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PubMed1977

[Transient behavior of the ammonium-limited chemostatic cultures of Escherichia coli ML 30].

In connection with the bistability of pyruvate formation in ammonium limited continuous cultures of E. coli ML 30 (Bergter u. Roth 1977) the transient behaviour of cell density and pyruvate concentration were studied. Immediately after a shift up in the dilution rate from D = 0.15 h-1 to D = 0.6 h-1 the bacteria excreted pyruvate into the medium, followed by a resumption of pyruvate. The specific pyruvate formation rate as well as the specific growth rate reached the new steady state with damped oscillations. Possibly the excretion of pyruvate after the shift is caused by the higher non limiting concentrations of ammonium during the first of the transition. This hypothesis is supported by the transient behaviour of an ammonium limited continuous culture after a pulse of ammonium to the culture. The relations between ammonium metabolism and pyruvate formation are discussed.

Escherichia coli↗

PubMedDate not supplied

[Device for analyzing roentgen images].

Analyzer of X-ray images includes a photodetector array with online memory, a single-purpose computer and an imager for the memory contents. The photodetector array incorporates 2 X (64 X 128) bistable photocells, their action potentials are adjusted for brightness. The analyzer is the most useful in verifying parallelism of absorbing plates in X-ray grids, in checking protective devices, and in processing mammograms.

Computers↗

PubMedDate not supplied

[Characteristics of the stoichiometric regulation of glycolysis in prokaryotic cells. A model].

A model is developed to investigate the steady state of the glycolytic system of procaryotic cells which have the hexose transport into the cell coupled with phosphoenolpyruvic acid-dependent phosphorylation. Analysis of the model shows that this phosphorylation results: 1) in stoichiometric connection between the rate of hexose consumption and the utilization of ATP and 2) the stoichiometric positive feedback. The latter can be the cause of bistability, oscillations and other nonlinear phenomena.

Bacteria↗

PubMed1984

[Abdominal ultrasonography in infants and children].

Ultrasonography is very useful and suitable for evaluating abdominal masses especially in infants and children, because it is a noninvasive, painless method and does not utilize ionizing radiation. Since 1962 we have examined various abdominal diseases in the pediatric field using ultrasonic machines with A-mode, bistable and gray scale contact compound scanners. Currently, it has become much easier to find very small lesions in the abdomen with a real time B-scanner. Hepatocarcinoma, choledochal cyst, neuroblastoma, ovarian teratoma, Wilm's tumor and hydronephroureter were presented and their echographic features were discussed. Pediatric abdominal ultrasonography is a very important modality for making accurate and quick decisions in the treatment of diseased infants and children.

Abdominal Neoplasms↗

PubMed1982

Transitions between alternate ATP-producing and ATP-consuming stationary states in a reconstituted enzyme system containing phosphofructokinase.

The kinetic behavior of a reconstituted eyzyme system containing purified phosphofructokinase, pyruvate kinase, adenylate kinase, and glucose 6-phosphate isomerase was investigated. Experimentally the approach is based on a stirred flow-through reactor containing gel entrapped enzymes. The experiments were performed on the basis of a mathematical model developed from the kinetic properties of the individual enzymes involved. The system is able to exhibit alternative stable stationary states for one set of experimental conditions (bistability) originating mainly from the allosteric character of the phosphofructokinase. From a functional point of view, these states are either ATP-generating or ATP-consuming. Theoretically, the appearance of alternate steady states gives rise to hysteretic behavior of the system. In fact, transitions between alternate ATP generating stationary states as well as between ATP-generating and ATP-consuming steady states were observed experimentally.

Adenosine Triphosphate↗

PubMedDate not supplied

[Qualitative analysis of a mathematical model of a open enzyme reaction].

A general case of the set of two differential equations, describing an open reaction v1 leads to S v reversible E P v2 leads to, has been considered. The requirements to the character of the functions v1([S]), v2([P]) and v([S], [P]) were formulated for the case of existence and absence of alternative steady states and sustained oscillations. The formulae were derived to determine the slope of the unstable portion of the quasi-steady state characteristic. The generalized model of Monod, Wyman and Changeux has been considered as an example of v([S], [P]). It has been shown that with monotonically decreasing v1 and monotonically increasing v2, the alternative steady states and oscillations are possible only in the presence of substrate inhibition or product activation. However, under the joint action of substrate inhibition and product activation, the system will exhibit bistability rather than an oscillatory behavior. In the case of an irreversible two-substrate reaction which can be described by a similar mathematical model, inhibition by the first and second substrate is equivalent to substrate inhibition and product activation.

Catalysis↗

PubMedDate not supplied

[Stability of spatially nonuniform states of diffuse systems].

