Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Stability analysis”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 469 records · Page 26Linked to original sources
PubMed2005

Bacterial community succession in natural river biofilm assemblages.

Temporal bacterial community changes in river biofilms were studied using 16S rRNA gene-based polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) followed by sequence analysis. Naturally occurring biofilms were sampled in 2001 during an undisturbed 7-month low-water period in the River Garonne (SW France). During the sampling period epilithic biomass exhibited a particular pattern: two 3-month periods of accumulation that resulted in two peaks in summer and fall, each at about 25 g ash-free dry mass per square meter. Bacterial community DGGE profiles differed between the summer and fall biomass peaks and shared only 30% common operational taxonomic units (OTUs), suggesting the influence of seasonal factors on these communities. During the second biomass accrual phase, bacterial richness and the appearance of new OTUs fitted a conceptual model of bacterial biofilm succession. During succession, five OTUs (corresponding to Dechloromonas sp., Nitrospira sp., and three different Spirosoma spp.) exhibited particular patterns and were present only during clearly defined successional stages, suggesting differences in life-history strategies for epilithic bacteria. Co-inertia analysis of DGGE banding patterns and physical-chemical data showed a significant relationship between community structure and environmental conditions suggesting that bacterial communities were mainly influenced by seasonal changes (temperature, light) and hydrodynamic stability. Within the periods of stability, analysis of environmental variables and community patterns showed the dominant influence of time and maturation on bacterial community structure. Thus, succession in these naturally occurring epilithic biofilm assemblages appears to occur through a combination of allogenic (seasonal) and autogenic changes.

Bacteria↗
PubMed2001

Radiographic analysis of lumbar lordosis: centroid, Cobb, TRALL, and Harrison posterior tangent methods.

STUDY DESIGN: Delayed, repeated measures, with three examiners each twice digitizing thirty lateral lumbar radiographs. OBJECTIVES: To determine the reliability and clinical utility of the centroid, Cobb, tangential radiologic assessment of lumbar lordosis (TRALL), and Harrison posterior tangent line-drawing methods for analysis of lumbar lordosis. BACKGROUND DATA: Cobb's method is commonly used for curvature analysis on lateral lumbar radiographs, whereas the centroid, TRALL, and Harrison posterior tangent methods are not widely used. METHODS: Thirty lateral lumbar radiographs were digitized twice by each of three examiners. To evaluate reliability of determining global and segmental alignment, all four vertebral body corners of T12-S1 and the superior margin of the femur head were digitized. Angles created were segmental and global centroid, (two-line) Cobb angles, and intersections of posterior tangents. A global TRALL angle was determined. Means, standard deviations, mean absolute differences, interclass and intraclass correlation coefficients (ICC), and confidence intervals were calculated. RESULTS: The interobserver and intraobserver reliabilities of measuring all segmental and global angles were in the high range (ICCs > 0.83). The mean absolute differences of observers' measurements were small (0.6 degrees -2.0 degrees ). Distal segmental (L4-S1) and global angles of lumbar curvature were dependent on the method of measurement. CONCLUSIONS: All four radiographic methods had high reliability and low mean absolute differences of observers' measurements. Because it lacks a segmental analysis, the TRALL method is not recommended. The centroid, Cobb, and Harrison posterior tangent methods provide global and segmental angles. However, the centroid segmental method requires three segments and is less useful for a stability analysis.

Humans↗
PubMed2003

High cell density induces spontaneous bifurcations of dissolved oxygen controllers during CHO cell fermentations.

High cell density cultures of CHO cells growing in a bioreactor under dissolved oxygen control were found to undergo spontaneous bifurcations and a subsequent loss of stability some time into the fermentation. This loss of stability was manifested by sustained and amplified oscillations in the bioreactor dissolved oxygen concentration and in the oxygen gas flow rate to the reactor. To identify potential biological and operational causes for the phenomenon, linear stability analysis was applied in a neighborhood of the experimentally observed bifurcation point. The analysis revealed that two steady state process gains, K(P1) and K(P2), regulated k(l)a and gas phase oxygen concentration inputs, respectively, and the magnitude of K(P1) was found to determine system stability about the bifurcation point. The magnitude of K(P1), and hence the corresponding open-loop steady state gain K(OL1), scaled linearly with the bioreactor cell density, increasing with increasing cell density. These results allowed the generation of a fermentation stability diagram, which partitioned K(C)-N operating space into stable and unstable regions separated by the loci of predicted critically stable controller constants, K(C,critical), as a function of bioreactor cell density. This consistency of this operating diagram with experimentally observed changes in system stability was demonstrated. We conclude that time-dependent increases in cell density are the cause of the observed instabilities and that cell density is the critical bifurcation parameter. The results of this study should be readily applicable to the design of a more robust controller.

