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Biomedical subjects

C Wilhelm

Publications and source records attributed to C Wilhelm.

At least 19 recordsLinked to original sources
PubMed2006

Antibacterial activity of cefquinome against equine bacterial pathogens.

Cefquinome is known for its use as an antibacterial drug in cattle and pigs. The objective of this study was to evaluate the antibacterial activity of cefquinome against equine pathogenic bacteria. The minimum inhibitory concentration (MIC) of cefquinome was determined for a total of 205 strains, which had recently been isolated in Europe from diseased horses (respiratory infection, foal septicaemia). The bactericidal activity was tested against 19 strains using the time killing method. The post-antibiotic effect (PAE) and post-antibiotic sub-MIC effect (PA SME) were determined against 12 strains. Cefquinome showed high activity against Actinobacillus equuli and streptococci (MIC(90) of 0.016 and 0.032microg/mL), Enterobacteriaceae (MIC(90)=0.125microg/mL) and staphylococci (MIC(90)=0.5microg/mL). The activity was limited against Rhodococcus spp. and Pseudomonas spp. Cefquinome was shown to be a time dependent bactericidal antibiotic against the target pathogens, killing occurring at a concentration close to the MIC. A PAE of 0.5-10h was calculated against streptococci whereas no PAE was observed for Escherichia coli. A longer PA SME was determined for streptococci (3.3 to >24h with a killing effect) and E. coli (0.5-13.9h). Cefquinome was shown to have a broad spectrum of activity which covers many equine pathogens.

Animals↗
PubMed2006

Signaling through the phosphatidylinositol 3-kinase regulates mechanotaxis induced by local low magnetic forces in Entamoeba histolytica.

In micro-organisms, as well as in metazoan cells, cellular polarization and directed migration are finely regulated by external stimuli, including mechanical stresses. The mechanisms sustaining the transduction of such external stresses into intracellular biochemical signals remain mainly unknown. Using an external magnetic tip, we generated a magnetic field gradient that allows migration analysis of cells submitted to local low-intensity magnetic forces (50 pN). We applied our system to the amoeba Entamoeba histolytica. Indeed, motility and chemotaxis are key activities that allow this parasite to invade and destroy the human tissues during amoebiasis. The magnetic force was applied either inside the cytoplasm or externally at the rear pole of the amoeba. We observed that the application of an intracellular force did not affect cell polarization and migration, whereas the application of the force at the rear pole of the cell induced a persistent polarization and strongly directional motion, almost directly opposed to the magnetic force. This phenomenon was completely abolished when phosphatidylinositol 3-kinase activity was inhibited by wortmanin. This result demonstrated that the applied mechanical stimulus was transduced and amplified into an intracellular biochemical signal, a process that allows such low-intensity force to strongly modify the migration behavior of the cell.

Animals↗
PubMed2006

Balancing the energy flow from captured light to biomass under fluctuating light conditions.

The balance of energy flow from light absorption into biomass was investigated under simulated natural light conditions in the diatom Phaeodactylum tricornutum and the green alga Chlorella vulgaris. The energy balance was quantified by comparative analysis of carbon accumulation in the new biomass with photosynthetic electron transport rates per absorbed quantum, measured both by fluorescence quenching and oxygen production. The difference between fluorescence- and oxygen-based electron flow is defined as 'alternative electron cycling'. The photosynthetic efficiency of biomass production was found to be identical for both algae under nonfluctuating light conditions. In a fluctuating light regime, a much higher conversion efficiency of photosynthetic energy into biomass was observed in the diatom compared with the green alga. The data clearly show that the diatom utilizes a different strategy in the dissipation of excessively absorbed energy compared with the green alga. Consequently, in a fluctuating light climate, the differences between green algae and diatoms in the efficiency of biomass production per photon absorbed are caused by the different amount of alternative electron cycling.

Acclimatization↗
PubMed2003

Deformation of intracellular endosomes under a magnetic field.

We present a non-invasive method to monitor the membrane tension of intracellular organelles using a magnetic field as an external control parameter. By exploiting the spontaneous endocytosis of anionic colloidal ferromagnetic nanoparticles, we obtain endosomes possessing a superparamagnetic lumen in eukaryotic cells. Initially flaccid, the endosomal membrane undulates because of thermal fluctuations, restricted in zero field by the resting tension and the curvature energy of the membrane. When submitted to a uniform magnetic field, the magnetized endosomes elongate along the field, resulting in the flattening of the entropic membrane undulations. The quantification of the endosome deformation for different magnetic fields allows in situ measurement of the resting tension and the bending stiffness of the membrane enclosing the intracellular organelle.

Cell Membrane↗
PubMed2003

Rotational magnetic endosome microrheology: viscoelastic architecture inside living cells.

