PubMed Health⌕ Search

Biomedical subjects

M L Jiménez

Publications and source records attributed to M L Jiménez.

At least 19 recordsLinked to original sources
PubMed2006

Optical and electro-optical derivation of the pretransitional behavior of orientational and shear viscosities in the isotropic phase of liquid crystals.

The pretransitional equilibrium properties of isotropic liquids in the proximity of the isotropic-nematic phase transition are well known and successfully modeled. Much less is known about the dynamic behavior, and in particular about the pretransitional viscosity. In this work we combine two techniques [dynamic light scattering (DLS) and electric birefringence spectroscopy (EBS)] offering complementary insights into both static and dynamic pretransitional behavior of the homologous nCB family (n-alkyl cyanobiphenyl). EBS explores the single molecule flipping dynamics retarded by a paranematic potential barrier and enables extracting the associated transport coefficient, which is found to be of Arrhenius type in the whole temperature range explored. DLS reflects the collective dynamics of correlated domains and depends on the viscous damping of the orientational order. Such a viscosity displays Arrhenius behavior only sufficiently far from the transition temperature, with deviations growing as a power law of the appropriate reduced temperature with exponents around 0.1.

Journal Article↗
PubMed2005

Influence of cell-model boundary conditions on the conductivity and electrophoretic mobility of concentrated suspensions.

In the last few years, different theoretical models and analytical approximations have been developed addressing the problem of the electrical conductivity of a concentrated colloidal suspension. Most of them are based on the cell model concept, and coincide in using Kuwabara's hydrodynamic boundary conditions, but there are different possible approaches to the electrostatic boundary conditions. We will call them Levine-Neale's (LN, they are Neumann type, that is they specify the gradient of the electrical potential at the boundary), and Shilov-Zharkikh's (SZ, Dirichlet type). The important point in our paper is that we show by direct numerical calculation that both approaches lead to identical evaluations of the conductivity of the suspensions if each of them is associated to its corresponding evaluation of the macroscopic electric field. The same agreement between the two calculations is reached for the case of electrophoretic mobility. Interestingly, there is no way to reach such identity if two possible choices are considered for the boundary conditions imposed to the field-induced perturbations in ionic concentrations on the cell boundary (r = b), deltan(i) (r = b). It is demonstrated that the conditions deltan(i)(b) = 0 lead to consistently larger conductivities and mobilities. A qualitative explanation is offered to this fact, based on the plausibility of counter-ion diffusion fluxes favoring both the electrical conduction and the motion of the particles.

Colloids↗
PubMed2005

Determination of stagnant layer conductivity in polystyrene suspensions: temperature effects.

In the classical theory of electrokinetic phenomena, it is admitted that the whole electrokinetic behavior of any colloidal system is fully determined by the zeta potential, zeta, of the interface. However, both experimental data and theoretical models have shown that this is an incomplete picture, as ions in the stagnant layer (the region between the solid surface and the slip plane--the plane where the equilibrium potential equals zeta) may respond to the field. In this paper, we aim at the evaluation of this contribution by the estimation of both K(SL)(sigma) (the surface conductivity of the stagnant layer) and K(d)(sigma) (the conductivity associated with the diffuse layer). This will be done by measuring the high-frequency dielectric dispersion (HFDD) in polystyrene suspensions; here "high-frequency" means the frequency interval where Maxwell-Wagner-O'Konski relaxation takes place (typically at MHz frequencies). Prior to any conclusions, a treatment of electrode polarization effects in the measurements was needed: we used two methods, and both led to similar results. Simulating the existence of surface conductivity by bulk conductivity, we reached the conclusion that no consistent explanation can be given for our HFDD and additional electrophoresis data based on the existence of diffuse-layer conductivity alone. We thus show how K(SL)(sigma) can be estimated and demonstrate that it can be explained by an ionic mobility very close to that characteristic of ions in the bulk solution. Such mobility, and hence also the values of K(SL)(sigma), increases with temperature as expected on simple physical grounds.

Journal Article↗
PubMed2002

Analysis of the dielectric permittivity of suspensions by means of the logarithmic derivative of its real part.

