on Golgi apparatus (GA) architecture. We demonstrated that individual subunits UPIb and UPIIIa are capable of trafficking from the endoplasmic reticulum to the GA in UCs. Moreover, UPIb, UPIIIa or UPIb/UPIIIa expressing UCs revealed fragmentation and peripheral redistribution of Golgi-units. Notably, expression of UPIb or UPIb/UPIIIa triggered similar GA fragmentation in MDCK and HeLa cells that do not express UPs endogenously. The colocalization analysis of UPIb/UPIIIa-EGFP and COPI, COPII or clathrin suggested that UPs follow constitutively the post-Golgi route to the apical PM. Depolymerisation of microtubules leads to complete blockade of the UPIb/UPIIIa-EGFP post-Golgi transport, while disassembly of actin filaments shows significantly reduced delivery of UPIb/UPIIIa-EGFP to the PM. Our findings show the significant effect of the UPs expression on the GA fragmentation, which enables secretory Golgi-outpost to be distributed as close as possible to the sites of cargo delivery at the PM.

Lithofacies, architecture, and reservoir heterogeneity of tidal-dominated platform marginal oolitic shoal: An analogue of oolitic reservoirs of Lower Triassic Feixianguan Formation, Sichuan Basin, SW China

Abstract The Yudongliang Outcrop, located in the northern part of the Sichuan Basin, nicely exposes oolitic shoal complexes of the Lower Triassic Feixianguan Formation. The outcrop is an analogue for the subsurface Longgang and Yuanba gas fields and other Permo-Triassic reservoirs with dolomitized and nondolomitized oolite-dominated grainstone bodies as the main producing zones. Based on five measured sedimentological sections, and petrographic and petrophysical analysis of 200 plugs, the architecture of the reservoir bodies has been reconstructed in detail and placed within a high-frequency sequence-stratigraphic framework. The studied interval belongs to the highstand system tract (HST) of a single composite sequence and comprises three shallowing-upward high-frequency sequences, which consist from base to top of oncoid/peloid/ooid grain-dominated packstone, cross-bedded ooid grainstone, dolograinstone, fenestra-laminated dolomite, and crystalline dolomite. The internal architecture of the oolitic shoal is the result of complex tidal-dominated hydrodynamics. Facies and diagenetic partitioning with each high-frequency sequence are the primary controls on pore types and reservoir properties. Original lithofacies texture and subsequent cementation resulted in interbedded thin effective reservoirs and permeability barriers that created a complex internal architecture of reservoir bodies. The shapes of reservoir bodies generally follow those of the lithofacies, but with the addition of intercalated thin flow barriers.

Colistin as a potentiator of anti-TB drug activity against Mycobacterium tuberculosis.

The mycobacterial cell wall is an effective permeability barrier that limits intracellular concentrations of anti-TB drugs and hampers the success of treatment. We hypothesized that colistin might enhance the efficacy of anti-TB drugs by increasing mycobacterial cell wall permeability. In this study, we investigated the additional effect of colistin on the activity of anti-TB drugs against Mycobacterium tuberculosis in vitro. The concentration-dependent and time-dependent killing activity of isoniazid, rifampicin or amikacin alone or in combination with colistin against M. tuberculosis H37Rv was determined. Mycobacterial populations with both high and low metabolic activity were studied, and these were characterized by increasing or steady levels of ATP, respectively. With exposure to a single drug, striking differences in anti-TB drug activity were observed when the two mycobacterial populations were compared. The addition of colistin to isoniazid and amikacin resulted in sterilization of the mycobacterial load, but only in the M. tuberculosis population with high metabolic activity. The emergence of isoniazid and amikacin resistance was completely prevented by the addition of colistin. The results of this study emphasize the importance of investigating mycobacterial populations with both high and low metabolic activity when evaluating the efficacy of anti-TB drugs in vitro. This is the first study showing that colistin potentiates the activity of isoniazid and amikacin against M. tuberculosis and prevents the emergence of resistance to anti-TB drugs. These results form the basis for further studies on the applicability of colistin as a potentiator of anti-TB drugs.

Simulation study of the permeability of a model lipid membrane at the fluid-solid phase transition.

