bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow structure and dynamic particle accumulation in thermocapillary convection in a liquid bridge
Authors: Tanaka S. , Kawamura H. , Ueno I. , Schwabe D.
Published: July 2006

Thermocapillary convection is induced in a liquid bridge by a nonuniform surface tension distribution caused by an axial temperature difference. A toroidal vortex is formed by the thermocapillary force over the free surface. The induced flow is visualized by using fine particles as tracers. At a sufficiently high Marangoni number, three-dimensional standing and traveling oscillatory flows appear, and under certain flow conditions, the tracer particles form particle accumulation structures (PAS). In the present study, the conditions for the occurrence of PAS have been carefully investigated with focus on the spiral loop PAS (SL-PAS) that appears when the flow exhibits a traveling mode. The particles gather along a closed spiral loop that winds itself around the toroidal vortex. Observed from above, the spiral loop looks as if it is rotating azimuthally. The number of spirals corresponds with the azimuthal wave number of the traveling wave and each spiral consists of either single or double turns. The azimuthal traveling direction of the particles trapped on the SL-PAS is opposite to that of the SL-PAS pattern and of the hydrothermal wave under the presently focused conditions. By varying particle diameter and density within a certain range, it was revealed that the SL-PAS appears almost independently of the particle properties. The path line of each particle trapped in the SL-PAS is different from the shape of the SL-PAS itself. The Stokes number of a particle is examined and found to be much smaller than unity. Furthermore, a structure similar to the SL-PAS was also visualized by injecting colored dye. Thus, the shape of the SL-PAS is primarily determined not by the particle-particle interaction but by the flow field itself.
Effect of nanoscale bubbles on viscous flow and rupture in thin liquid films
Authors: Ajaev Vladimir S.
Published: July 2006

Nanoscale bubbles between a thin liquid film and a hydrophobic solid substrate affect rupture by relaxing the no-slip boundary condition for the viscous flow in the liquid. They also cause appreciable changes in the shape of the flow domain and the value of disjoining pressure when the film thickness is comparable to the bubble size. We investigate all three effects systematically for a single gas bubble on the substrate under the liquid film in the framework of a lubrication-type model. A scaling law for film thickness versus time is found, but the scaling exponent is different from the case when no bubble is present. Rupture time is studied as a function of bubble size and disjoining pressure parameters. Presence of bubbles is shown to significantly decrease rupture time and can also be the cause of instability and rupture in otherwise stable films.
Energy decay of rotating turbulence with confinement effects
Authors: Morize C. , Moisy F.
Published: July 2006

The energy decay of grid-generated turbulence in a rotating tank is experimentally investigated by means of particle image velocimetry. For times smaller than the Ekman time scale, a range of approximate self-similar decay is found, in the form u2(t)∝t−n, with the exponent n decreasing from 2 to values close to 1 as the rotation rate is increased. Even at very weak rotation rates, rotation is shown to have a strong indirect influence on the decay law, by making the integral length scale to quickly saturate to the experiment size through the propagation of inertial waves. The experimental decay exponents are found in good agreement with the predicted values from a phenomenological model based on the exponent of the energy spectrum, in which both the effects of the rotation and the confinement are taken into account.
Separating the effects of wall blocking and near-wall shear in the interaction between the wall and the free shear layer in a wall jet
Authors: Dejoan A. , Leschziner M. A.
Published: July 2006

The statistical and structural characteristics of two plane jets, one developing along a real wall and the other along a frictionless wall (equivalent to a zero-shear, nondeformable free surface) are compared by way of highly resolved LES (large eddy simulation) solutions at computational conditions close to those of DNS (direct numerical simulation). The aim is to distinguish between two types of influence of the wall on the outer shear layer: one inviscid, arising from wall blocking, and the other, associated with the near-wall shear in the boundary layer. Results are presented for mean-flow properties, second moments and budgets thereof, structural characteristics, and the integral length scale. The comparisons demonstrate that the wall affects a significant proportion of the outer shear layer to a depth of approximately 3 times the thickness of the boundary layer, with or without wall shear. Outside the immediate near-wall layer, in the interaction region, the influence of the wall is affected by an interplay between turbulence diffusion toward the wall and inviscid processes associated with pressure fluctuations and their reflection from the wall. The addition of shear modifies substantially the statistical behavior and structure within the thin sheared region. The state of anisotropy and the energy-redistribution process among the normal-stress components change drastically, the structure is dominated by small scale, elongated eddies, and the near-wall layer is observed to ‘shield’ the wall from the penetration of large-scale vortices from the outer shear layer. The role of diffusion, in particular, renders the near-wall shear layer very different from a conventional equilibrium boundary layer, its integral scale being considerably enhanced by the influence of large-scale eddies originating in the outer shear layer and migrating toward the wall.
On the behavior of two-equation models in nonequilibrium homogeneous turbulence
Authors: Pettersson-Reif B. A. , Gatski T. B. , Rumsey C. L.
Published: July 2006