The problem of stability of inhomogeneous stationary states in kinetic systems with nonlinear local dynamics and diffusion is studied. Stability criterion applicable in the case of one rate equation or two coupled rate equations with the diffusion term presented at one of them is formulated in the integral form. It is made the conclusion on the absence of stable inhomogeneous stationary states in the unicomponent system with zero fluxes at the boundaries and the case of periodic boundary conditions. In the case of non-zero fluxes at the boundaries the condition of stability of exfoliated stationary structures in the systems with bistable local dynamics is given.

Biophysical Phenomena↗

PubMed1980

[Use of ultrasound in presurgical differential diagnosis of gynaecological tumours (author's translation)].

Ultrasonographic bistable compound A-B scanning was used in presurgical examinations of 53 gynaecological tumour patients. Ultrasonographic diagnosis proved to be superior to palpation for its better characterisation of the tumour by size, location, shape, consistency, and general description. It provided safe presurgical information in 23 per cent of those cases in which palpation had failed to yield satisfactory findings. -Tumours were correctly described by ultrasonography in 63 per cent of all cases, largely accurately in 28 per cent, and largely inaccurately in nine per cent. -Interpretation of typical ultrasonograms and of those likely to give problems in differential diagnosis as well as their proper integration with the overall clinical approach are discussed.

Diagnosis, Differential↗

PubMed1993

Dynamic properties of cortical evoked (10 Hz) oscillations: theory and experiment.

Experiments probed the dynamic properties of stimulus-evoked (approximately 10 Hz) oscillations in somatosensory cortex of anesthetized rats. Experimental paradigms and statistical time series analysis were based on theoretical ideas from a dynamic approach to temporal patterns of neuronal activity. From the results of a double-stimulus paradigm we conclude that the neuronal response contains two components with different dynamics and different coupling to the stimulus. Based on this result a quantitative dynamic model is derived, making use of normal form theory for bifurcating vector fields. The variables used are abstract, but measurable, dynamic components. The model parameters capture the dynamic properties of neuronal response and are related to experimental results. A structural interpretation of the model can be given in terms of the collective dynamics of neuronal groups, their mutual interaction, and their coupling to peripheral stimuli. The model predicts the stimulus-dependent lifetime of the oscillations as observed in experiment. We show that this prediction relies on the basic concept of dynamic bistability and does not depend on the modeling details.

Animals↗

PubMedDate not supplied

[Recording, consolidation, and reproduction of information in the neuronal memory].

The electroexcitable mosaic of neuron SD-membrane is considered to be a hysteresis medium: a set of bistable memory units (MU) that capture or give back the energy in state transitions. The consolidation is shown to be achieved by inactivation of a part of MU (which had lost their energy) by means of transit of their volt-conformative characteristics to the non-hysteresis form. The MU hysteresis features are the result of moderate mobility of the membrane charges. The consolidation results in immobilization of the charges. It seems that immobilization of gate charges is the same phenomenon. Great importance of time delay pertaining to this process is emphasized. The time delay and working point shift beyond the hysteresis loop limits into the hyperpolarization side ensures the non-destructive reading.

Cell Membrane↗

PubMed1996

Two-neuro networks. II. Leaky integrator pacemaker models.

The behavior of two pacemaker neurons simulated by leaky integrators and connected reciprocally by synapses was studied. In every case the firing of both neurons phase-locks. The resulting limit cycle may or may not show simultaneous firing of both neurons. When both synapses are excitatory, phase-locking with simultaneous neuronal firing is always present. When one synapse is excitatory and the other inhibitory, phase-locking is also present always, while the neurons may or may not fire simultaneously. For a restricted set of parameters, bistability appears; the initial conditions determine whether or not the limit cycle presents simultaneous firing. When both synapses are inhibitory, the system phase-locks without simultaneous firing for almost every set of parameters.

Biological Clocks↗

PubMed1977

Grey-scale ultrasound in the diagnosis of abdominal mass lesions.

With the introduction of grey-scale imaging, the diagnostic capability of ultrasound has been greatly increased. Previously, ultrasound images were limited to a bistable presentation that did not distinguish between high- and low-level echoes. As a result the images could be interpreted by only a few experienced ultrasonographers. Grey-scale imaging with the use of a television scan converter allows both the strong and the weaker echoes to be displayed, so that both the outline and the internal texture of normal abdominal organs and lesions can be visualized on a single scan. Abnormalities are now more readily identified and their relation to normal organs is more easily appreciated so that the scans can be interpreted not only by the ultrasonographer but also by the clinician. Several cases are presented in which grey-scale ultrasound was used.

Abdomen↗

PubMed1997

Molecular clues to pathogenesis in prion diseases.