When a range of lipid bilayers are melted to the disordered fluid phase from the (much less permeable) ordered gel phase, their permeability to a variety of permeants shows a peak at the transition temperature and drops off with increasing temperature, rather than just rising as melting proceeds. To explore this anomalous behavior, a simulated coarse-grained lipid membrane model that exhibits a phase transition upon expansion or compression was studied to determine how the permeation rate of a simple particle depends on the phase composition in the two-phase region and on particle size. The permeation rate and each phase's area fraction and area density could be directly calculated, along with the probability that the permeant would cross in either phase or in interfacial regions. For large permeants and system sizes, conditions could be found where permeability increases upon compression of the bilayer. Permeation was negligible in the gel phase and, in contrast to the predictions of the "leaky interface" hypothesis, was not enriched in interfacial regions. The anomalous effect could instead be attributed to an increase in the area per lipid of fluid-phase domains. This result motivated a model for the decrease in effective permeability barrier through fluid-phase domains arising from a decrease in the length of the gel/fluid interface at the midpoint of a permeation event.

The Mycobacterium tuberculosis outer membrane channel protein CpnT confers susceptibility to toxic molecules.

Mycobacterium tuberculosis, the causative agent of tuberculosis, is protected from toxic solutes by an effective outer membrane permeability barrier. Recently, we showed that the outer membrane channel protein CpnT is required for efficient nutrient uptake by M. tuberculosis and Mycobacterium bovis BCG. In this study, we found that the cpnT mutant of M. bovis BCG is more resistant than the wild type to a large number of drugs and antibiotics, including rifampin, ethambutol, clarithromycin, tetracycline, and ampicillin, by 8- to 32-fold. Furthermore, the cpnT mutant of M. bovis BCG was 100-fold more resistant to nitric oxide, a major bactericidal agent required to control M. tuberculosis infections in mice. Thus, CpnT constitutes the first outer membrane susceptibility factor in slow-growing mycobacteria. The dual functions of CpnT in uptake of nutrients and mediating susceptibility to toxic molecules are reflected in macrophage infection experiments: while loss of CpnT was detrimental for M. bovis BCG in macrophages that enable bacterial replication, presumably due to inadequate nutrient uptake, it conferred a survival advantage in macrophages that mount a strong bactericidal response. Importantly, the cpnT gene showed a significantly higher density of nonsynonymous mutations in drug-resistant clinical M. tuberculosis strains, indicating that CpnT is under selective pressure in human tuberculosis and/or during chemotherapy. Our results indicate that the CpnT channel constitutes an outer membrane gateway controlling the influx of nutrients and toxic molecules into slow-growing mycobacteria. This study revealed that reducing protein-mediated outer membrane permeability might constitute a new drug resistance mechanism in slow-growing mycobacteria.

MicroRNA-205 Targets Tight Junction-related Proteins during Urothelial Cellular Differentiation

The mammalian bladder urothelium classified as basal, intermediate, and terminally differentiated umbrella cells offers one of the most effective permeability barrier functions known to exist in nature because of the formation of apical uroplakin plaques and tight junctions. To improve our understanding of urothelial differentiation, we analyzed the microRNA (miRNA) expression profiles of mouse urinary tissues and by TaqMan miRNA analysis of microdissected urothelial layers and in situ miRNA-specific hybridization to determine the dependence of these miRNAs on the differentiation stage. Our in situ hybridization studies revealed that miR-205 was enriched in the undifferentiated basal and intermediate cell layers. We then used a quantitative proteomics approach to identify miR-205 target genes in primary cultured urothelial cells subjected to antagomir-mediated knockdown of specific miRNAs. Twenty-four genes were reproducibly regulated by miR-205; eleven of them were annotated as cell junction- and tight junction-related molecules. Western blot analysis demonstrated that antagomir-induced silencing of miR-205 in primary cultured urothelial cells elevated the expression levels of Tjp1, Cgnl1, and Cdc42. Ectopic expression of miR-205 in MDCK cells inhibited the expression of tight junction proteins and the formation of tight junctions. miR-205- knockdown urothelial cells showed alterations in keratin synthesis and increases of uroplakin Ia and Ib, which are the urothelial differentiation products. These results suggest that miR-205 may contribute a role in regulation of urothelial differentiation by modulating the expression of tight junction-related molecules.