The class of turbulence models that utilize two-equation differential transport equations is investigated. A dynamical systems analysis is performed on these transport equations to identify the various critical points and associated stability characteristics. The goal is to understand the transient solution behavior and to identify the possible solution fixed points. The analysis is restricted to homogeneous flows with constant and time-dependent mean shear. The transient behavior of the turbulence variables identifies possible pseudolaminar solutions that can be generated. These solutions are unphysical and need to be avoided. The present study provides the necessary framework for analyzing such two-equation turbulence models. Finally it is shown that models rigorously derived from Reynolds-stress transport models by utilizing the algebraic stress approximation, e.g., explicit algebraic stress models, still constitute a justifiable method also when the mean shear varies in time, provided that the time variation is sufficiently slow. Closed form algebraic time dependent solutions are derived for some commonly used models in homogeneous shear flows.
Batchelor’s spectrum from an axisymmetric strained scalar field
Authors: Higgins Keith , Ooi Andrew , Chong M. S.
Published: July 2006

The Pullin and Lundgren [Phys. Fluids 13, 2553 (2001)] model of passive scalar transport together with an axisymmetric solution of the advection diffusion equation is used to model the scalar variance spectrum in the viscous-convective subrange. When the Schmidt number is large, the resulting spectrum shows k−1 scaling before an ultimate diffusive cutoff, in agreement with Batchelor’s [J. Fluid Mech. 5, 113 (1959)] earlier result. The present analysis shows how the k−1 range gradually emerges as the Schmidt number is steadily increased, and it provides an estimate of the Batchelor constant qB=215. This value, which depends on assumptions discussed within the paper, is large compared to most other theoretical, experimental, and numerical results.
Magnetic field effects on gas discharge plasmas
Authors: Sternberg Natalia , Godyak Valery , Hoffman Daniel
Published: July 2006

A study of the fluid model for cylindrical weakly ionized quasineutral plasmas in an axial magnetic field is presented. The model takes into account ionization, ion and electron inertia, as well as frictional forces for ions and electrons. The behavior of the plasma parameters for arbitrary magnitudes of the magnetic field, arbitrary gas pressure, and plasma size is presented, making the model applicable for a wide range of discharge conditions. A magnetic field parameter is introduced, which specifies a parameter range for the magnetic field, gas pressure, and plasma size where the Boltzmann equilibrium with the ambipolar field for the electron distribution is satisfied. In addition, a parametric relation for the magnetic field, gas pressure, and plasma size is obtained, which separates the region of weak magnetic field effects from the region of strong magnetic field effects. For strongly magnetized plasmas, an asymptotic solution with nonzero plasma density at the plasma boundary is presented. Analytical approximations for the ionization frequency and the plasma density at the plasma boundary are found for arbitrary external discharge parameters. The theoretical results are supported by numerical computations.
On the application of confined twin-jet instability to micro-mixing enhancement
Authors: Nahum A. , Seifert A.
Published: July 2006

The confined twin-jet hydrodynamic instability is exploited to enhance mixing of low Reynolds number laminar flows. The baseline flow is investigated numerically and experimentally for three configurations, differing by the jet spacing and wall confinement. Using hot-wire anemometry and particle image velocimetry, the Reynolds and Strouhal numbers associated with flow bifurcations were detected and are in good agreement with current and previously published numerical simulations. Superimposing weak harmonic excitation on the baseline twin-jets flow is shown to significantly enhance the naturally unstable modes, and to generate considerable perturbations at the jets merging area. The influence of excitation frequency and amplitude on the resulting unsteadiness is also studied. It is found that applying excitation with the same Strouhal number to subcritical Reynolds number flow creates similar perturbation structures as for supercritical Reynolds number flows. These findings demonstrate the potential of the confined twin-jet geometry to be utilized to enhance mixing for a wide range of applications where turbulent mixing is absent.
Ion-acoustic dressed solitons in a dusty plasma
Authors: Tiwari R. S. , Mishra M. K.
Published: July 2006

Using the reductive perturbation method, equations for ion-acoustic waves governing the evolution of first- and second-order potentials in a dusty plasma including the dynamics of charged dust grains have been derived. The renormalization procedure of Kodama and Taniuti is used to obtain a steady state nonsecular solution of these equations. The variation of velocity and width of the Korteweg-de Vries (KdV) as well as dressed solitons with amplitude have been studied for different concentrations and charge multiplicity of dust grains. The higher-order perturbation corrections to the KdV soliton description significantly affect the characteristics of the solitons in dusty plasma. It is found that in the presence of positively charged dust grains the system supports only compressive solitons. However, the plasma with negatively charged dust grains can support compressive solitons only up to a certain concentration of dust. Above this critical concentration of negative charge, the dusty plasma can support rarefactive solitons. An expression for the critical concentration of negatively charged dust in terms of charge and mass ratio of dust grains with plasma ions is also derived.
Direct numerical simulation of impinging shock wave/turbulent boundary layer interaction at M=2.25
Authors: Pirozzoli Sergio , Grasso Francesco
Published: July 2006