Measurement of the dielectric permittivity of colloidal suspensions in the kilohertz frequency range (the so-called low-frequency dielectric dispersion) is a promising tool for the electrokinetic characterization of colloids. However, this technique is less used than would be desirable because of the difficulties associated with the measurements, the most important of which is the electrode polarization (EP). Recently (M. Wübbenhorst and J. Van Turnhout, Dielectrics Newsl. November (2000)) a method was proposed that appears capable of separating the unwanted electrode effects from the double-layer relaxation that we are interested in. The method, based on the logarithmic derivative of raw epsilon'(omega) data (epsilon'(omega) is the real part of the permittivity of the suspension for a frequency omega of the applied AC field), is first checked against the well-known theory of the AC permittivity of colloidal suspensions developed by DeLacey and White (E. H. B. DeLacey and L. R. White, J. Chem. Soc. Faraday Trans. 277, 2007 (1981)). We show that the derivative epsilon''(D)(omega)=-(pi/2)(partial differential epsilon'/partial differential ln omega) gives an excellent representation of the true imaginary part of the permittivity, epsilon''(omega). The technique is then applied to experimental data of the dielectric constant of polystyrene and ethylcellulose suspensions. We found that epsilon''(D) displays two separated behaviors when plotted against log omega in the frequency range 100 Hz-1 MHz: a monotonous decrease (associated with EP) followed by an absorption peak (associated with the double-layer relaxation, or alpha-relaxation). Interestingly, they are separated enough to make it possible to easily find the characteristic frequency of the alpha-relaxation. Fitting a relaxation function to epsilon''(D)(omega) after eliminating the part due to EP, we could calculate the real part epsilon'(omega) and compare it to the DeLacey and White (DW) theoretical predictions. A significantly better agreement between DW calculations and experimental epsilon'(omega) data is obtained when the logarithmic derivative method is used, as compared to the classical electrode-separation techniques.

Journal Article↗
PubMed2002

Aging effects in the electrokinetics of colloidal iron oxides.

We analyze in this contribution the effect of aging on the electrokinetic properties of magnetite (Fe(3)O(4)) and hematite (alpha-Fe(2)O(3)). In both cases, high-purity commercial samples and monodisperse synthetic particles were studied. Commercial magnetite showed a rather erratic dependence of its electrophoretic mobility u(e) with the concentration of NaCl. Furthermore, sufficient concentrations of the latter were able to change the sign of the mobility. When KNO(3) solutions were used, although no such change was observed, no clear effect of [KNO(3)] on the mobility was found, and, in addition, an intense aging effect was detected, as the mobility became increasingly positive in suspensions that were stored over 1 day. The picture was radically different with synthetic magnetite spheres, as the expected overall decrease of u(e) with either NaCl or KNO(3) concentration was measured. However, also in this case the aging effect was clearly observed: u(e) tended in this case to more negative values upon suspension storage, and a steady value of the mobility was reached only after 5 days in NaCl (and even longer in KNO(3) solutions). Because of the crystal structure similarities between magnetite and maghemite (gamma-Fe(2)O(3)), it has been shown that the final step of magnetite oxidation is maghemite. This is confirmed in the present study, as the mobility-pH trends of magnetite progressively approach those of maghemite after about 7 days of storage. Since hematite is chemically more stable than magnetite, our study focused in this case on the comparison between commercial and synthetic particles. The former showed a negative mobility at pH 5.5 under all conditions, suggesting an isoelectric point well below the value accepted for hematite (>/=7). The effect of aging on commercial samples was again very significant, as u(e) decreased in absolute value, apparently without limit as the time since preparation was longer. In contrast, synthetic hematite showed a more predictable dependence on ionic strength, and more limited aging effects, as u(e) reached equilibrium values after around 5 days in NaCl; longer times were required in KNO(3) solutions.

Journal Article↗
PubMed2000

Comparison of three methods for identification of Streptococcus milleri group isolates to species level.

A collection of 180 clinical isolates of Streptococcus milleri group were identified to species level using two phenotypic methods (a commercial system and the reference method based on differential phenotypic reactions) and a genotypic method (hybridisation of the 16S rRNA gene with species-specific probes) in order to evaluate the performance of the respective methods. A high level of agreement (80%) was observed between the results of the reference method and the genotypic method. The highest level of agreement was found for the species Streptococcus anginosus (83%), a high level of agreement (76%) also being achieved for Streptococcus constellatus and Streptococcus intermedius. The sensitivity of the commercial system compared to the genotypic method was 76% overall, but it was low (57.5%) for Streptococcus intermedius. Twenty-five strains belonged to the recently described CI strains.

Bacterial Typing Techniques↗
PubMed1999

Five-year analysis of antimicrobial susceptibility of the Streptococcus milleri group.

