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Biomedical subjects

Michael Mayer

Publications and source records attributed to Michael Mayer.

At least 19 recordsLinked to original sources
PubMed2006

Toward a global phylogeny of the Brassicaceae.

The Brassicaceae is a large plant family (338 genera and 3,700 species) of major scientific and economic importance. The taxonomy of this group has been plagued by convergent evolution in nearly every morphological feature used to define tribes and genera. Phylogenetic analysis of 746 nrDNA internal transcribed spacer (ITS) sequences, representing 24 of the 25 currently recognized tribes, 146 genera, and 461 species of Brassicaceae, produced the most comprehensive, single-locus-based phylogenetic analysis of the family published to date. Novel approaches to nrDNA ITS analysis and extensive taxonomic sampling offered a test of monophyly for a large complement of the currently recognized tribes and genera of Brassicaceae. In the most comprehensive analysis, tribes Alysseae, Anchonieae plus Hesperideae, Boechereae, Cardamineae, Eutremeae, Halimolobeae, Iberideae, Noccaeeae, Physarieae, Schizopetaleae, Smelowskieae, and Thlaspideae were all monophyletic. Several broadly defined genera (e.g., Draba and Smelowskia) were supported as monophyletic, whereas others (e.g., Sisymbrium and Alyssum) were clearly polyphyletic. Analyses of ITS data identified several problematic sequences attributable to errors in sample identification or database submission. Results from parsimony ratchet and Bayesian analyses recovered little support for the backbone of the phylogeny, suggesting that many lineages of Brassicaceae have undergone rapid radiations that may ultimately be difficult to resolve with any single locus. However, the development of a preliminary supermatrix including the combination of 10 loci for 65 species provides an initial estimate of intertribal relations and suggests that broad application of such a method will provide greater understanding of relationships in the family.

Animals↗
PubMed2006

Estimation of solid phase affinity constants using resistive-pulses from functionalized nanoparticles.

This paper describes a method for estimating the solid phase affinity constant of antibodies by using resistive-pulse (Coulter counting) data from spherical nanoparticles that expose antigens. We developed this technique by analyzing data published recently by Saleh, O.A., Sohn, L.L., 2003a. Proc. Natl. Acad. Sci. U.S.A. 100, 820-824. These authors used resistive-pulse sensing to detect an increase in the diameter of streptavidin-functionalized colloids due to the binding of monoclonal anti-streptavidin antibodies. Based on further analysis of their data, we were able to determine the number of antibodies bound to the colloids at various antibody concentrations. This information made it possible to estimate the solid phase affinity constant of the interaction by fitting the data with binding isotherms that describe the binding equilibrium between antibody and antigen. We calculated a value of 2.6x10(8)+/-0.8x10(8) M-1 which is in agreement with the specifications of the supplier of the antibody.

Antibody Affinity↗
PubMed2006

Direct reconstruction of tissue parameters from differential multifrequency EIT in vivo.

The basic purpose of electrical impedance tomography (EIT) is the reconstruction of conductivity distributions. While multifrequency measurements are of common use, the majority of reconstructed images are still conductivity distributions from one single frequency. More interesting than conductivities at each frequency are electrical tissue parameters, which describe the frequency-dependent conductivity changes of tissue. These parameters give information about physiological or electrical properties of tissues. By using this spectral information, a classification of different tissue types is possible. To get a distribution of tissue parameters, usually a posterior fitting of a tissue model to the conductivity spectra obtained with classical reconstruction algorithms at various frequencies is used. In this work, a single-step reconstruction algorithm for differential imaging was developed for the direct estimation of Cole parameters. This method is termed differential parametric reconstruction. The Cole model was used as the underlying tissue model, where only the relative changes of the two conductivity parameters sigma(0) and sigma(infinity) were reconstructed and the other two parameters of the model which are less identifiable were set to constant values. The reconstruction algorithm was tested with simulated noisy datasets and real measurement data from EIT measurements on the human thorax. These measurements were taken from healthy subjects and from patients with a serious lung injury. The new method yields a good image quality and higher robustness against noise compared to conventional reconstruction methods.

Algorithms↗
PubMed2006

Triggering and visualizing the aggregation and fusion of lipid membranes in microfluidic chambers.