The interaction of a spatially developing adiabatic boundary layer flow at M∞=2.25 and Reθ=3725 with an impinging oblique shock wave (β=33.2°) is analyzed by means of direct numerical simulation of the compressible Navier-Stokes equations. Under the selected flow conditions the incoming boundary layer undergoes mild separation due to the adverse pressure gradient. Coherent structures are shed near the average separation point and the flow field exhibits large-scale low-frequency unsteadiness. The formation of the mixing layer is primarily responsible for the amplification of turbulence, which relaxes to an equilibrium state past the interaction. Complete equilibrium is attained in the inner part of the boundary layer, while in the outer region the relaxation process is incomplete. Far from the interaction zone, turbulence exhibits a universal behavior and it shows similarities with the incompressible case. The interaction of the coherent structures with the incident shock produces acoustic waves that propagate upstream, thus inducing an oscillatory motion of the separation bubble and a subsequent flapping motion of the reflected shock. The simulation indicates the occurrence of low-frequency tones in the interaction zone associated with peaks in the pressure spectra at discrete frequencies. We propose that such large-scale low-frequency unsteadiness is due to a resonance mechanism that establishes in the interaction region, and which has close similarities with those responsible for the generation of tones in cavity flows and screeching jets. In order to support our claim, we develop a simplified model for the acoustic resonance that is capable to predict the characteristic frequencies of the tones.
Electron acceleration due to small pulses in shock waves in inhomogeneous plasmas: Perpendicular propagation
Authors: Sato Masatoshi , Ohsawa Yukiharu
Published: July 2006

Electron acceleration in shock waves propagating perpendicular to an external magnetic field, which is inhomogeneous and reversed in a finite region, is studied with relativistic particle simulations. By the time a shock wave reaches the inhomogeneous region, some electrons are energized by a small compressive pulse generated in the shock wave. After penetrating this region, the original shock wave is damped. However, a new shock wave with the magnetic polarity opposite to that of the original shock wave is excited. In this secondary shock wave, particle-accelerating compressive pulses are also generated. Further, after the encounter with the original shock wave, the magnetic null point starts to move with a slightly slower speed than the secondary shock wave, and electric fields are formed around it. Some electrons also gain energy in meandering orbits around the field-reversed pulse.
Relaxation of a relativistic electron beam in plasma in the trapping regime
Authors: Timofeev I. V. , Lotov K. V.
Published: July 2006

A model for collective relaxation of high-power relativistic electron beams in plasmas is proposed, which describes beam-plasma interaction in the regime when amplitudes of unstable waves are large enough to trap beam electrons. The distinctive feature of this regime is that the power lost by beam electrons as they interact with exited wave packets weakly depends on the energy of these packets. This makes the model insensitive to the nature of nonlinear processes in plasma, which are responsible for saturation of the beam instability. The model is thus rather universal and suitable for quantitative comparison with the experimental data. The predicted profiles of energy release along the plasma column are in a good quantitative agreement with the ones measured at various experimental facilities.
Plant cells recognize chitin fragments for defense signaling through a plasma membrane receptor
Authors: Kaku Hanae , Nishizawa Yoko , Ishii-Minami Naoko , Akimoto-Tomiyama Chiharu , Dohmae Naoshi , Takio Koji , Minami Eiichi , Shibuya Naoto
Published: July 2006

Chitin is a major component of fungal cell walls and serves as a molecular pattern for the recognition of potential pathogens in the innate immune systems of both plants and animals. In plants, chitin oligosaccharides have been known to induce various defense responses in a wide range of plant cells including both monocots and dicots. To clarify the molecular machinery involved in the perception and transduction of chitin oligosaccharide elicitor, a high-affinity binding protein for this elicitor was isolated from the plasma membrane of suspension-cultured rice cells. Characterization of the purified protein, CEBiP, as well as the cloning of the corresponding gene revealed that CEBiP is actually a glycoprotein consisting of 328 amino acid residues and glycan chains. CEBiP was predicted to have a short membrane spanning domain at the C terminus. Knockdown of
CEBiP
gene by RNA interference resulted in the suppression of the elicitor-induced oxidative burst as well as the gene responses, showing that CEBiP plays a key role in the perception and transduction of chitin oligosaccharide elicitor in the rice cells. Structural analysis of CEBiP also indicated the presence of two LysM motifs in the extracellular portion of CEBiP. As the LysM motif has been known to exist in the putative Nod-factor receptor kinases involved in the symbiotic signaling between leguminous plants and rhizobial bacteria, the result indicates the involvement of partially homologous plasma membrane proteins both in defense and symbiotic signaling in plant cells.
Hydrogen-bond switching through a radical pair mechanism in a flavin-binding photoreceptor
Authors: Gauden Magdalena , van Stokkum Ivo H. M. , Key Jason M. , Lührs Daniel Ch. , van Grondelle Rienk , Hegemann Peter , Kennis John T. M.
Published: July 2006

