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At least 775 records · Page 43Linked to original sources
PubMed1977

Spatial frequency and light-spread descriptions of visual acuity and hyperacuity.

Resolution (visual acuity) and differential spatial localization (hyperacuity) targets were selected to allow rigorous psychophysical measurements as well as ready expression of both their spatial frequency spectrum and their retinal image light distribution. Thresholds were about 1 arc min for acuity and 4-6 arc sec for hyperacuity. As is consistent with the reciprocal relationship between the space and spatial frequency domains, the small locally restricted spatial differences between just distinguishable patterns are represented in the frequency domain by equally small differences, which are distributed over the entire spatial frequency spectrum. While they occur in many test situations, phase variations of spatial frequency components are not necessary for achieving optimum acuity and hyperacuity.

Female↗
PubMed2003

Phase-coherent generation of tunable visible femtosecond pulses.

Stable interference between the outputs of two noncollinearly phase-matched optical parametric amplifiers seeded by separate white-light continua has been observed. This means that the tunable visible pulses have a well-defined relative phase and that the temporal jitter between them is less than 1 fs. The residual phase variations are due to fluctuations of the pump power.

Journal Article↗
PubMed2006

Optical inclinometer based on a single long-period fiber grating combined with a fused taper.

A new concept to measure rotation angles based on a fiber-optic modal Mach-Zehnder interferometer is demonstrated by using a nonadiabatic taper cascaded with a long-period fiber grating. Information about the magnitude of the rotation angle can be obtained from the measurement of the interference pattern visibility, and under certain conditions it is also possible to obtain the sign of the rotation angle from the induced phase variation in the fiber interferometer.

Journal Article↗
PubMed2002

Protein expression in Mycobacterium tuberculosis differs with growth stage and strain type.

Different phenotypes are displayed by Mycobacterium tuberculosis (M. tuberculosis) strains, fuelling speculation that certain strains are "hypervirulent" and able to evade host defenses better than others. Furthermore, differential antigen expression by M. tuberculosis strains may explain why certain patients are susceptible to a repeat episode of tuberculosis. The objective of this study was to compare protein expression by M. tuberculosis H37Rv and clinical isolates in order to determine whether differential protein expression contributes to the different phenotypes expressed by these strains. Expression of alpha-crystallin, the antigen 85 complex, PstS-1, L-alanine dehydrogenase and the 65 kDa antigen was analysed by Western blotting and enzyme-linked immunosorbent assays, using mouse monoclonal antibodies. We found no significant difference in the growth rate of the M. tuberculosis strains in vitro, and although M. tuberculosis protein expression showed phase variation during growth, expression seemed to be qualitatively, but not quantitatively, conserved in the strains investigated. These results have potentially important implications for vaccine development and serodiagnosis.

Animals↗
PubMed2004

Dynamic light scattering microscopy. A novel optical technique to image submicroscopic motions. I: theory.

The theoretical basis of an optical microscope technique to image dynamically scattered light fluctuation decay rates (dynamic light scattering microscopy) is developed. It is shown that relative motions between scattering centers even smaller than the optical resolution of the microscope are sufficient to produce significant phase variations resulting in interference intensity fluctuations in the image plane. The timescale and time dependence for the temporal autocorrelation function of these intensity fluctuations is derived. The spatial correlation distance, which reports the average distance between constructive and destructive interference in the image plane, is calculated and compared with the pixel size, and the distance dependence of the spatial correlation function is derived. The accompanying article in this issue describes an experimental implementation of dynamic light scattering microscopy.

Cell Movement↗
PubMed2005

Markov Chain modeling of pyelonephritis-associated pili expression in uropathogenic Escherichia coli.

Pyelonephritis-associated pili (Pap) expression in uropathogenic Escherichia coli is regulated by a complex phase variation mechanism involving the competition between leucine-responsive regulatory protein (Lrp) and DNA adenine methylase (Dam). Population dynamics of pap gene expression has been studied extensively and the detailed molecular mechanism has been largely elucidated, providing sufficient information for mathematical modeling. Although the Gillespie algorithm is suited for modeling of stochastic systems such as the pap operon, it becomes computationally expensive when detailed molecular steps are explicitly modeled in a population. Here we developed a Markov Chain model to simplify the computation. Our model is analytically derived from the molecular mechanism. The model presented here is able to reproduce results presented using the Gillespie method, but since the regulatory information is incorporated before simulation, our model runs more efficiently and allows investigation of additional regulatory features. The model predictions are consistent with experimental data obtained in this work and in the literature. The results show that pap expression in uropathogenic E. coli is initial-state-dependent, as previously reported. However, without environment stimuli, the pap-expressing fraction in a population will reach an equilibrium level after approximately 50-100 generations. The transient time before reaching equilibrium is determined by PapI stability and Lrp and Dam copy numbers per cell. This work demonstrates that the Markov Chain model captures the essence of the complex molecular mechanism and greatly simplifies the computation.