LptE binds to and alters the physical state of LPS to catalyze its assembly at the cell surface.

The assembly of lipopolysaccharide (LPS) on the surface of Gram-negative bacterial cells is essential for their viability and is achieved by the seven-protein LPS transport (Lpt) pathway. The outer membrane (OM) lipoprotein LptE and the β-barrel membrane protein LptD form a complex that assembles LPS into the outer leaflet of the OM. We report a crystal structure of the Escherichia coli OM lipoprotein LptE at 2.34 Å. The structure reveals homology to eukaryotic LPS-binding proteins and allowed for the prediction of an LPS-binding site, which was confirmed by genetic and biophysical experiments. Specific point mutations at this site lead to defects in OM biogenesis. We show that wild-type LptE disrupts LPS-LPS interactions in vitro and that these mutations decrease the ability of LptE to disaggregate LPS. Transmission electron microscopic imaging shows that LptE can disrupt LPS aggregates even at substoichiometric concentrations. We propose a model in which LptE functions as an LPS transfer protein in the OM translocon by disaggregating LPS during transport to allow for its insertion into the OM.

Holarrhena antidysenterica as a resistance modifying agent against Acinetobacter baumannii: Its effects on bacterial outer membrane permeability and efflux pumps

Increasing rates of infections caused by multidrug resistant Acinetobacter baumannii (MDRAB) and extensively drug resistant A. baumannii (XDRAB) have caused the need for searching alternative agents. The purposed of this project was to search plant-derived natural products that act as resistant modifying agents (RMAs) against A. baumannii. In this study, we further evaluated the activity of Holarrhena antidysenterica that has been previously proposed as RMA of novobiocin for a model strain, A. baumannii ATCC 19606 on clinically isolated non-MDRAB, MDRAB, and XDRAB. Effects of H. antidysenterica on outer membrane permeability and efflux pumps of the pathogen were conducted to preliminary elucidate mechanisms of this resistant modifier. Novobiocin was selected as a model antibiotic because it is well-established as an effective agent against Gram-positive pathogens. But, it possessed low level of antibacterial activity against Gram-negative pathogens due to an effective permeability barrier of these pathogens. H. antidysenterica ethanol extract possessed weak intrinsic antibacterial activity with minimum inhibitory concentration (MIC) more than 1000 μg/mL. The extract, at concentrations of 250, 125, and 62.5 μg/mL, remarkably enhanced the inhibitory effects of novobiocin (1/4 × MIC; 1-4 μg/mL) against XDRAB isolates. Synergistic effects of novobiocin at 1/4 × MIC and 1/8 × MIC in combination with H. antidysenterica either at 31.2, 15.6, or 7.8 μg/mL against clinical isolates non-MDRAB, MDRAB, and XDRAB were evidenced for 80% of the combinations (189 out of 234 combinations). Although, no enhancement of the accumulation of ethidium bromide was observed after treated with H. antidysenterica, this plant extract weakened the outer membrane of the pathogen as indicated by an increase in the N-phenyl-1-naphthylamine uptake. Our results suggested that H. antidysenterica which primarily interrupts membrane permeability should be further investigated as a promising resistant modifier for A. baumannii.

Fabrication of Polysulfone/ZnO Membrane: Influence of ZnO Nanoparticles on Membrane Characteristics

ABSTRACTA mixed matrix membrane consisting of a polysulfone and nano‐ZnO particles has been fabricated by a combination of solution dispersion blending and phase‐inversion method. The study is focused on the influence of ZnO on membrane morphology, surface roughness, and hydrophilicity, which were investigated through the methods of scanning electron microscopy, atomic force microscopy, and contact angle meter, respectively. In addition, the changes in membrane characteristics with the addition nano‐ZnO particles were examined by pure water flux, water content and porosity, and salts rejection including NaCl and MgSO4. From the results, it was revealed that nano‐ZnO particles constructed a membrane having more hydrophilic, smooth, and tightly packed surface, which offered a high flux and an effective permeability barrier. Moreover, it was observed that nano‐ZnO particles produced spongy a membrane with uniform and well‐interconnecting pores. © 2013 Wiley Periodicals, Inc. Adv Polym Technol 2013, 32, 21369; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21369