BLUF (blue light sensing using FAD) domains constitute a recently discovered class of photoreceptor proteins found in bacteria and eukaryotic algae, where they control a range of physiological responses including photosynthesis gene expression, photophobia, and negative phototaxis. Other than in well known photoreceptors such as the rhodopsins and phytochromes, BLUF domains are sensitive to light through an oxidized flavin rather than an isomerizable cofactor. To understand the physicochemical basis of BLUF domain photoactivation, we have applied femtosecond transient absorption spectroscopy to the Slr1694 BLUF domain of
Synechocystis PCC6803
. We show that photoactivation of BLUF domains proceeds by means of a radical-pair mechanism, driven by electron and proton transfer from the protein to the flavin, resulting in the transient formation of anionic and neutral flavin radical species that finally result in the long-lived signaling state on a 100-ps timescale. A pronounced deuteration effect is observed on the lifetimes of the intermediate radical species, indicating that proton movements underlie their molecular transformations. We propose a photoactivation mechanism that involves a successive rupture of hydrogen bonds between a conserved tyrosine and glutamine by light-induced electron transfer from tyrosine to flavin and between the glutamine and flavin by subsequent protonation at flavin N5. These events allow a reorientation of the conserved glutamine, resulting in a switching of the hydrogen-bond network connecting the chromophore to the protein, followed by radical-pair recombination, which locks the glutamine in place. It is suggested that the redox potential of flavin generally defines the light sensitivity of flavin-binding photoreceptors.
Homeostatic shutdown of long-term potentiation in the adult hippocampus
Authors: Roth-Alpermann Claudia , Morris Richard G. M. , Korte Martin , Bonhoeffer Tobias
Published: July 2006

Homeostasis is a key concept in biology. It enables ecosystems, organisms, organs, and cells to adjust their operating range to values that ensure optimal performance. Homeostatic regulation of the strength of neuronal connections has been shown to play an important role in the development of the nervous system. Here we investigate whether mature neurons also possess mechanisms to prevent the strengthening of input synapses once the limit of their operating range has been reached. Using electrophysiological recordings in hippocampal slices, we show that such a mechanism exists but comes into play only after a considerable number of synapses have been potentiated. Thus, adult neurons can sustain a substantial amount of synaptic strengthening but, once a certain threshold of potentiation is exceeded, homeostatic regulation ensures that no further strengthening can occur.
A molecular correlate to the Gleason grading system for prostate adenocarcinoma
Authors: True Lawrence , Coleman Ilsa , Hawley Sarah , Huang Ching-Ying , Gifford David , Coleman Roger , Beer Tomasz M. , Gelmann Edward , Datta Milton , Mostaghel Elahe , Knudsen Beatrice , Lange Paul , Vessella Robert , Lin Daniel , Hood Leroy , Nelson Peter S.
Published: July 2006

Adenocarcinomas of the prostate can be categorized into tumor grades based on the extent to which the cancers histologically resemble normal prostate glands. Because grades are surrogates of intrinsic tumor behavior, characterizing the molecular phenotype of grade is of potential clinical importance. To identify molecular alterations underlying prostate cancer grades, we used microdissection to obtain specific cohorts of cancer cells corresponding to the most common Gleason patterns (patterns 3, 4, and 5) from 29 radical prostatectomy samples. We paired each cancer sample with matched benign lumenal prostate epithelial cells and profiled transcript abundance levels by microarray analysis. We identified an 86-gene model capable of distinguishing low-grade (pattern 3) from high-grade (patterns 4 and 5) cancers. This model performed with 76% accuracy when applied to an independent set of 30 primary prostate carcinomas. Using tissue microarrays comprising >800 prostate samples, we confirmed a significant association between high levels of monoamine oxidase A expression and poorly differentiated cancers by immunohistochemistry. We also confirmed grade-associated levels of defender against death (DAD1) protein and HSD17β4 transcripts by immunohistochemistry and quantitative RT-PCR, respectively. The altered expression of these genes provides functional insights into grade-associated features of therapy resistance and tissue invasion. Furthermore, in identifying a profile of 86 genes that distinguish high- from low-grade carcinomas, we have generated a set of potential targets for modulating the development and progression of the lethal prostate cancer phenotype.
Symmetry breaking in individual plasmonic nanoparticles
Authors: Wang Hui , Wu Yanpeng , Lassiter Britt , Nehl Colleen L. , Hafner Jason H. , Nordlander Peter , Halas Naomi J.
Published: July 2006

The plasmon resonances of a concentric metallic nanoshell arise from the hybridization of primitive plasmon modes of the same angular momentum on its inner and outer surfaces. For a nanoshell with an offset core, the reduction in symmetry relaxes these selection rules, allowing for an admixture of dipolar components in all plasmon modes of the particle. This metallodielectric nanostructure with reduced symmetry exhibits a core offset-dependent multipeaked spectrum, seen in single-particle spectroscopic measurements, and exhibits significantly larger local-field enhancements on its external surface than the equivalent concentric spherical nanostructure.
Human impacts on the rates of recent, present, and future bird extinctions
Authors: Pimm Stuart , Raven Peter , Peterson Alan , Şekercioğlu Çağan H. , Ehrlich Paul R.
Published: July 2006