Algorithms↗
PubMed2004

Stochastic gene expression in fluctuating environments.

Stochastic mechanisms can cause a group of isogenic bacteria, each subject to identical environmental conditions, to nevertheless exhibit diverse patterns of gene expression. The resulting phenotypic subpopulations will typically have distinct growth rates. This behavior has been observed in several contexts, including sugar metabolism and pili phase variation. Under fixed environmental conditions, the net growth rate of the population is maximized when all cells are of the fastest growing phenotype, so it is unclear what fitness advantage is conferred by population heterogeneity. However, unlike ideal laboratory conditions, natural environments tend to fluctuate, either periodically or randomly. Here we use a stochastic population model to show that, during growth in such fluctuating environments, a dynamically heterogenous bacterial population can sometimes achieve a higher net growth rate than a homogenous one. By using stochastic mechanisms to sample several distinct phenotypes, the bacteria are able to anticipate and take advantage of sudden changes in their environment. However, this heterogeneity is beneficial only if the bacterial response rate is sufficiently low. Our results could be useful in the design of artificial evolution experiments and in the optimization of fermentation processes.

Bacterial Physiological Phenomena↗
PubMed1989

Dynamic behavior of the middle ear based on sweep frequency tympanometry.

A measuring apparatus was developed; its probe tip, which exhibits flat frequency characteristics, enables this apparatus to measure the absolute sound pressure and absolute phase variations versus both sweeping frequency and external auditory canal pressure. Although it is difficult to diagnose ossicular chain separation and fixation from the commonly used tympanograms with a low probe tone frequency (e.g.f = 220 Hz), the results obtained with this apparatus enable one to clearly distinguish patients with ossicular chain disorders from normal subjects. Therefore, it seems to be highly useful in the clinical diagnosis of ossicular chain disorders.

Acoustic Impedance Tests↗
PubMedDate not supplied

Current sheet applicator arrays for superficial hyperthermia of chestwall lesions.

The current sheet applicator is an electromagnetic heating device whose size may be chosen virtually independent of frequency even though practical limitations may restrict it to VHF and UHF bands. In this paper we investigate absorbed power distributions in muscle tissue from current sheet applicators when used as elements of a planar array intended for superficial hyperthermic treatment of tumours. Advantages offered by current sheet applicators for tissue heating include compact size, a linear polarization of the induced electric field and relatively large heating area. It is shown that the effective field produced by a pair of these elements is continuous regardless of whether the common edges of the elements are perpendicular or parallel to the direction of impressed current. The feasibility of customizing the shape and size of the field is also illustrated. The absorbed power distribution patterns due to a coherently driven array operating around 434 MHz is relatively insensitive to phase variations of about 20 degrees but is sensitive to relative power level variations as low as 10%. Mutual coupling between array elements may be reduced to acceptable levels by incorporating suitable spacing between them. It is also demonstrated that there is good agreement between measurements of absorbed power distributions and predictions using the Gaussian beam model.

Biomedical Engineering↗
PubMed1997

Nucleotide sequence analysis of the Streptococcus gordonii glucosyltransferase gene, gtfG.

Streptococcus gordonii has an extracellular glucosyltransferase (GTF) that polymerizes the glucose moiety of sucrose to form both water-soluble and water-insoluble glucans. Whereas multiple gtf genes have been identified in strains of mutans streptococci and Streptococcus salivarius, a single gene, designated gtfG, encodes the GTF of S. gordonii Challis. gtfG is also unique among the characterized gtfs in that it has a described regulatory determinant, rgg. Furthermore, the GTF activity in S. gordonii undergoes reversible phase variation between high and low levels. In order to gain insight into this novel GTF system, the nucleotide sequence of gtfG was determined and found to consist of a 4,734 base pair open reading frame encoding a protein with a deduced molecular weight of ca. 174,000. gtfG was similar to other sequenced gtfs with a conserved signal sequence followed by a ca. 600-bp region distinctive for gtfG, a conserved region encoding a putative catalytic active site and a series of six direct repeats in the carboxyl terminal region implicated in glucan binding. Although comparison of gtfG to other gtfs did not show a basis for the primer-independence of the encoded enzyme or the nature of the glucan products, the gtfG sequence data provide an important basis for further studies of these enzymes.