We present a method that makes it possible to trigger, observe, and quantify membrane aggregation and fusion of giant liposomes in microfluidic chambers. Using electroformation from spin-coated films of lipids on transparent indium tin oxide electrodes, we formed two-dimensional networks of closely packed, surface-attached giant liposomes. We investigated the effects of fusogenic agents by simply flowing these molecules into the chambers and analyzing the resulting shape changes of more than 100 liposomes in parallel. We used this setup to quantify membrane fusion by several well-studied mechanisms, including fusion triggered by Ca2+, polyethylene glycol, and biospecific tethering. Directly observing many liposomes simultaneously proved particularly useful for studying fusion events in the presence of low concentrations of fusogenic agents, when fusion was rare and probabilistic. We applied this microfluidic fusion assay to investigate a novel 30-mer peptide derived from a recently identified human receptor protein, B5, that is important for membrane fusion during the entry of herpes simplex virus into host cells. This peptide triggered fusion of liposomes at an approximately 6 times higher probability than control peptides and caused irreversible interactions between adjacent membranes; it was, however, less fusogenic than Ca2+ at comparable concentrations. Closely packed, surface-attached giant liposomes in microfluidic chambers offer a method to observe membrane aggregation and fusion in parallel without requiring the use of micromanipulators. This technique makes it possible to characterize rapidly novel fusogenic agents under well-defined conditions.

Electrochemistry↗
PubMed2005

Microoxen: microorganisms to move microscale loads.

It is difficult to harness the power generated by biological motors to carry out mechanical work in systems outside the cell. Efforts to capture the mechanical energy of nanomotors ex vivo require in vitro reconstitution of motor proteins and, often, protein engineering. This study presents a method for harnessing the power produced by biological motors that uses intact cells. The unicellular, biflagellated algae Chlamydomonas reinhardtii serve as "microoxen." This method uses surface chemistry to attach loads (1- to 6-microm-diameter polystyrene beads) to cells, phototaxis to steer swimming cells, and photochemistry to release loads. These motile microorganisms can transport microscale loads (3-microm-diameter beads) at velocities of approximately 100-200 microm.sec(-1) and over distances as large as 20 cm.

Animals↗
PubMed2005

Bacterial printing press that regenerates its ink: contact-printing bacteria using hydrogel stamps.

This paper describes the use of micropatterned agarose stamps prepared by molding against PDMS masters to print patterns of bacteria on agar plates. Topographically patterned agarose stamps were inked with suspensions of bacteria; these stamps generated patterns of bacteria with features as small as 200 microm over areas as large as 50 cm2. Stamps with many small features (>200 microm) were used to study patterns of bacteria growing on media containing gradients of small molecules; stamps with larger features (>750 microm) were used to print different strains of bacteria simultaneously. The stamp transfers only a small percentage of cells that are on its surface to the agar at a time; it is thus possible to replica-pattern hundreds of times with a single inking. The use of soft stamps provides other useful functions. Stamps are easily customized to provide a range of patterns. When culture media is included in the agarose stamp, cells divide and thrive on the surface. The resulting "living stamp" regenerates its "ink" and can be used to pattern surfaces repetitively for a month. This method is rapid, reproducible, convenient, and can be used to control the pattern, spacing, and orientation between colonies of different bacteria.

Aliivibrio fischeri↗
PubMed2005

Escherichia coli swim on the right-hand side.

The motion of peritrichously flagellated bacteria close to surfaces is relevant to understanding the early stages of biofilm formation and of pathogenic infection. This motion differs from the random-walk trajectories of cells in free solution. Individual Escherichia coli cells swim in clockwise, circular trajectories near planar glass surfaces. On a semi-solid agar substrate, cells differentiate into an elongated, hyperflagellated phenotype and migrate cooperatively over the surface, a phenomenon called swarming. We have developed a technique for observing isolated E. coli swarmer cells moving on an agar substrate and confined in shallow, oxidized poly(dimethylsiloxane) (PDMS) microchannels. Here we show that cells in these microchannels preferentially 'drive on the right', swimming preferentially along the right wall of the microchannel (viewed from behind the moving cell, with the agar on the bottom). We propose that when cells are confined between two interfaces--one an agar gel and the second PDMS--they swim closer to the agar surface than to the PDMS surface (and for much longer periods of time), leading to the preferential movement on the right of the microchannel. Thus, the choice of materials guides the motion of cells in microchannels.

Agar↗
PubMed2005

Electroformation of giant liposomes from spin-coated films of lipids.