Unqualified, the statement that ≈1.3% of the ≈10,000 presently known bird species have become extinct since A.D. 1500 yields an estimate of ≈26 extinctions per million species per year (or 26 E/MSY). This is higher than the benchmark rate of ≈1 E/MSY before human impacts, but is a serious underestimate. First, Polynesian expansion across the Pacific also exterminated many species well before European explorations. Second, three factors increase the rate: (
i
) The number of known extinctions before 1800 is increasing as taxonomists describe new species from skeletal remains. (
ii
) One should calculate extinction rates over the years since taxonomists described the species. Most bird species were described only after 1850. (
iii
) Some species are probably extinct; there is reluctance to declare them so prematurely. Thus corrected, recent extinction rates are ≈100 E/MSY. In the last decades, the rate is <50 E/MSY, but would be 150 E/MSY were it not for conservation efforts. Increasing numbers of extinctions are on continents, whereas previously most were on islands. We predict a 21st century rate of ≈1,000 E/MSY. Extinction threatens 12% of bird species; another 12% have small geographical ranges and live where human actions rapidly destroy their habitats. If present forest losses continue, extinction rates will reach 1,500 E/MSY by the century’s end. Invasive species, expanding human technologies, and global change will harm additional species. Birds are poor models for predicting extinction rates for other taxa. Human actions threaten higher fractions of other well known taxa than they do birds. Moreover, people take special efforts to protect birds.
Sub-parts-per-billion level detection of NO2using room-temperature quantum cascade lasers
Authors: Pushkarsky Michael , Tsekoun Alexei , Dunayevskiy Ilya G. , Go Rowel , Patel C. Kumar N.
Published: July 2006

We report the sub-parts-per-billion-level detection of NO2using tunable laser-based photoacoustic spectroscopy where the laser radiation is obtained from a room-temperature continuous-wave high-power quantum cascade laser operating in an external grating cavity configuration. The continuously tunable external grating cavity quantum cascade laser produces maximum single-frequency output of ≈300 mW tunable over ≈350 nm centered at 6.25 μm. We demonstrate minimum detection level of ≈0.5 parts per billion of NO2in the presence of humidified air.
Single molecules of the bacterial actin MreB undergo directed treadmilling motion in
Caulobacter
crescentus
Authors: Kim So Yeon , Gitai Zemer , Kinkhabwala Anika , Shapiro Lucy , Moerner W. E.
Published: July 2006

The actin cytoskeleton represents a key regulator of multiple essential cellular functions in both eukaryotes and prokaryotes. In eukaryotes, these functions depend on the orchestrated dynamics of actin filament assembly and disassembly. However, the dynamics of the bacterial actin homolog MreB have yet to be examined
in vivo
. In this study, we observed the motion of single fluorescent MreB–yellow fluorescent protein fusions in living
Caulobacter
cells in a background of unlabeled MreB. With time-lapse imaging, polymerized MreB [filamentous MreB (fMreB)] and unpolymerized MreB [globular MreB (gMreB)] monomers could be distinguished: gMreB showed fast motion that was characteristic of Brownian diffusion, whereas the labeled molecules in fMreB displayed slow, directed motion. This directional movement of labeled MreB in the growing polymer provides an indication that, like actin, MreB monomers treadmill through MreB filaments by preferential polymerization at one filament end and depolymerization at the other filament end. From these data, we extract several characteristics of single MreB filaments, including that they are, on average, much shorter than the cell length and that the direction of their polarized assembly seems to be independent of the overall cellular polarity. Thus, MreB, like actin, exhibits treadmilling behavior
in vivo
, and the long MreB structures that have been visualized in multiple bacterial species seem to represent bundles of short filaments that lack a uniform global polarity.
A phospho-signaling pathway controls the localization and activity of a protease complex critical for bacterial cell cycle progression
Authors: Iniesta Antonio A. , McGrath Patrick T. , Reisenauer Ann , McAdams Harley H. , Shapiro Lucy
Published: July 2006

Temporally and spatially controlled master regulators drive the
Caulobacter
cell cycle by regulating the expression of >200 genes. Rapid clearance of the master regulator, CtrA, by the ClpXP protease is a critical event that enables the initiation of chromosome replication at specific times in the cell cycle. We show here that a previously unidentified single domain-response regulator, CpdR, when in the unphosphorylated state, binds to ClpXP and, thereby, causes its localization to the cell pole. We further show that ClpXP localization is required for CtrA proteolysis. When CpdR is phosphorylated, ClpXP is delocalized, and CtrA is not degraded. Both CtrA and CpdR are phosphorylated via the same CckA histidine kinase phospho-signaling pathway, providing a reinforcing mechanism that simultaneously activates CtrA and prevents its degradation by delocalizing the CpdR/ClpXP complex. In swarmer cells, CpdR is in the phosphorylated state, thus preventing ClpXP localization and CtrA degradation. As swarmer cells differentiate into stalked cells (G
1
/S transition), unphosphorylated CpdR accumulates and is localized to the stalked cell pole, where it enables ClpXP localization and CtrA proteolysis, allowing the initiation of DNA replication. Dynamic protease localization mediated by a phospho-signaling pathway is a novel mechanism to integrate spatial and temporal control of bacterial cell cycle progression.
Gastrointestinal effects of prebiotics
Authors: Cummings J. H. , Macfarlane G. T.
Published: July 2006