The previously developed technique of magnetic rotational microrheology [Phys. Rev. E 67, 011504 (2003)] is proposed to investigate the rheological properties of the cell interior. An endogeneous magnetic probe is obtained inside living cells by labeling intracellular compartments with magnetic nanoparticles, following the endocytosis mechanism, the most general pathway used by eucaryotic cells to internalize substances from an extracellular medium. Primarily adsorbed on the plasma membrane, the magnetic nanoparticles are first internalized within submicronic membrane vesicles (100 nm diameter) to finally concentrate inside endocytotic intracellular compartments (0.6 microm diameter). These magnetic endosomes attract each other and form chains within the living cell when submitted to an external magnetic field. Here we demonstrate that these chains of magnetic endosomes are valuable tools to probe the intracellular dynamics at very local scales. The viscoelasticity of the chain microenvironment is quantified in terms of a viscosity eta and a relaxation time tau by analyzing the rotational dynamics of each tested chain in response to a rotation of the external magnetic field. The viscosity eta governs the long time flow of the medium surrounding the chains and the relaxation time tau reflects the proportion of solidlike versus liquidlike behavior (tau=eta/G, where G is the high-frequency shear modulus). Measurements in HeLa cells show that the cell interior is a highly heterogeneous structure, with regions where chains are embedded inside a dense viscoelastic matrix and other domains where chains are surrounded by a less rigid viscoelastic material. When one compound of the cell cytoskeleton is disrupted (microfilaments or microtubules), the intracellular viscoelasticity becomes less heterogeneous and more fluidlike, in the sense of both a lower viscosity and a lower relaxation time.

Adsorption↗
PubMed2003

Rotational magnetic particles microrheology: the Maxwellian case.

An experimental method based on the rotational dynamics of a magnetic probe is reported to measure the local viscoelasticity of soft materials on microscopic scales. The technique is based on the alignment of dipolar chains of submicrometer magnetic particles in the direction of an applied magnetic field. On one hand, light scattering is used to detect the chains' oscillations over a 0.001-100 Hz frequency range when submitted to an oscillating magnetic field and leads to global microrheological measurements. On the other hand, the chains' rotation toward a permanent magnetic field is observed with a microscope, allowing a local determination of viscoelastic properties on the scale of the chains of particles. We demonstrate the accuracy of both assays with a micellar Maxwellian solution and validate theoretical predictions.

Journal Article↗
PubMed2003

Cell internalization of anionic maghemite nanoparticles: quantitative effect on magnetic resonance imaging.

Anionic iron oxide nanoparticles are efficiently internalized into macrophages where they concentrate within micrometric endosomes, conferring on them a high magnetic susceptibility. The uptake of anionic maghemite nanoparticles by macrophages was quantified by an electron spin resonance (ESR) experiment. MR spin-echo sequences were performed with various TEs and TRs. The contrast enhancement was compared between two types of agarose phantoms with the same equivalent ferrite concentrations but containing either dispersed isolated nanoparticles or magnetically labeled macrophages. It is shown that the intracellular confinement of maghemite nanoparticles within micrometric endosomes results in a significant decrease of the longitudinal relaxivity and a moderate decrease of the transverse relaxivity compared to the relaxivities of the dispersed isolated nanoparticles. As a consequence, the signature of endosomal magnetic labeling consists of a negative contrast on T(1)-weighted images in the whole ferrite concentration range, whereas the presence of extracellular isolated nanoparticles can result in a positive enhancement.

Animals↗
PubMed2003

Intracellular uptake of anionic superparamagnetic nanoparticles as a function of their surface coating.

A new class of superparamagnetic nanoparticles bearing negative surface charges is presented. These anionic nanoparticles show a high affinity for the cell membrane and, as a consequence, are captured by cells with an efficiency three orders of magnitude higher than the widely used dextran-coated iron oxide nanoparticles. The surface coating of anionic particle with albumin strongly reduces the non specific interactions with the plasma membrane as well as the overall cell uptake and at the same time restores the ability to induce specific interactions with targeted cells by the coadsorption on the particle surface of a specific ligand. Kinetics of cellular particle uptake for different cell lines are quantitated using two new complementary assays (Magnetophoresis and Electron Spin Resonance).

Animals↗
PubMed2002

Local rheological probes for complex fluids: application to Laponite suspensions.

We present an experimental method allowing a direct measurement of the local rheological behavior of complex fluids. A magnetic probe is inserted into the bulk of the fluid and submitted to a controlled magnetic force or torque, which induces a mechanical perturbation of the fluid. The geometry of the perturbation can be varied using two kinds of probes: a magnetic bead submitted to a homogeneous magnetic force in one direction, and a magnetic needle that can turn inside the material under the effect of an applied magnetic torque. Two complex viscoelastic fluids are investigated. First, a surfactant solution, which has a linear mechanical behavior in the range of the applied stresses, is used to test and validate the experimental methodology. We then use the local probes to investigate a Laponite colloidal suspension, which exhibits nonlinear behavior such as thixotropy, shear thinning, and aging. In this latter fluid, we find an exponential growth of the rheological relaxation time versus the system age, a power-law dependence of the fluid viscosity on the applied stress, and a dynamical yield stress which saturates with the fluid aging time.