Algorithms↗
PubMed2000

Robust PID tuning strategy for uncertain plants based on the Kharitonov theorem

In this paper, the Kharitonov theorem for interval plants is exploited for the purpose of synthesizing a stabilizing controller. The aim here is to develop a controller to simultaneously stabilize the four Kharitonov-defined vortex polynomials. Different from the prevailing works, the controller is designed systematically and graphically through the search of a non-conservative Kharitonov region in the controller coefficient parameter plane. The region characterizes all stabilizing PID controllers that stabilize an uncertain plant. Thus the relationship between the Kharitonov region and the stabilizing controller parameters is manifest. Extensively, to further guarantee the system with certain robust safety margins, a virtual gain phase margin tester compensator is added. Stability analysis is carried out. The control system is proved to maintain robustness at least to the pre-specified margins. The synthesized controller with coefficients selected from the obtained non-conservative Kharitonov region can stabilize the concerned uncertain plants and fulfill system specifications in terms of gain margins and phase margins.

Journal Article↗
PubMed1997

Plume dynamics in quasi-2D turbulent convection.

We have studied turbulent convection in a vertical thin (Hele-Shaw) cell at very high Rayleigh numbers (up to 7x10(4) times the value for convective onset) through experiment, simulation, and analysis. Experimentally, convection is driven by an imposed concentration gradient in an isothermal cell. Model equations treat the fields in two dimensions, with the reduced dimension exerting its influence through a linear wall friction. Linear stability analysis of these equations demonstrates that as the thickness of the cell tends to zero, the critical Rayleigh number and wave number for convective onset do not depend on the velocity conditions at the top and bottom boundaries (i.e., no-slip or stress-free). At finite cell thickness delta, however, solutions with different boundary conditions behave differently. We simulate the model equations numerically for both types of boundary conditions. Time sequences of the full concentration fields from experiment and simulation display a large number of solutal plumes that are born in thin concentration boundary layers, merge to form vertical channels, and sometimes split at their tips via a Rayleigh-Taylor instability. Power spectra of the concentration field reveal scaling regions with slopes that depend on the Rayleigh number. We examine the scaling of nondimensional heat flux (the Nusselt number, Nu) and rms vertical velocity (the Peclet number, Pe) with the Rayleigh number (Ra(*)) for the simulations. Both no-slip and stress-free solutions exhibit the scaling NuRa(*) approximately Pe(2) that we develop from simple arguments involving dynamics in the interior, away from cell boundaries. In addition, for stress-free solutions a second relation, Nu approximately nPe, is dictated by stagnation-point flows occurring at the horizontal boundaries; n is the number of plumes per unit length. No-slip solutions exhibit no such organization of the boundary flow and the results appear to agree with Priestley's prediction of Nu approximately Ra(1/3). (c) 1997 American Institute of Physics.

Journal Article↗
PubMed1999

Analysis of genomic integrity and p53-dependent G1 checkpoint in telomerase-induced extended-life-span human fibroblasts.

Life span determination in normal human cells may be regulated by nucleoprotein structures called telomeres, the physical ends of eukaryotic chromosomes. Telomeres have been shown to be essential for chromosome stability and function and to shorten with each cell division in normal human cells in culture and with age in vivo. Reversal of telomere shortening by the forced expression of telomerase in normal cells has been shown to elongate telomeres and extend the replicative life span (H. Vaziri and S. Benchimol, Curr. Biol. 8:279-282, 1998; A. G. Bodnar et al., Science 279:349-352, 1998). Extension of the life span as a consequence of the functional inactivation of p53 is frequently associated with loss of genomic stability. Analysis of telomerase-induced extended-life-span fibroblast (TIELF) cells by G banding and spectral karyotyping indicated that forced extension of the life span by telomerase led to the transient formation of aberrant structures, which were subsequently resolved in higher passages. However, the p53-dependent G1 checkpoint was intact as assessed by functional activation of p53 protein in response to ionizing radiation and subsequent p53-mediated induction of p21(Waf1/Cip1/Sdi1). TIELF cells were not tumorigenic and had a normal DNA strand break rejoining activity and normal radiosensitivity in response to ionizing radiation.