The defining effect of prebiotics is to stimulate selectively the growth of bifidobacteria and lactobacilli in the gut and, thereby, increase the body's natural resistance to invading pathogens. Prebiotic carbohydrates may also have additional, less specific, benefits because they are fermented in the large intestine. The prebiotic carbohydrates that have been evaluated in humans at the present time largely consist of fructans or galactans. There is consistent evidence from in vitro and in vivo studies that these are not digested by normal human enzymes, but are readily fermented by anaerobic bacteria in the large intestine. There are no reports of faecal recovery of measurable quantities of prebiotic carbohydrates. Through fermentation in the large intestine, prebiotic carbohydrates yield short-chain fatty acids, stimulate the growth of many bacterial species in addition to the selective effects on lactobacilli and bifidobacteria, they can also produce gas. Along with other fermented carbohydrates, prebiotics have mild laxative effects, although this has proved difficult to demonstrate in human studies because the magnitude of laxation is small. Potentially, the most important effect of prebiotic carbohydrates is to strengthen the body's resistance to invading pathogens and, thereby, prevent episodes of diarrhoea. At the present time, this effect has not been convincingly demonstrated in either adults or children, although there have been attempts to ameliorate the diarrhoea associated with antibiotics and travel, but without success. However, prebiotic carbohydrates clearly have significant and distinctive physiological effects in the human large intestine, and on the basis of this it is likely that they will ultimately be shown to be beneficial to health.
Fermentation and bulking capacity of indigestible carbohydrates: the case of inulin and oligofructose
Authors: Nyman M.
Published: July 2006

The bulking index (i.e. the increase in faecal fresh weight in gram per gram indigestible carbohydrate ingested) with oligofructose and inulin is similar to that produced with other easily fermented fibres such as pectins and gums. Most studies in man have been performed at a level of 15 g/d and more investigations on lower intakes are needed to appoint the least intake for an effect. Concerning short-chain fatty acids (SCFA) most studies have been using oligofructose and points at an increased butyric acid formation in the caecum of rats. In one study on rats with inulin high caecal proportions of propionic acid were obtained. As inulin has a higher molecular weight than oligofructose it might be speculated if this could be a reason to the different SCFA-profile formed. No effects on faecal concentrations of SCFA in humans have been revealed with inulin and oligofructose, which neither is expected as most of the SCFA formed during the fermentation already has been absorbed or utilized by the colonic mucosa.
Calcium intake, calcium bioavailability and bone health
Authors: Cashman K. D.
Published: July 2006

Calcium accounts for 1–2 % of adult human body weight. Over 99 % of total body Ca is found in the teeth and bones. Therefore, in addition to the obvious structural role of the skeleton, it also serves as a reservoir for Ca. Dietary Ca intake has an important impact on bone metabolism and bone health. Chronic Ca deficiency resulting from inadequate intake or poor intestinal absorption is one of several important causes of reduced bone mass and osteoporosis. It is vital, therefore, that adequate dietary Ca is consumed at all stages of life — in early life so that the genetically programmed peak bone mass can be reached and in later adulthood so that the skeletal mass can be maintained and age-related bone loss minimised. Unfortunately, there is wide variation in the estimates of daily Ca requirements made by different expert authorities. Furthermore, there is evidence that many individuals are not consuming these recommended levels. The consequence of this for bone health will be discussed in the present review. Besides the amount of Ca in the diet, the absorption of dietary Ca in foods is also a critical factor in determining the availability of Ca for bone development and maintenance. Thus, there is a need to identify food components and/or functional food ingredients that may positively influence Ca absorption in order to ensure that Ca bioavailability from foods can be optimised. This approach may be of particular value in individuals who fail to achieve the dietary recommended level of Ca.
Non-digestible oligosaccharides and calcium absorption in girls with adequate calcium intakes
Authors: Griffin I. J. , Davila P. M. , Abrams S. A.
Published: July 2006

Non-digestible oligosaccharides such as inulin and oligofructose have been shown to consistently increase calcium absorption in experimental animals, but data in humans are less clear-cut. The objective of this study was to assess the effect of 8 g/d of oligofructose or a mixture of inulin and oligofructose on calcium absorption in girls at or near menarche. A total of fifty-nine subjects were studied using a balanced, randomized, cross-over design. They received, in random order, 8 g/d placebo (sucrose), oligofructose or the mixture inulin+oligofructose for 3 weeks, separated by a 2-week washout period. Throughout the study, subjects consumed a total of approximately 1500 mg/d dietary calcium, by adding two glasses of calcium-fortified orange juice to their diet. Four grams of placebo, oligofructose or the mixture inulin+oligofructose was added to each glass of orange juice immediately before it was consumed. At the end of each 3-week adaptation period, calcium absorption was measured, using a dual stable isotope technique, from the cumulative fractional excretion of an oral and an intravenous tracer over 48 hours. Calcium absorption was significantly higher in the group receiving the inulin+oligofructose mixture than in the placebo group (38·2±9·8 % v. 32·3±9·8 %; P=0·01), but no significant difference was seen between the oligofructose group and the placebo group (31·8±9·3 % v. 31·8±10·0 %, P=NS). We conclude that modest intakes of an inulin+oligofructose mixture increases calcium absorption in girls at or near menarche.
Molecular methods for exploring the intestinal ecosystem
Authors: Tannock G. W.
Published: July 2006