Skin Membrane Electrical Impedance Properties under the Influence of a Varying Water Gradient

The stratum corneum (SC) is an effective permeability barrier. One strategy to increase drug delivery across skin is to increase the hydration. A detailed description of how hydration affects skin permeability requires characterization of both macroscopic and molecular properties and how they respond to hydration. We explore this issue by performing impedance experiments on excised skin membranes in the frequency range 1Hz to 0.2 MHz under the influence of a varying gradient in water activity (aw). Hydration/dehydration induces reversible changes of membrane resistance and effective capacitance. On average, the membrane resistance is 14 times lower and the effective capacitance is 1.5 times higher when the outermost SC membrane is exposed to hydrating conditions (aw= 0.992), as compared to the case of more dehydrating conditions (aw= 0.826). Molecular insight into the hydration effects on the SC components is provided by natural-abundance (13)C polarization transfer solid-state NMR and x-ray diffraction under similar hydration conditions. Hydration has a significant effect on the dynamics of the keratin filament terminals and increases the interchain spacing of the filaments. The SC lipids are organized into lamellar structures with ∼ 12.6 nm spacing and hexagonal hydrocarbon chain packing with mainly all-trans configuration of the acyl chains, irrespective of hydration state. Subtle changes in the dynamics of the lipids due to mobilization and incorporation of cholesterol and long-chain lipid species into the fluid lipid fraction is suggested to occur upon hydration, which can explain the changes of the impedance response. The results presented here provide information that is useful in explaining the effect of hydration on skin permeability.

Exposure of Mycobacteria to Cell Wall-inhibitory Drugs Decreases Production of Arabinoglycerolipid Related to Mycolyl-arabinogalactan-peptidoglycan Metabolism

The "cell wall core" consisting of a mycolyl-arabinogalactan-peptidoglycan (mAGP) complex represents the hallmark of the mycobacterial cell envelope. It has been the focus of intense research at both structural and biosynthetic levels during the past few decades. Because it is essential, mAGP is also regarded as a target for several antitubercular drugs. Herein, we demonstrate that exposure of Mycobacterium bovis Bacille Calmette-Guérin or Mycobacterium marinum to thiacetazone, a second line antitubercular drug, is associated with a severe decrease in the level of a major apolar glycolipid. This inhibition requires MmaA4, a methyltransferase reported to participate in the activation process of thiacetazone. Following purification, this glycolipid was subjected to detailed structural analyses, combining gas-liquid chromatography, mass spectrometry, and nuclear magnetic resonance. This allowed to identify it as a 5-O-mycolyl-β-Araf-(1→2)-5-O-mycolyl-α-Araf-(1→1)-Gro, designated dimycolyl diarabinoglycerol (DMAG). The presence of DMAG was subsequently confirmed in other slow growing pathogenic species, including Mycobacterium tuberculosis. DMAG production was stimulated in the presence of exogenous glycerol. Interestingly, DMAG appears structurally identical to the terminal portion of the mycolylated arabinosyl motif of mAGP, and the metabolic relationship between these two components was provided using antitubercular drugs such as ethambutol or isoniazid known to inhibit the biosynthesis of arabinogalactan or mycolic acid, respectively. Finally, DMAG was identified in the cell wall of M. tuberculosis. This opens the possibility of a potent biological function for DMAG that may be important to mycobacterial pathogenesis.