Journal Article↗
PubMed2002

Binding of biological effectors on magnetic nanoparticles measured by a magnetically induced transient birefringence experiment.

We have investigated the relaxation of the magnetically induced birefringence in a suspension of magnetic nanoparticles in order to detect the binding reaction of polyclonal antibodies on the particle surface. The birefringence relaxation is driven by the rotational diffusion of the complex formed by the magnetic nanoparticles bound to the antibody and thus is directly related to the hydrodynamic size of this complex. Birefringence relaxations are well described by stretched exponential laws revealing a polydisperse distribution of hydrodynamic diameters. Comparing the size distribution of samples with different initial ratios of immunoglobuline added per magnetic nanoparticles, we evidence the graft of an antibody on particle and eventually the onset of particles aggregation. Measurements on samples separated in size by gel filtration demonstrate the robustness of our experiment for the determination of size distribution and its modification due to the adsorption of a macromolecule. The immunoglobuline binding assay is performed comparatively for ionic magnetic nanoparticles with different coatings.

Journal Article↗
PubMed2002

Magnetophoresis and ferromagnetic resonance of magnetically labeled cells.

We develop in this paper two methods, based on different physical concepts, to quantify the uptake of magnetic nanoparticles in biological cells. The first one, magnetophoresis, is based on the measurement of the velocity of magnetically labeled cells submitted to a magnetic field gradient. The second one quantitates the particles' electronic spin using an electron paramagnetic resonance experiment. We show a quantitative agreement between both methods for macrophagic cells. The uptake kinetics and uptake capacity are discussed for macrophagic cells and other cell lines.

Biophysics↗
PubMed2002

Identification and characterization of murine Brunol4, a new member of the elav/bruno family.

RNA-binding proteins are involved in post-transcriptional processes like mRNA stabilization, post-transcriptional modification, and transport and have been suggested to play an important role in developmental gene regulation. We report here the cloning and characterization of Brunol4, a novel mouse cDNA closely related to the elav-type family of genes encoding for RNA-binding proteins and a subfamily recently named after the bruno gene of Drosophila. Murine Brunol4 is localized near the centromere of chromosome 18. The cDNA sequence of Brunol4 is separated by 12 introns and the size of Brunol4 may be around 250 kb due to the large size of several introns. Brunol4 expression is detectable in the developing embryo and, later on becomes mainly restricted to cerebral structures, in particular the cerebellum where it persists in the adult organism. We predict a role of Brunol4 and the respective human homologue in differentiation and maintenance of neuronal structures.

Animals↗
PubMed2001

[Thrombophilia factors as inducers of retinal vascular occlusion].

PURPOSE: Thrombophilic abnormalities and defects in the anticoagulant system, such as protein C or protein S deficiency, activated protein C resistance and factor V Leiden mutation, may produce retinal vascular occlusions. PATIENTS: Blood samples from 66 consecutive retinal vascular occlusion patients were obtained and analysed for protein C resistance and fibrinolysis activity. Other thrombophilic and standard laboratory coagulant tests were also carried out. RESULTS: An increased activity of factor VIII was found in 72% of the collective. Furthermore, 24% of all patients and 32% of patients younger than 45 years old were resistant to activated protein C. Nevertheless only one patient showed a homozygous factor V Leiden mutation. CONCLUSION: Thrombophilic activity in cases of protein C resistance or factor V Leiden mutation may result in severe thrombotic manifestations in ocular vessels. Compared to the elderly, younger people showed a higher activated protein C resistance which seems be one of the most common causes for retinal vascular occlusion in this age group.

Activated Protein C Resistance↗
PubMed2001

Xanthophyll synthesis in diatoms: quantification of putative intermediates and comparison of pigment conversion kinetics with rate constants derived from a model.

