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Biomedical subjects

E de Boer

Publications and source records attributed to E de Boer.

At least 55 records · Page 3Linked to original sources
PubMed1996

[Verocytotoxin-producing Escherichia coli O157 in Dutch veal calves and beef cattle].

In 1994 (August-September) and in 1995 (July-October) faeces of Dutch veal calves and adult cattle was examined for the presence of verocytotoxin-producing Escherichia coli O157 (O157 VTEC). The samples were collected at slaughterhouses. In 1994, O157 VTEC were isolated from three (0.9%) of 365 fecal samples of veal calves. Faeces of adult cattle was not collected. In 1995, O157 VTEC were isolated from one (0.5%) of 18.3 fecal samples of veal calves and from 30 (11.1%) of 270 fecal samples of adult cattle. In 1994, the organisms were isolated by selective plating onto both sorbitol MacConkey agar (SMAC) and SMAC containing tellurite and cefixime (TC-SMAC) following selective enrichment in modified trypton soya broth with acriflavin (mTSB + a). In 1995, the samples were enriched in modified E. coli broth with novobiocin (mEC + n), and in addition to directly plating onto TC-SMAC the enriched cultures were plated after incorporation of an immunomagnetic separation step of E. coli O157. All 30 strains isolated from adult cattle were isolated with the immunomagnetic separation procedure. Four of the 30 samples were determined positive also by directly plating onto TC-SMAC. The sample from the veal calve was determined positive only by directly plating onto TC-SMAC. Plating onto SMAC resulted only in negative results. Characterization of the O157 VTEC isolates showed similarities between isolates of humans and cattle. Additional experiments need to be done concerning this aspect. From this study it appeared that also Dutch cattle are a reservoir of O157 VTEC.

Animals↗
PubMed1996

Heterochromatin effects on the frequency and duration of LCR-mediated gene transcription.

Locus control regions (LCRs) are responsible for initiating and maintaining a stable tissue-specific open chromatin structure of a locus. In transgenic mice, LCRs confer high level expression on linked genes independent of position in the mouse genome. Here we show that an incomplete LCR loses this property when integrated into heterochromatic regions. Two disruption mechanisms were observed. One is classical position-effect variegation, resulting in continuous transcription in a clonal subpopulation of cells. The other is a novel mechanism resulting in intermittent gene transcription in all cells. We conclude that only a complete LCR fully overcomes heterochromatin silencing and that it controls the level of transcription by ensuring activity in all cells at all times rather than directly controlling the rate of transcription.

Animals↗
PubMed1996

[Infections with verocytotoxin-producing Escherichia coli and hemolytic-uremic syndrome].

Infections with verocytotoxin-producing Escherichia coli and (VTEC), and especially with serotype O157, are the main cause of haemolytic-uraemic syndrome (HUS) in children in the Netherlands. 8% of the patients infected develop HUS; the incidence is 2/105/year in children under 5 years. The infection may be asymptomatic, but may also lead to mild to haemorrhagic diarrhoea and to haemorrhagic colitis. Up to 10% of the patients die in the acute phase of the disease and in up to another 10% the renal damage does not resolve completely. In 1993 bovine meat samples examined by polymerase chain reaction revealed VTEC in 16% of the cases; however none of the isolated serotypes was known to be pathogenic in humans. Epidemiological investigations are being carried out in the cattle population. Verocytotoxins are exotoxins that bind to surface receptors on cells after which part of the toxin is internalized where it inhibits protein synthesis. The functional receptor is glycosphingolipid globotriaosylceramide, a molecule normally only expressed in the renal glomeruli of children under three years of age.

Adolescent↗
PubMed1996

A selective medium for the isolation of Arcobacter from meats.

A method, including enrichment in Arcobacter Selective Broth (ASB) and isolation on semisolid Arcobacter Selective Medium (ASM) under aerobic conditions at 24 degrees C, is described for the isolation of Arcobacter from retail meat products. Selective agents used in ASB and ASM were cefoperazone, trimethoprim, piperacillin and cycloheximide. Arcobacters were isolated from 53 (24.1%) of 220 poultry meat products and also, at lower incidence from samples of beef and pork. The isolates were identified as A. butzleri or A. butzleri-like and belonged to a wide variety of serotypes and biotypes.