The infectious agent of the transmissible spongiform encephalopathies (TSE) resembles a virus in that it propagates in vivo and has distinct strains. However, compelling evidence strongly suggests that a posttranslational structural alteration in a glycoprotein PrPC (the normal, cellular isoform of the so-called prion protein) is responsible for pathogenesis of these diseases. According to this hypothesis-now close to being generally accepted-iatrogen, sporadic and familial forms of TSE would have the same molecular mechanism: the conversion of PrPC into a protease-resistant isoform PrPSc kinetically behaves as an autocatalytic process which, combined with the high turnover rate of the normal isoform, may endow the system with bistability properties and subsequent threshold behavior between normal and pathogenic steady-states. Normal prion protein seems to be necessary for long-term survival of Purkinje neurons, regulation of circadian rhythms and, more controversially, for normal synaptic function. At least part of the pathology might be due to the unavailability of normal isoform rather than to the accumulation of PrPSc. NMR structure of the normal mouse prion protein reveals a short, unexpected beta-sheet which might be a nucleation site for the conformational transition between PrPC and PrPSc. Prion diseases may challenge the edged distinction that we use to make between informational (DNA) and functional (proteins) macromolecules. Pathogenic mechanism of prions might also be involved in other proteins to achieve and pass on their conformation. Hence, structural inheritance at the molecular level might be the missing link for the understanding of the structural inheritance processes featured at the cellular level. Moreover, evolutionary paradigm postulating a primitive RNA world is weakened by the mechanism of prion diseases.

Animals↗

PubMed1998

Stability analysis of the FitzHugh-Nagumo differential equations driven by impulses: applied to the electrical firing of magnocellular neurons.

A stability analysis is carried out for a mathematical model which describes the electrical firing of a single vasopressin neuron. The model used in a FitzHugh-Nagumo-type system which is driven by impulses. The analysis is based on recent developments in the stability theory of impulsive differential equations. Conditions are derived under which the system of differential equations is stable at two of its equilibrium points. Biologically this bistability represents the cell alternating between periods of electrical activity and silence. The conditions for stability are specified in terms of the amplitude and frequency of the impulses perturbing the system. Both stochastic and deterministic impulses are considered.

Action Potentials↗

PubMed2025

The SinR·SlrR Heteromer Attenuates Transcription of a Long Operon of Flagellar Genes in Bacillus subtilis.

During growth, Bacillus subtilis differentiates into subpopulations of motile individuals and non-motile chains, associated with dispersal and biofilm formation, respectively. The two cell types are dictated by the activity of the alternative sigma factor SigD encoded as the penultimate gene of the 27-kb long fla/che flagellar operon. The frequency of SigD-ON motile cells is increased by the heteromeric transcription factor SwrA·DegU that activates the fla/che promoter. Conversely, the frequency of motile cells is decreased by the heteromeric transcription factor SinR·SlrR, but the mechanism and location of inhibition is poorly understood. Here, using ChIP-Seq analysis, we determine the binding sites of the SinR·SlrR heteromer on the genome. We identified two sites within the fla/che operon that were necessary and sufficient to attenuate transcript abundance by causing premature termination upstream of the gene that encodes SigD. Thus, cell motility and the transition to biofilm formation depend on the expression of a long operon governed by two opposing heteromeric transcription factors that operate at two different stages of the transcription cycle. More broadly, our study serves as a model for transcription factors that control transcriptional elongation and the regulation of long operons in bacteria.

Bacillus subtilis↗

PubMed2025

100+ years of phase variation: the premier bacterial bet-hedging phenomenon.

Stochastic, reversible switches in the expression of Salmonella flagella variants were first described by Andrewes in 1922. Termed phase variation (PV), subsequent research found that this phenomenon was widespread among bacterial species and controlled expression of major determinants of bacterial-host interactions. Underlying mechanisms were not discovered until the 1970s/1980s but were found to encompass intrinsic aspects of DNA processes (i.e. DNA slippage and recombination) and DNA modifications (i.e. DNA methylation). Despite this long history, discoveries are ongoing with expansions of the phase-variable repertoire into new organisms and novel insights into the functions of known loci and switching mechanisms. Some of these discoveries are somewhat controversial as the term 'PV' is being applied without addressing key aspects of the phenomenon such as whether mutations or epigenetic changes are reversible and generated prior to selection. Another 'missing' aspect of PV research is the impact of these adaptive switches in real-world situations. This review provides a perspective on the historical timeline of the discovery of PV, the current state-of-the-art, controversial aspects of classifying phase-variable loci and possible 'missing' real-world effects of this phenomenon.

Gene Expression Regulation, Bacterial↗