This paper describes spin-coating of solutions of lipids and using the resulting thin films for electroformation of giant liposomes. Spin-coating made it possible to generate uniform films of lipids with controllable thickness over large surfaces (>25 cm(2)) of indium tin oxide. Establishing a range of thicknesses optimal for electroformation (25-50 nm), we demonstrate formation of giant liposomes from lipids (such as asolectin, phosphatidylserine, and phosphatidylglycerol) that do not readily form giant liposomes from traditional, droplet-derived films. We compared liposomes from a spin-coated film of lipids to liposomes formed from traditional droplet-derived films and found that spin-coated films produced larger (by factor of 2-5) and more abundant liposomes than droplet-derived films of lipids. Electroformation from spin-coated, homogenous lipid films of optimal thickness provided a reproducible way to obtain liposomes with diameters that are predominantly larger than 30 microm over the entire surface of formation.

Biotechnology↗
PubMed2005

Giant liposomes in physiological buffer using electroformation in a flow chamber.

We describe a method to obtain giant liposomes (diameter 10-100 microm) in solutions of high ionic strength to perform a membrane-binding assay under physiological conditions. Using electroformation on ITO electrodes, we formed surface-attached giant liposomes in solutions of glycerol in a flow chamber and then introduced solutions of high ionic strength (up to 2 M KCl) into this chamber. The ionic solution exchanged with the isoosmolar glycerol solution inside and outside the liposomes. An initial mismatch in index of refraction between the inside and outside of liposomes allowed for the observation of solution replacement. Ions and small polar molecules exchanged into and out of surface-attached liposomes within minutes. In contrast, liposomes formed in solutions of macromolecules retained molecules larger than 4 kDa, allowing for encapsulation of these molecules for hours or days even if the solution outside the liposomes was exchanged. We propose that solutes entered liposomes through lipid tubules that attach liposomes to the film of lipids on the surface of the ITO electrode. The method presented here makes it straightforward to perform flow-through binding assays on giant liposomes under conditions of physiological ionic strength. We performed a membrane-binding assay for annexin V, a calcium-dependent protein that binds to phosphatidylserine (PS). The binding of annexin V depended on the concentration of PS and decreased as ionic strength increased to physiological levels.

Animals↗
PubMed2005

An administrative intervention to improve the utilization of laboratory tests within a university hospital.

BACKGROUND: Improving the appropriateness of testing behavior and reducing the number of laboratory tests have been recognized as essential parts of quality improvement. OBJECTIVE: To assess the effectiveness of an administrative and a short-term educational intervention aimed at reducing clinical biochemistry laboratory utilization. DESIGN: An analysis comparing utilization of laboratory tests performed on in-patients before and after the intervention. SETTING: Computerized database of all laboratory tests performed in Hadassah Ein Kerem Medical Center, Jerusalem, Israel during 1999-2003. INTERVENTION: The administrative intervention included restricting available emergency laboratory tests and frequency of repeated orders. The educational measures included: discussion of the misuse of laboratory tests and its consequences with the hospital medical staff, and presentation of the new restrictive policy. A feedback of the intervention's results was sent to the wards and reviewed with senior medical staff. MAIN OUTCOME MEASURES: Change in utilization (measured as rates per 100 hospital days) of clinical biochemistry tests by hospital division and by selected laboratory tests. RESULTS: An overall reduction of 19% in laboratory tests (95% CI: 18.8-19.2%) was observed in the year after the intervention. Utilization decreased significantly in all the hospital's medical divisions, within a range of 14.9-43.8%. During the intervention period, utilization of hematology tests was reduced by 7.6% (P = 0.009). Statistically significant reductions were noted in the ordering of all 12 selected clinical biochemistry tests. Although the orders of total cholesterol decreased by 72.2%, the utilization of 'high-volume' tests, such as glucose and electrolytes, showed only a modest decrease (7.9% and 6.9%, respectively). CONCLUSIONS: The present study included all hospital medical staff and covered all the available clinical biochemistry tests. This rather simple and low-cost intervention resulted in significant reductions in clinical biochemistry test orders as well as in the ordering of hematological blood tests.

Adult↗
PubMed2005

Monitoring of lung edema using focused impedance spectroscopy: a feasibility study.