Cell Survival↗
PubMed1999

Why do women choose nursing?

To answer the question, "Why do women choose nursing?", female nursing students were given an opportunity to speak "in their own voices" to reveal their needs, motivations, and expectations. In a study employing grounded theory methodology, 16 female nursing students were interviewed and audiotaped for initial constant comparison analysis. Further analyses employing coding, categorizing, comparing, and contrasting of data made it possible to isolate recurrent words and phrases, and to identify concepts for the purpose of theory building. Reliability was established by both stability analysis and reproducibility analysis. Validity was supported by construct (convergent and discriminant) and representational components. Not surprisingly, the construct of desiring to care for others was readily apparent. However, it was power and empowerment that emerged from the interviews as the most resounding and fully developed construct of the study. Students' references to practical motivations for choosing nursing were so notably scarce that their virtual omission constituted an important theme.

Adolescent↗
PubMed1996

Properties of an intracellular beta-glucosidase purified from the cellobiose-fermenting yeast Candida wickerhamii.

An intracellular beta-glucosidase was isolated from the cellobiose-fermenting yeast, Candida wickerhamii. Production of the enzyme was stimulated under aerobic growth, with the highest level of production in a medium containing cellobiose as a carbohydrate source. The molecular mass of the purified protein was approximately 94 KDa. It appeared to exist as a dimeric structure with a native molecular mass of about 180 KDa. The optimal pH ranged from 6.0 to 6.5 with p-nitrophenyl beta-D-glucopyranoside (NpGlc) as a substrate. The optimal temperature for short-term (15-min) assays was 35 degrees C, while temperature-stability analysis revealed that the enzyme was labile at temperatures of 28 degrees C and above. Using NpGlc as a substrate, the enzyme was estimated to have a Km of 0.28 mM and a Vmax of 525 mumol product min-1 mg protein-1. Similar to the extracellular beta-glucosidase produced by C. wickerhamii, this enzyme resisted end-product inhibition by glucose, retaining 58% of its activity at 100 mM glucose. The activity of the enzyme was highest against aryl beta-1,4-glucosides. However, p-nitrophenyl xylopyranoside, lactose, cellobiose, and trehalose also served as substrates for the purified protein. Activity of the enzyme was stimulated by long-chain n-alkanols and inhibited by ethanol, 2-propanol, and 2-butanol. The amino acid sequence, obtained by Edman degradation analysis, suggests that this beta-glucosidase is related to the family-3 glycosyl hydrolases.

Aerobiosis↗
PubMed2004

Predicting stability of mixed microbial cultures from single species experiments: 2. Physiological model.

In this paper, we study the equilibria of a physiological model describing the continuous culture in which two microbial populations compete for two substitutable resources. This work is an extension of the stability analysis of the phenomenological model of mixed microbial growth [M.M. Ballyk, G.S.K. Wolkowicz, Exploitative competition in the chemostat for two perfectly substitutable resources, Math. Biosci. 118 (1993) 127-180; S.S. Pilyugin, G.T. Reeves, A. Narang, Predicting stability of mixed microbial cultures from single species experiments: 2. Phenomenological model]. Here, we investigate the influence of the peripheral enzymes that catabolize the substrate uptake on the stability of the mixed culture. We show that, under steady state conditions, an increase in the concentration of one substrate inhibits the uptake of the other substrate(s). We present the criteria for existence, uniqueness, and stability of various types of equilibria. We formulate these criteria in terms of growth isoclines and consumption curves for each of the competing species. Since both types of curves can be obtained from a single species experiment, our approach provides a direct connection between theory and experiment and allows one to infer the dynamics of mixed cultures from the dynamics of single species cultures. By expressing the stability criteria in terms of intracellular properties, the model establishes a link between ecology and molecular biology.

Culture Media↗
PubMed2004

Clustering transition in a system of particles self-consistently driven by a shear flow.

We introduce a simple model of active transport for an ensemble of particles driven by an external shear flow. Active refers to the fact that the flow of the particles is modified by the distribution of particles itself. The model consists in that the effective velocity of every particle is given by the average of the external flow velocities felt by the particles located at a distance less than a typical radius, R. Numerical analysis reveals the existence of a transition to clustering depending on the parameters of the external flow and on R. A continuum description in terms of the number density of particles is derived, and a linear stability analysis of the density equation is performed in order to characterize the transitions observed in the model of interacting particles.