Amino Acid Sequence↗
PubMed1999

Pathogenesis of pneumococcal pneumonia.

Pneumococcal pneumonia is the most severe of the common community-acquired pulmonary infections. The recent release of the complete DNA sequence reveals the entire capability of the bacteria and the current challenge is to map gene products to the mechanics of disease. This process will reveal antibiotic targets and protein vaccine candidates crossing serotype boundaries. Choline is a major constituent of surfactant and it is also a required nutrient for the pneumococcus. It appears that choline incorporated into the cell wall can bind the bacteria to the receptor for platelet activating factor, the gateway to invasion. The choline also serves as an antenna to which multifunctional proteins dock, thereby decorating the bacterial surface. This set of 12 choline binding proteins is subject to phase variation of expression resulting in display of different combinations of proteins that adapt the bacteria to survival on the mucosa versus the blood stream. These changes affect protective antigens, adhesions, and lytic proteins tying together the major elements of pneumococcal physiology: natural DNA transformation, adherence and invasion of host cells, and autolysis. Taking these components and building an understanding of disease is challenging. Clearly, the toxin pneumolysin is a major mediator of cell damage in the lung. Inflammation is also incited by cell wall components. The signal transduction pathways that explain pneumococcal inflammation are more complex than those for gram-negative endotoxin.

Animals↗
PubMed2000

[Lipid peroxidation and activity of diagnostically important blood enzymes in rats during 35 days head-down suspension].

In the experiment with 35-d head-down suspension of rats concentrations of lipid peroxidation (LPO) products and values of antioxidant defense (AOD) parameters were determined in the lever, skeletal muscles, myocardium, and blood plasma. In addition, activity of 11 plasma enzymes characteristics of these tissues was evaluated. LPO was found to be activated in the lever, myocardium and skeletal muscles in the first and 4-5 weeks of suspension. The period between weeks 2 and 4 marked phase variation of LPO intensity which could have baseline or increased values. Plasma LPO was inhibited due to AOD activation pointing to compensatory processes on the level of organism and absence of pathologies in the organs and tissues under study. As a rule, activity of the plasma enzymes directly or indirectly participating in energy metabolism was reduced. Enzymes-catalysts of other types of metabolism predominantly elevated activity during LPO intensification. Canonic analysis established a significant correlation of LPO, AOD and activity of plasma enzymes, and LPO and AOD in the rat's lever and plasma enzymes. These correlations between LPO, AOD and activity of diagnostically important enzymes can be used to diagnose and predict changes in the state of organism caused by simulated factors of space flight.

Animals↗
PubMed2000

[Dynamics of the non-specific reactivity of the organism in time].

Experiments with 10 mongrel rats and dogs showed that individual reactivity of animals to hypobaric hypoxia was same during repeated exposure in two weeks. Investigations of 20 dogs demonstrated relatively permanent types of group reactivity, i.e. hyper-, normal, and hyporeactivity to adrenaline injections every 2 months over a year period. Each type of group reactivity was marked by phase variation in the count of peripheral blood leukocytes with the relation of group mean values unchanged. Analysis of mean values of adaptive levels in 340 subjects with varying individual reactivity over 10 years also pointed to undulation of these values in each group. At the same time, the difference in mean values of this parameter among the groups was kept constant. Hence, the group specific reactivity has a long-range character as determined by testing.

Adaptation, Physiological↗
PubMed2001

Changes in fatty acid composition of Pseudomonas aeruginosa ATCC 15442 induced by growth conditions: consequences of resistance to quaternary ammonium compounds.