Animals↗
PubMed1995

TBP binding and the rate of transcription initiation from the human beta-globin gene.

DNA-protein interaction studies in vitro revealed several factors binding over the TATA box and the region of transcription initiation (cap) site of the human beta-globin promoter; TATA binding protein TBP at -30, Sp1 at -19, GATA-1 at -12 and +5, YY1 at -9 and a novel factor C1 over the site of initiation (-4 to +7). Point mutants which specifically abolish the binding of each of these proteins were tested in a beta-globin locus control region (LCR) construct which allows quantitative comparisons at physiological levels of transcription. Only mutants which drastically affect the binding of TBP resulted in decreased levels of transcription. A threshold value of TBP binding of 15-30% of wild type was sufficient to give normal levels of transcription. This indicates that the association of TF IID with the TATA box is not limiting in the rate of initiation of transcription.

Animals↗
PubMed1995

High fraction of penta-coordinated aluminium in amorphous and crystalline aluminium borates.

27Al and 11B magic-angle spinning nuclear magnetic resonance (MAS NMR) results are reported on aluminium borates of composition Al2(1 - x)B2xO3 (0 < or = x < or = 0.7). It is shown that starting from liquid precursors with hexa-coordinated Al and tri- and tetra-coordinated B, porous aluminium borates are obtained containing high fractions of penta-coordinated Al. The presence of boron in the crystalline phases stabilizes penta-coordinated Al, existing already in their amorphous precursors, and at the same time distorts the surroundings of the hexa-coordinated Al. The reversible effect of air exposure on low-coordinated Al (Al4C, Al5C) and B (B3C) proved that at least these species are situated at the surfaces of the pores. The A9B2 phase still contains a small fraction of tetra-coordinated B. The 27Al and 11B MAS NMR spectra of A9B2 show a well-resolved powder spectrum with no indication of broadening due to a distorted configuration.

Aluminum Compounds↗
PubMed1995

The "inverse problem" solved for a three-dimensional model of the cochlea. I. Analysis.

With nonactive cochlear models of the "classical" type, it is impossible to stimulate the characteristic type of frequency selectivity that is revealed by modern mechanical measurements of the motion of the basilar membrane (BM). Locally active models of the cochlea have been proposed to alleviate this problem but it remains uncertain whether the real cochlea is active in this sense. The present study was undertaken to investigate this subject in a more general and systematic way than has hitherto been done. The "inverse" problem is solved for a three-dimensional (3-D) model and a procedure is developed for recovering the BM impedance needed to stimulate the given BM response function. In the present paper the theoretical basis of the procedure is presented, and an analysis is given of the validity of the method and the errors involved. It is shown why the inverse problem is "ill-posed" and why the results of our procedure are more accurate in the region of the response peak than in the more basal region of the model. The latter finding serves to delimit the domain within which results of an inverse procedure (for instance, in the short- or long-wave approximation) are to be judged. In a subsequent paper the technique developed will be applied to actual data sets from the literature on mechanical BM measurements.

Auditory Perception↗
PubMed1995

The "inverse problem" solved for a three-dimensional model of the cochlea. II. Application to experimental data sets.

With "classical" nonactive models of the cochlea it is impossible to simulate the degree of frequency selectivity that is revealed by modern mechanical measurements of the motion of the basilar membrane (BM). Locally active models have been proposed to alleviate this problem, but it remains uncertain whether the actual cochlea is active in this sense. In the first paper of this series [E. de Boer, J. Acoust. Soc. Am. 98, 896-903 (1995)], the "inverse" problem is solved for a (classical) three-dimensional model and a procedure is developed for recovering the BM impedance needed to simulate a given BM response function. It was found that the results of this procedure will be more accurate in the region of the response peak than in the more basal region of the model. In the present paper the same procedure is applied to data of recent mechanical experiments. For the peak region the outcome is unequivocal: Recent measurement results can only be simulated by the classical model when it is made locally active. Resynthesis of the model response, on the basis of the recovered BM impedance, confirms the validity of the method in the peak region and strengthens this conclusion.