Journal Article↗
PubMed1995

Nonlinear autoregressive analysis of the 3/s ictal electroencephalogram: implications for underlying dynamics.

In a previous study, nonlinear autoregressive (NLAR) models applied to ictal electroencephalogram (EEG) recordings in six patients revealed nonlinear signal interactions that correlated with seizure type and clinical diagnosis. Here we interpret these models from a theoretical viewpoint. Extended models with multiple nonlinear terms are employed to demonstrate the independence of nonlinear dynamical interactions identified in the 'NLAR fingerprint' of patients with 3/s seizure discharges. Analysis of the role of periodicity in the EEG signal reveals that the fingerprints reflect the dynamics not only of the periodic discharge itself, but also of the fluctuations of each cycle about an average waveform. A stability analysis is used to make qualitative inferences concerning the network properties of the ictal generators. Finally, the NLAR fingerprint is analyzed in the context of Volterra-Weiner theory.

Electroencephalography↗
PubMed2005

Sugar edge/sugar edge base pairs in RNA: stabilities and structures from quantum chemical calculations.

Cis and trans sugar edge/sugar edge (SE/SE) binding patterns are essential building units of RNAs. For example, SE/SE interactions form the A-minor motifs, the most important tertiary interaction type in functional RNAs. This study provides an in-depth structure and stability analysis for these two base pair families. Gas-phase-optimized geometries are reported for 12 cis and 7 trans SE/SE base pairs and contrasted to their X-ray counterparts. Interaction energies are computed at the RIMP2 level of theory using the density-functional-theory-optimized geometries. There is a good overall agreement between the optimized and X-ray geometries of the cis SE/SE base pairs. In contrast, only three of the seven trans SE/SE binding patterns could be optimized without a significant distortion of the X-ray geometry. Note, however, that many SE/SE base pairs participate in broader networks of interactions; thus it is not surprising to see some of them to deviate from the X-ray geometry in a complete isolation. Computed interaction energies reveal that all 12 known cis SE/SE binding patterns are very stable. Among the trans SE/SE binding patterns, only the rG/rG, rG/rC, and rA/rG base pairs are sufficiently stable in the crystal geometry. Prediction has been made for some structures not yet detected by crystallography, namely, cis rC/rC, rG/rC, rG/rU, and rU/rU and trans rG/rA base pairs. Interestingly, the new cis SE/SE binding patterns are not necessarily isosteric with the remaining 12 members of this family. The trans rG/rA base pair represents a viable option for base pairing in RNA to be identified by future X-ray studies. In a complete lack of structural information, prediction of other unknown members of the trans SE/SE family was not attempted. Analysis of the interaction energies shows a very large electron correlation component of the interaction energy, pointing at the elevated role of dispersion energy as compared to other types of base pairs. This likely is profitable for stabilization of SE/SE binding patterns in polar environments and could be one of the reasons why the A-minor motif is the leading type of tertiary interactions in RNAs.

Base Pairing↗
PubMed1997

First-order differential-delay equation for the baroreflex predicts the 0.4-Hz blood pressure rhythm in rats.

We have described a 0.4-Hz rhythm in renal sympathetic nerve activity (SNA) that is tightly coupled to 0.4-Hz oscillations in blood pressure in the unanesthetized rat. In previous work, the relationship between SNA and fluctuations in mean arterial blood pressure (MAP) was described by a set of two first-order differential equations. We have now modified our earlier model to test the feasibility that the 0.4-Hz rhythm can be explained by the baroreflex without requiring a neural oscillator. In this baroreflex model, a linear feedback term replaces the sympathetic drive to the cardiovascular system. The time delay in the feedback loop is set equal to the time delay on the efferent side, approximately 0.5 s (as determined in the initial model), plus a time delay of 0.2 s on the afferent side for a total time delay of approximately 0.7 s. A stability analysis of this new model yields feedback resonant frequencies close to 0.4 Hz. Because of the time delay in the feedback loop, the proportional gain may not exceed a value on the order of 10 to maintain stability. The addition of a derivative feedback term increases the system's stability for a positive range of derivative gains. We conclude that the known physiological time delay for the sympathetic portion of the baroreflex can account for the observed 0.4-Hz rhythm in rat MAP and that the sensitivity of the baroreceptors to the rate of change in blood pressure, as well as average blood pressure, would enhance the natural stability of the baroreflex.