The effects of changes in the fatty acid composition of Pseudomonas aeruginosa induced by growth conditions on its resistance to two quaternary ammonium compounds (QAC) were investigated. The temperature and growth phase were the most influential parameters affecting the fatty acid composition of this bacterium. Furthermore, the formation of saturated fatty acids and cyclopropane fatty acids was stimulated by increasing the temperature, whereas the proportion of unsaturated fatty acids fell. The degree of saturation and the proportion of cyclopropane fatty acids increased in the course of the exponential and stationary phases. These modifications mostly concerned the inner membrane of the bacterium. Resistance of P. aeruginosa to both QAC tested was not significantly influenced by temperature and growth phase variations. Thus, resistance to the two QAC did not seem to be dependent on modifications of the fatty acid composition of the inner membrane.

Benzalkonium Compounds↗
PubMed2001

[Activity of proteolytic enzymes and their inhibitors during proliferation of P-815 mastocytoma cell proliferation in vitro].

Proliferation of mastocytoma P-815 cells in vitro was accompanied by a rise in cathepsin D, elastase- and trypsin-like proteinase activity during 6 hours of culturing and a decline by hour 24. Yet alpha 1-proteinase inhibitor activity was inversely proportional to proteinase concentration. Antiproliferative action of actinomycin D disrupted phase variation of proteinase activity and, consequently, the level of alpha 1-proteinase inhibitor rose after 6 hours of cell culturing while that of alpha 2-macroglobulin--after 48 hr. Antiproliferative effect of actinomycin D was eliminated by reduced inhibitor level brought about under the influence of exogenous trypsin. When trypsin was added cathepsin D activity reached its peak 6 hr later while that of alpha 1-proteinase inhibitor declined. That effect and the actomycin D-proteinase inhibitor mechanism were retained when trypsin and actomycin D were present together. It is suggested that cathepsin D and alpha 1-proteinase inhibitor activity plays a key role in realizing the proliferative potential of mastocytoma P-815 cells.

Animals↗
PubMed2002

Phase variable changes in genes lgtA and lgtC within the lgtABCDE operon of Neisseria gonorrhoeae can modulate gonococcal susceptibility to normal human serum.

The alpha-chain of the core oligosaccharide of the lipo-oligosaccharide (LOS) produced by Neisseria gonorrhoeae can undergo reversible and rapid changes in structure due to phase-variable production of certain enzymes employed in the biosynthesis of the lacto-N-neotetraose structure. Five of these enzymes are encoded by the lgtABCDE operon, and polynucleotide tracts within three of these genes (lgtA, lgtC and lgtD) can be substrates for slipped-strand mispairing events that lead to nucleotide insertions or deletion events which result in variable production of their respective gene products. We now report that phase-variable synthesis of the lgtA and lgtC gene products in strain FA19 results in the production of elongated LOS alpha-chains and that the presence of these LOS species can result in gonococci being sensitive to the bacteriolytic action of serum-antibody and complement. Hence, phase variation within the lgtABCDE operon can significantly impact the ability of gonococci to subvert this important host defense system.

Amino Acid Sequence↗
PubMedDate not supplied

[Modeling of the forest insect population size: a game theory approach].

A game theory model of insect population dynamics is proposed. For the case when the population may be in one of two states: when physiological processes are directed to growth and reproduction, and when physiological processes are directed to the development of defense reactions, outbreaks of mass reproduction of insect populations may occur in conditions when population and environment have the "memory", and the state of population and environment depends on their state at the previous time moment. In the framework of the model, the well known effect of insect phase variation during the outbreak of reproduction is explained.

Algorithms↗
PubMed1992

Molecular characterization of cloned variants of Coxiella burnetii isolated in China.

To study the molecular properties of Coxiella burnetii phase variants we cloned the phase variants of C. burnetii Qiyi (CBQY) strain by the red plaque technique. Three cloned strains, CBQYIC3 (phase I), CBQYIIC7 (phase II) and CBQYIIC5 (semirough-phase) were analysed by SDS-PAGE, immunoblot assay, plasmid isolation and agarose gel electrophoresis of DNA restriction fragments. The results suggest that the unique phase-dependent substance is a lipopolysaccharide and that most protein components of phase I and phase II cells are shared. No significant differences of DNA restriction fragments were found between clonal isolates of phase I and phase II C. burnetii CBQY strains. A plasmid of approximately 56 Kb was isolated from both phase I and phase II variants indicating that phase variation probably could not be attributed to its presence or absence.

Bacterial Proteins↗