Currently only ionizing or invasive methods are used in clinical applications for the monitoring of extracellular lung water. Alternatively a method called focused conductivity spectroscopy (FCS) is suggested, which aims at reconstructing a pulmonary edema index (PEIX) by measuring the electrical conductivity of the region of interest (ROI) at several frequencies. In contrast to electrical impedance tomography (EIT) a minimum number of strategically placed electrodes is used. The goals of this study were the analysis of the sensitivity for the PEIX, an estimate of the optimal electrode configuration and the determination of the required frequencies. In order to calculate the solution of the FCS forward problem a realistic 3D model of a human torso was developed containing both lungs, the heart, the liver and the thorax musculature. The bioelectrical properties for each compartment were described with appropriate tissue models which relate the conductivity spectra to physiological parameters. The PEIX was defined as the interstitial volume fraction of the alveolar septa. Furthermore the model includes 48 electrodes subdivided into three layers. The optimal electrode configuration was selected by minimizing the number of electrodes, among certain subsets of these electrodes. The analysis shows that eight to ten electrodes and six frequencies are theoretically sufficient to obtain a coefficient of variation.

Adult↗
PubMed2005

Protein production and crystallization at SECSG -- an overview.

Using a high degree of automation, the Southeast Collaboratory for Structural Genomics (SECSG) has developed high throughput pipelines for protein production, and crystallization using a two-tiered approach. Primary, or tier-1, protein production focuses on producing proteins for members of large Pfam families that lack a representative structure in the Protein Data Bank. Target genomes are Pyrococcus furiosus and Caenorhabditis elegans. Selected human proteins are also under study. Tier-2 protein production, or target rescue, focuses on those tier-1 proteins, which either fail to crystallize or give poorly diffracting crystals. This two tier approach is more efficient since it allows the primary protein production groups to focus on the production of new targets while the tier-2 efforts focus on providing additional sample for further studies and modified protein for structure determination. Both efforts feed the SECSG high throughput crystallization pipeline, which is capable of screening over 40 proteins per week. Details of the various pipelines in use by the SECSG for protein production and crystallization, as well as some examples of target rescue are described.

Animals↗
PubMed2005

Direct patterning of mammalian cells onto porous tissue engineering substrates using agarose stamps.

This paper describes simple, inexpensive, and potentially generic methodology for generating patterns of mammalian cells on porous scaffolds for tissue engineering using replica printing. Circular patterns (diameter: 200, 700, and 1000 microm) of human osteoblasts were transferred directly from topographically patterned agarose stamps onto porous hydroxyapatite scaffolds or onto fibronectin-coated glass slides. The use of hydrogel stamps provided a "wet", biocompatible surface and maintained the viability of cells adsorbed on stamps during the patterning process. Stamps inked once with suspensions of cells allowed the repeated patterning of substrates. Direct stamping of human osteoblasts (and, potentially other mammalian cells) can be used to control the size, spacing, and geometry of patterns of cells printed on porous tissue engineering substrates. This approach may find use in controlling the spatial invasion of scaffolds, promoting the hierarchical organization of cells, and in controlling cell-cell interactions as a step in preservation of phenotypes of cells.

Actins↗
PubMed2005

Fat and hydration monitoring by abdominal bioimpedance analysis: data interpretation by hierarchical electrical modeling.

In a previous publication, it was demonstrated that the abdominal subcutaneous fat layer thickness (SFL) is strongly correlated with the abdominal electrical impedance when measured with a transversal tetrapolar electrode arrangement. This article addresses the following questions: 1) To which extent do different abdominal compartments contribute to the impedance? 2) How does the hydration state of tissues affect the data? 3) Can hydration and fat content be assessed independently? For simulating the measured data a hierarchical electrical model was built. The abdomen was subdivided into three compartments (subcutaneous fat, muscle, mesentery). The true anatomical structure of the compartment boundaries was modeled using finite-element modeling (FEM). Each compartment is described by an electrical tissue model parameterized in physiological terms. Assuming the same percent change of the fat fraction in the mesentery and the SFL the model predicts a change of 1,24 omega/mm change of the SFL compared to 1,1 omega/mm measured. 42% of the change stem from the SFL, 56% from the mesentery and 2% from changes of fat within the muscle compartment. A 1% increase of the extracellular water in the muscle is not discernible from a 1% decrease of the SFL. The measured data reflect not only the SFL but also the visceral fat. The tetrapolar electrode arrangement allows the measurement of the abdominal fat content only if the hydration remains constant.

Abdomen↗