Molecular methods have provided renewed impetus for the analysis of the composition of the intestinal microflora in health and disease. The polymerase chain reaction coupled with denaturing gradient gel electrophoresis provides a method whereby the bacterial communities in large numbers of samples can be compared efficiently and effectively. Altered bacterial populations associated with disease states can then be targeted for further investigation. In the long-term, an ‘abnormal microflora’ might be rectified by the use of probiotics or prebiotics.
Oligofructose and experimental model of neonatal necrotising enterocolitis
Authors: Butel M.-J. , Waligora-Dupriet A.-J. , Szylit O.
Published: July 2006

The gut of preterm neonates is colonised with a paucity of bacterial species originating more from the environment than from the mother. Furthermore, a delayed colonisation by bifidobacteria promotes colonisation by potentially pathogenic bacteria. This may contribute towards the development of neonatal necrotising enterocolitis (NEC). The physiopathology of NEC is still unclear but immaturity of the gut, enteral feeding and bacterial colonisation are all thought to be involved. None of the current preventive treatments are considered satisfactory. Modulating the autochthonous microflora by probiotics or prebiotics could be a more reliable approach to prevention. Using gnotobiotic quails as an experimental model of NEC we have shown that onset of intestinal lesions requires a combination of low endogenous lactase activity, lactose in diet, and colonisation by lactose-fermenting bacteria such as the clostridia. The protective role of bifidobacteria was demonstrated in this model through a decrease in clostridial populations and in butyric acid. Oligofructose dietary supplementation was shown to enhance this effect with an increase in the bifidobacterial level and consequently a greater decrease in clostridia. However, oligofructose was unable to promote a bifidobacterial acquisition when the microflora was initially deprived of this group. Nevertheless, oligofructose can act as an anti-infective agent and decrease the occurrence or severity of the lesions depending on the bacteria involved. According to these results and to the fact that oligosaccharides are a major component of breast milk, the addition of oligofructose in formula milks may be a nutritional approach to favouring colonisation by a beneficial flora.
The immune-enhancing effects of dietary fibres and prebiotics
Authors: Schley P. D. , Field C. J.
Published: July 2006

The gastrointestinal tract is subjected to enormous and continual foreign antigenic stimuli from food and microbes. This organ must integrate complex interactions among diet, external pathogens, and local immunological and non-immunological processes. It is critical that protective immune responses are made to potential pathogens, while hypersensitivity reactions to dietary antigens are minimised. There is increasing evidence that fermentable dietary fibres and the newly described prebiotics can modulate various properties of the immune system, including those of the gut-associated lymphoid tissues (GALT). This paper reviews evidence for the immune-enhancing effects of dietary fibres. Changes in the intestinal microflora that occur with the consumption of prebiotic fibres may potentially mediate immune changes via: the direct contact of lactic acid bacteria or bacterial products (cell wall or cytoplasmic components) with immune cells in the intestine; the production of short-chain fatty acids from fibre fermentation; or by changes in mucin production. Although further work is needed to better define the changes, mechanisms for immunomodulation, and the ultimate impact on immune health, there is convincing preliminary data to suggest that the consumption of prebiotics can modulate immune parameters in GALT, secondary lymphoid tissues and peripheral circulation. Future protocols on the physiological impact of consuming prebiotics should be designed to include assessments of the gut microflora, gut physiology and the function and composition of the various regions of GALT.
Dietary carbohydrate's effects on lipogenesis and the relationship of lipogenesis to blood insulin and glucose concentrations
Authors: Parks E. J.
Published: July 2006

The process by which dietary carbohydrate is transformed into fat in the human body is termedde novolipogenesis. New methods for the measurement of this process in humans are available and have been used to investigate the role of the carbohydrate form (fed as a liquid or solid), the level of processing of carbohydrate in foods, and the role of lipogenesis in the control of liver triacylglycerol secretion. The present paper will discuss how research results are affected by both the physical state of the carbohydrate in the diet and by the metabolic state of individual research subjects. Of interest is the relationship between the glycemic index of a food (or indicators of a food's glycemic index) and that food's ability to stimulate lipogenesis in humans. Given the increasing prevalence of obesity worldwide, future scientific emphasis will expand methods to quantitate the lipogenic potential of specific foods and dietary patterns and investigate how the metabolic state of insulin resistance affects lipogenesis and/or contributes to obesity.
Inulin and oligofructose modulate lipid metabolism in animals: review of biochemical events and future prospects
Authors: Delzenne N. M. , Daubioul C. , Neyrinck A. , Lasa M. , Taper H. S.
Published: July 2006