How hydrophobic molecules traverse the outer membranes of Gram-negative bacteria

The outer membrane (OM) of Gram-negative bacteria provides an effective barrier to their often-harsh extracellular milieu. In particular, the outer leaflet of the OM is not a canonical monolayer of phospholipids. Rather, it is composed of lipopolysaccharide (LPS), a molecule generally consisting of a core of Lipid A decorated with inner and outer core oligosaccharides. The oligosaccharides extend ∼30 A above the plane of the lipid headgroups of the outer leaflet. As such, it is an effective permeability barrier against potentially harmful compounds (1). However, obviously, permeability is required for bacterial survival; no bacterium is an island, as it were. For example, uptake of nutrients is essential, and OM transport proteins are required to conduct this function. The recent paper of Lepore et al. (2) has significantly extended our understanding of how hydrophobic molecules are transported across the OM. With rare exception (e.g., ref. 3), virtually all OM proteins are β-barrels, consisting of an even number of eight to twenty-four of β-strands forming a pore-like structure. Many of these OM pore-like β-barrels are classified as porins, and most nutrient uptake is accomplished by them. The effective aperture of the porin is dictated by the number of β-strands, and the aperture size then dictates the size (and shape) of the solutes that can diffuse through them. Porins function passively, permitting the energy-independent diffusion of solute molecules with a molecular mass of 600 Da or less downhill across a concentration gradient, through the porin's β-barrel, and into the periplasm. Another class of energy-independent OM transporters uses low-affinity binding sites that effectively serve to amplify small concentration gradients at the site of the transporter (4).

Generation of permeability barriers during melt extraction at mid‐ocean ridges

Melt focusing at mid‐ocean ridges is necessary to explain the narrowness of the zone of crustal accretion and the formation of large but localized on‐axis seamounts at slow and ultraslow spreading centers. It has been proposed that melt focusing is facilitated by the presence of a barrier to upward melt migration at the base of the thermal boundary layer (TBL). We assess the development of a melt impermeable boundary by modeling the geochemical evolution and crystallization history of melts as they rise into the TBL of mid‐ocean ridges with different spreading rates. A permeability barrier, associated with a crystallization front controlled by the conductive thermal regime, exists for melt trajectories at slow to fast spreading ridges (≥10 mm/yr half rate). The effective lateral scope of the barrier, where the slope of the barrier exceeds a critical value that allows buoyant melt transport to the axis, generally increases with spreading rate. At all distances from the axis at ultraslow ridges and off‐axis at slow spreading ridges, the weak crystallization front may prohibit formation of an efficient barrier and lead to the possibility that some fraction of melt may be incorporated into the lithospheric mantle, allowing refertilization. The protracted crystallization history and potential absence of an effective permeability barrier may explain the dearth of volcanism at ultraslow ridges and calls for a revision of lateral melt focusing scenarios at ultraslow spreading rates.

A Novel Polymyxin Derivative That Lacks the Fatty Acid Tail and Carries Only Three Positive Charges Has Strong Synergism with Agents Excluded by the Intact Outer Membrane

Polymyxins are cationic lipopeptides (five cationic charges) and the last resort for the treatment of serious Gram-negative infections caused by multiresistant strains. NAB741 has a cyclic peptide portion identical to that of polymyxin B but carries in the linear peptide portion a threonyl-D-serinyl residue (no cationic charges) instead of the diaminobutyryl-threonyl-diaminobutyryl residue (two cationic charges). At the N terminus of the peptide, NAB741 carries an acetyl group instead of a mixture of methyl octanoyl and methyl heptanoyl residues. NAB741 sensitized Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, and Acinetobacter baumannii to antibiotics against which the intact outer membrane is an effective permeability barrier. When tested by using Etest strips on plates containing increasing concentrations of NAB741, the fractional inhibition concentration index (FICI) of the combination of NAB741 with rifampin ranged from <or=0.111 to 0.158 and that with clarithromycin from <or=0.094 to 0.292. When tested by the checkerboard method, the corresponding FICI values against E. coli ATCC 25922 were <or=0.141 to <or=0.155 with rifampin and 0.094 with clarithromycin. In addition, at 4 microg/ml, NAB741 decreased the MICs of azithromycin, mupirocin, fusidic acid, and vancomycin for E. coli strains and E. cloacae by factors ranging from 8 to 200. A sister peptide, NAB752, carrying a threonyl-aminobutyryl residue as the linear peptide portion, was inactive. Furthermore, NAB741 sensitized E. coli to the bactericidal activity of fresh guinea pig serum. The renal clearance of NAB741 was approximately 400-fold, 16-fold, and 8-fold higher than those measured for colistin, NAB7061, and NAB739, respectively.