Recently, we reported the presence of the violaxanthin-antheraxanthin-zeaxanthin cycle in diatoms, and showed that violaxanthin is the putative precursor of both diadinoxanthin and fucoxanthin in the diatom Phaeodactylum tricornutum Bohlin (M. Lohr and C. Wilhelm, 1999, Proc. Natl. Acad. Sci. USA 96: 8784-8789). In the present study, two possible intermediates in the synthesis of violaxanthin from beta-carotene were identified in P. tricornutum, namely beta-cryptoxanthin and beta-cryptoxanthin epoxide. In low light, the latter pigment prevails, but in high light beta-cryptoxanthin accumulates, probably as the result of an increased activity of the xantophyll-cycle de-epoxidase. The apparent kinetics of several xanthophyll conversion steps were determined for P. tricornutum and Cyclotella meneghiniana Kuitzing. The experimentally determined conversion rates were used to evaluate the hypothetical pathway of xanthophyll synthesis in diatoms. For this purpose a mathematical model was developed which allows the calculation of theoretical rates of pigment conversion for microalgae under steady-state growth conditions. A comparison between measured and calculated conversion rates agreed well with the proposal of a sequential synthesis of fucoxanthin via violaxanthin and diadinoxanthin. The postulation of zeaxanthin as an obligatory intermediate in the synthesis of violaxanthin, however, resulted in large discrepancies between the measured and calculated rates of its epoxidation. Instead of zeaxanthin, beta-cryptoxanthin epoxide may be involved in the biosynthesis of violaxanthin in diatoms.

Antioxidants↗
PubMed2001

The de-epoxidase and epoxidase reactions of Mantoniella squamata (Prasinophyceae) exhibit different substrate-specific reaction kinetics compared to spinach.

In vivo the prasinophyceaen alga Mantoniella squamata Manton et Parke uses an incomplete violaxanthin (Vx) cycle, leading to a strong accumulation of antheraxanthin (Ax) under conditions of high light. Here, we show that this zeaxanthin (Zx)-depleted Vx/Ax cycle is caused by an extremely slow second de-epoxidation step from Ax to Zx, and a fast epoxidation from Ax back to Vx in the light. The rate constant of Ax epoxidation is 5 to 6 times higher than the rate constant of Zx formation, implying that Ax is efficiently converted back to Vx before it can be de-epoxidated to Zx. It is, however, only half the rate constant of the first de-epoxidation step from Vx to Ax, thus explaining the observed net accumulation of Ax during periods of strong illumination. When comparing the rate constant of the second de-epoxidation step in M. squamata with Zx formation in spinach (Spinacia oleracea L.) thylakoids, we find a 20-fold reduction in the reaction kinetics of the former. This extremely slow Ax de-epoxidation, which is also exhibited by the isolated Mantoniella violaxanthin de-epoxidase (VDE), is due to a reduced substrate affinity of M. squamata VDE for Ax compared with the VDE of higher plants. Mantoniella VDE, which has a similar Km value for Vx, shows a substantially increased Km for the substrate Ax in comparison with spinach VDE. Our results furthermore explain why Zx formation in Mantoniella cells can only be found at low pH values that represent the pH optimum of VDE. A pH of 5 blocks the epoxidation reaction and, consequently, leads to a slow but appreciable accumulation of Zx.

Chlorophyta↗
PubMed2000

Organization of the pigment molecules in the chlorophyll a/b/c containing alga Mantoniella squamata (Prasinophyceae) studied by means of absorption, circular and linear dichroism spectroscopy.

In order to obtain information on the organization of the pigment molecules in chlorophyll (Chl) a/b/c-containing organisms, we have carried out circular dichroism (CD), linear dichroism (LD) and absorption spectroscopic measurements on intact cells, isolated thylakoids and purified light-harvesting complexes (LHCs) of the prasinophycean alga Mantoniella squamata. The CD spectra of the intact cells and isolated thylakoids were predominated by the excitonic bands of the Chl a/b/c LHC. However, some anomalous bands indicated the existence of chiral macrodomains, which could be correlated with the multilayered membrane system in the intact cells. In the red, the thylakoid membranes and the LHC exhibited a well-discernible CD band originating from Chl c, but otherwise the CD spectra were similar to that of non-aggregated LHC II, the main Chl a/b LHC in higher plants. In the Soret region, however, an unusually intense (+) 441 nm band was observed, which was accompanied by negative bands between 465 and 510 nm. It is proposed that these bands originate from intense excitonic interactions between Chl a and carotenoid molecules. LD measurements revealed that the Q(Y) dipoles of Chl a in Mantoniella thylakoids are preferentially oriented in the plane of the membrane, with orientation angles tilting out more at shorter than at longer wavelengths (9 degrees at 677 nm, 20 degrees at 670 nm and 26 degrees at 662 nm); the Q(Y) dipole of Chl c was found to be oriented at 29 degrees with respect to the membrane plane. These data and the LD spectrum of the LHC, apart from the presence of Chl c, suggest an orientation pattern of dipoles similar to those of higher plant thylakoids and LHC II. However, the tendency of the Q(Y) dipoles of Chl b to lie preferentially in the plane of the membrane (23 degrees at 653 nm and 30 degrees at 646 nm) is markedly different from the orientation pattern in higher plant membranes and LHC II. Hence, our CD and LD data show that the molecular organization of the Chl a/b/c LHC, despite evident similarities, differs significantly from that of LHC II.

Bacterial Proteins↗