Inulin and oligofructose, besides their effect on the gastro-intestinal tract, are also able to exert ‘systemic’ effect, namely by modifying the hepatic metabolism of lipids in several animal models. Feeding male Wistar rats on a carbohydrate-rich diet containing 10 % inulin or oligofructose significantly lowers serum triacylglycerol (TAG) and phospholipid concentrations. A lower hepatic lipogenesis, through a coordinate reduction of the activity and mRNA of lipogenic enzymes is a key event in the reduction of very low-density lipoprotein-TAG secretion by oligofructose. Oligofructose is also able to counteract triglyceride metabolism disorder occurring through dietary manipulation in animals, and sometimes independently on lipogenesis modulation: oligofructose reduces post-prandial triglyceridemia by 50 % and avoids the increase in serum free cholesterol level occurring in rats fed a Western-type high fat diet. Oligofructose protects rats against liver TAG accumulation (steatosis) induced by fructose, or occurring in obese Zucker fa/fa rats. The protective effect of dietary inulin and oligofructose on steatosis in animals, would be interesting, if confirmed in humans, since steatosis is one of the most frequent liver disorders, occurring together with the plurimetabolic syndrome, in overweight people. The panel of putative mediators of the systemic effects of inulin and oligofructose consists in either modifications in glucose/insulin homeostasis, the end-products of their colonic fermentation (i.e. propionate) reaching the liver by the portal vein, incretins and/or the availability of other nutrients. The identification of the key mediators of the systemic effects of inulin and oligofructose is the key to identify target function(s) (or dysfunction(s)), and finally individuals who would take an advantage of increasing their dietary intake.
Experimental evidences on the potential of prebiotic fructans to reduce the risk of colon cancer
Authors: Pool-Zobel B. , van Loo J. , Rowland I. , Roberfroid M. B.
Published: July 2006

Inulin is extracted from the chicory root. It is a set of fructans with its monomers (n=2–65) linked by means of β(2–1) bonds. This linkage cannot be hydrolysed by either pancreatic or by brush border digestive enzymes in the upper intestinal tract of humans. As such the carbohydrates arrive in the colon, where they are fermented by bifidobacteria and other lactic acid producing bacteria, thus enhancing their relative populations in the gut. Recent research in experimental animal models revealed that inulin has significant anticarcinogenic properties. It acts chemopreventively by reducing the incidence of azoxymethane (AOM) — induced aberrant crypt foci and tumours in the colon. These effects may be due to the stimulation of bifidobacteria, which themselves have been shown to act as antigenotoxic in the colon and to reduce AOM-induced tumours. Also fermentation products, including the short-chain fatty acid butyrate, could contribute to the protective effects. In this case a mechanism may be the induction of apoptosis of already transformed cells. The experimental evidence from animal studies and from studies elucidating potential mechanisms strongly supports the possibility that inulin will contribute to reducing risks for colon cancer in humans. In order to obtain more insight into this possibility, human dietary intervention studies relating biomarkers of reduced risk to inulin consumption are needed.
Inulin/oligofructose and anticancer therapy
Authors: Taper H. S. , Roberfroid M. B.
Published: July 2006

The results of our investigations indicate that dietary treatment with inulin or oligofructose incorporated in the basal diet for experimental animals: (i) reduced the incidence of mammary tumors induced in Sprague-Dawley rats by methylnitrosourea; (ii) inhibited the growth of transplantable malignant tumors in mice; and (iii) decreased the incidence of lung metastases of a malignant tumor implanted intramuscularily in mice. Moreover, besides such cancer risk reduction effects, the dietary treatment with inulin or oligofructose significantly potentiated the effects of subtherapeutic doses of six different cytotoxic drugs commonly utilized in human cancer treatment. If confirmed, such dietary treatment with inulin or oligofructose potentiating cancer therapy might become an interesting approach to complement classical protocols of human cancer treatment without any additional risk for the patients.
Oligosaccharides in infant formula
Authors: Vandenplas Y.
Published: July 2006

Breast-feeding is the golden standard for infant feeding. However, the majority of a few week old infants are fed with a second choice infant feeding, cow's milk based formula. Amongst the multiple differences between human and cow's milk regards the development of the gastro-intestinal flora: the flora of the breast-fed infant being richer in bifidobacteria and lactobacilli. Both species are known to be potentially beneficial for the health of the host. The absence of oligosaccharides, the third largest component in human milk, in cow's milk is likely to account for the differences in colonic flora. The oligosaccharide content and concentration in breast milk is — just as for the other macronutrients — a dynamic process, making it impossible for industry to mimic nature. However, if the composition cannot be mimicked, the effect and function can be imitated. The addition of two oligosaccharides, galacto-oligosaccharides and inulin, to cow's milk based infant formula has been shown to have a bifidogenic effect, and to stimulate the growth of bifidi and lactobacilli. In conclusion, the addition of oligosaccharides to cow's milk based infant formula brings this alternative, second choice infant feeding one step closer to the golden standard of human milk. But, prolonged breast-feeding should still be promoted with maximum effort.