Activity Cycles↗
PubMed2000

Journal club as a supplement to the undergraduate biochemistry laboratory.

After purification of lysozyme, our biochemistry students write a research proposal that outlines a strategy for studying this enzyme after alteration by site-directed mutagenesis. Despite a literature search that yielded a wealth of background information, students were often overwhelmed by the assignment because they were not familiar with advanced techniques of protein analysis. We therefore developed a series of journal clubs in which teams of students present methods and data found in papers dealing with lysozyme. The five topics for journal clubs include; substrate binding and mechanism; spectroscopic techniques; stability analysis; two-dimensional NMR; and X-ray crystallography. After the adoption of the group talks, the quality of the research proposals improved immensely and students found the assignment to be an educationally rewarding exercise.

Journal Article↗
PubMed1996

Singular perturbation analysis of competitive neural networks with different time scales.

The dynamics of complex neural networks must include the aspects of long- and short-term memory. The behavior of the network is characterized by an equation of neural activity as a fast phenomenon and an equation of synaptic modification as a slow part of the neural system. The main idea of this paper is to apply a stability analysis method of fixed points of the combined activity and weight dynamics for a special class of competitive neural networks. We present a quadratic-type Lyapunov function for the flow of a competitive neural system with fast and slow dynamic variables as a global stability method and a modality of detecting the local stability behavior around individual equilibrium points.

Algorithms↗
PubMed2002

A new approach to stability of neural networks with time-varying delays.

The stability of neural networks is a prerequisite for successful applications of the networks as either associative memories or optimization solvers. Because the integration and communication delays are ubiquitous, the stability of neural networks with delays has received extensive attention. However, the approach used in the previous investigation is mainly based on Liapunov's direct method. Since the construction of Liapunov function is very skilful, there is little compatibility among the existing results. In this paper, we develop a new approach to stability analysis of Hopfield-type neural networks with time-varying delays by defining two novel quantities of nonlinear function similar to the matrix norm and the matrix measure, respectively. With the new approach, we present sufficient conditions of the stability, which are either the generalization of those existing or new. The developed approach may be also applied for any general system with time delays rather than Hopfield-type neural networks.

Neural Networks, Computer↗
PubMed1999

Theory of ocular dominance pattern formation.

We investigate a general and analytically tractable model for the activity-dependent formation of neuronal connectivity patterns. Previous models are contained as limiting cases. As an important example we analyze the formation of ocular dominance patterns in the visual cortex. A linear stability analysis reveals that the model undergoes a Turing-type instability as a function of interaction range and receptive field size. The phase transitions is of second order. After the linear instability the patterns may reorganize which we analyze in terms of a potential for the dynamics. Our analysis demonstrates that the experimentally observed dependency of ocular dominance patterns on interocular correlations of visual experience during development can emerge according to two generic scenarios: either the system is driven through the phase transition during development thereby selecting and stabilizing the first unstable mode or a primary pattern reorganizes towards larger wavelength according their lower energy. Experimentally observing the time course of ocular dominance pattern formation will decide which scenario is realized in the brain.

Algorithms↗
PubMedDate not supplied

Effect of drug stability on the analysis of release data from controlled release microspheres.

The objective of this study was to determine the effect of drug stability on the analysis of in vitro release data from controlled release microspheres. Amoxicillin was chosen as a model drug and its stability in various media determined. The in vitro release of amoxicillin from PLGA microspheres was investigated in phosphate buffered saline (PBS) containing either Tween-80 or Tween-80 + Na-azide. Drug concentration in the media and the amount remaining inside the microspheres were analysed using HPLC. Amoxicillin degraded rapidly in PBS. The rate of degradation was reduced in the presence of Tween 80 and Na-azide. Drug release from the microspheres was analysed with and without considering the drug degradation rate. Mass balance between released drug, drug remaining in the microspheres and initial drug loading was only achieved when drug degradation was accounted for. Drug stability should be considered in analysis of in vitro extended release data, since released drug may degrade with time. These results have implications for in vitro release studies for controlled release products that include drugs and other compounds which are susceptible to degradation during the course of the release study.

Amoxicillin↗