Auditory Perception↗
PubMed1995

On equivalence of locally active models of the cochlea.

In the literature on locally active models of the cochlea several forms of the model have been worked out in which elements of the organ of Corti are assumed to produce acoustical energy. In these models use is made of a secondary resonance or a place-dependent delay to achieve activity over a limited part of the length of the basilar membrane, and outer hair cells are postulated as sources of energy production. In the present paper it will be shown that several of these models are formally equivalent. That means that they can be reduced to a standard form, and that the only difference resides in the choice and meaning of the parameters. The results of the analysis serve to facilitate the analysis of function and structure in cochlear models containing sources of local activity. In particular, questions of positive versus negative feedback, and the parts played by resonance and impedance-level differences can be resolved unambiguously. The treatment can be extended to models with higher dimensionality, and can also serve to obtain insight in nonlinear phenomena occurring in cochlear models.

Basilar Membrane↗
PubMed1994

Two-tone suppression in a locally active nonlinear model of the cochlea.

In auditory nerve, inner-hair cell and basilar-membrane responses, it has been found that the response to one tone can be suppressed by another tone. This phenomenon, called two-tone suppression, is examined on the level of the basilar membrane with a locally active long-wave model of the cochlea in which the active mechanism is nonlinear. The model is solved in the frequency domain by means of a quasi-linear solution method. Several phenomena, such as difference in growth of suppression for low-side and high-side suppressors and critical dependence of the phase of the probe response on suppressor parameters, have been replicated. The attenuation hypothesis which states that the presence of a suppressor has the same effect on the probe response as attenuation of probe level is shown to be insufficient in explaining the experimental data. This model, in which suppression is simply reduction of power amplification due to saturation of the active mechanism, is more successful in this respect.

Auditory Perception↗
PubMed1993

The sulcus connection. On a mode of participation of outer hair cells in cochlear mechanics.

Motile outer hair cells (OHCs) can only participate well in cochlear mechanics when one end of the hair cells is more restrained in its movements than the other. On this thought a model of the organ of Corti (OoC) is developed in which (in every cross section) the tectorial membrane (TM) is considered to consist of two stiff segments connected by a hinge. Movements of the TM then induce movements of the fluid contained in the inner spiral sulcus (ISS) and the fluid dynamics of the sulcus will play an important part in restraining the top ends of the hair cells. Since the model is meant to reflect low-level phenomena, it is conceived as linear. For simplicity, it operates in the long-wave mode. The sulcus is considered as a narrow channel that is closed at both ends. Since the sulcus fluid is an independent energy-storage element of the system, the entire cochlear model, consisting of two main fluid channels, the organ of Corti (OoC), the three-element TM and the sulcus, and equipped with motile OHCs, can be described by a fourth-order differential equation, with four boundary conditions. It is shown that the model can ideally produce response curves that reflect more than 40 dB of cochlear gain. When the actual effect of the sulcus is taken into account, however, the maximum gain is reduced to approximately 30 dB. The main reason for the reduction is that the sulcus cannot produce a large enough pressure to restrain the upper ends of the OHCs.

Acoustics↗
PubMed1993

Self-suppression in a locally active nonlinear model of the cochlea: a quasilinear approach.

Mechanical input-output functions of the cochlea for pure-tone stimuli are nonlinear for frequencies around the characteristic frequency. To simulate these functions, a long-wave model of the cochlea containing a saturating pressure generator (located at the site of the outer hair cells) is solved in the frequency domain with a quasilinear method. In this method distortion products in the basilar-membrane (BM) response are treated as perturbations and the nonlinear pressure waveform is approximated by the first-order Fourier component. Because the saturating pressure generator forms part of a feedback loop the solution of the model is achieved in a number of iteration steps. Model results show flattening of the BM response at higher input pressures; this property, called self-suppression, is due to saturation of the pressure generator. The resulting input-output functions display the main features of experimental curves. The third-order distortion product in the BM velocity is always more than 25 dB below the primary BM velocity and does not influence the results of the computation; this justifies the use of the quasilinear method.

Acoustic Impedance Tests↗