Susceptibility to 17 antibiotics was studied in 180 strains of the Streptococcus milleri group (88 Streptococcus anginosus, 63 Streptococcus constellatus, and 29 Streptococcus intermedius) isolated over a 5-year period. Minimum inhibitory concentrations of penicillin were in the intermediate range for 5.6% of the strains. Resistance to erythromycin and clindamycin was found in 17.1% and 16.6% of the isolates, respectively. A steady increase in the susceptibility to ciprofloxacin was observed over the study period. Imipenem was the most active beta-lactam agent tested. Glycopeptide antibiotics showed excellent activity. Only slight differences between the three species were found in terms of antibiotic susceptibility. Intermediate resistance to penicillin is appearing among the Streptococcus milleri group in our area; consequently, care must be taken when choosing a macrolide for the management of infections caused by these microorganisms.

Anti-Bacterial Agents↗
PubMed1998

[Phenotypic characterization of erythromycin resistance in strains of the genus Streptococcus isolated from clinical specimens].

There are presently two recognized mechanisms of resistance to macrolide antibiotics in species of the genus Streptococcus: target modification and efflux. Target modification occurs in the ribosomes through the production of a ribosomal methylase codified for the erm genes and it is phenotypically known as MLSB resistance. Expression of MLSB resistance can be inducible or constitutive. Active efflux has been recently described in S. pyogenes and S. pneumoniae, and its expression is known as M phenotype. We studied the susceptibility, by determining the MIC using agar dilution, to penicillin, erythromycin and clindamycin, of 295 strains of the genus Streptococcus clinically isolated from 1993 to 1996 (70 S. pyogenes, 81 S. pneumoniae, 37 viridans group, and 107 S. milleri). We also studied the phenotype pattern of erythromycin-clindamycin resistance in the strains resistant to erythromycin by using the double-disc test. We found a 17.2% resistance to erythromycin and a 12.5% resistance to clindamycin. The M phenotype, initially described in S. pyogenes, was also observed in 6.6% of our S. pneumoniae strains, 25% of S. viridans spp., and 4.8% in the S. milleri group.

Anti-Bacterial Agents↗
PubMed1996

First record of a louse fly, Stilbometopa impressa (Bigot), and new host for Microlynchia pusilla (Speiser) (Hippoboscidae) from the Cape Region, Baja California Sur, México.

Nine of thirty California quail (Callipepla californica achrustera) captured in autumn of 1992, 17 km west of La Paz, Baja California Sur, México, were parasitized by louse flies. We identified eight Microlynchia pusilla and three Stilbometopa impressa from 30 quails in the ratio of 2.75:1. These are the first records of S. impressa for Cape Region and the first time either fly has been reported from the California quail in Baja California Sur.

Animals↗
PubMed1996

[Imipenem combined with teicoplanin or vancomycin in the initial empirical treatment of febrile neutropenia. Analysis of the primary response and of a global sequential strategy in 126 episodes].

The results of empiric antibiotic therapy in 126 episodes of febrile neutropenia in patients with hematologic neoplasms postchemotherapy and bone marrow transplantation are presented. The main objective of this work was the study of the initial control of infection comparing two glycopeptidic antibiotics: vancomycin and teicoplanin combined with imipenem in first line of empiric therapy. The secondary objective was to analyze the overall control of infection during the complete episode of neutropenia using a sequential empiric antibiotic therapy course which included the addition of amikacin followed by intravenous amphotericin B when fever persisted or recurred without microbiological documentation. Both initial courses (no guidelines), imipenem + vancomycin (arm A) and imipenem + teicoplanin (arm B) resulted in a similar percentage of response at 72 hours, both in episodes of fever of unknown origin (FUO) (55% and 68%, respectively; p = NS) and in those microbiologically documented (54% and 34.5%, p = NS); 58% and 79% of these episodes, respectively, were caused by gram-positive organisms. About 60% of patients in both arm ultimately required the empiric addition of amikacin, with or without amphotericin B, because of persistence or recurrence of fever; the percentage of overall responses in both arm did not differ significantly, both in FUO (70% and 86%, p = NS) and in microbiologically documented episodes (71% and 45%, p = NS). The overall infectious mortality for the whole group was 1.58%. In conclusion, no significant differences were observed in the clinical response or in toxicity between the combination of imipenem with any of the two glycopeptides: vancomycin or teicoplanin, for the initial empiric therapy of febrile neutropenia. The sequential empiric use of amikacin followed by amphotericin B assured an adequate overall control of infection in a group of patients with prolonged severe neutropenia.

Adolescent↗
PubMed1994

[Clinical spectrum of hypoxanthine-guanine phosphoribosyltransferase deficiency: study of 12 cases].

BACKGROUND: The hypoxanthine-guanine phosphoribosyltransferase deficiency (HGPRT) may have two clinical forms: that of the Lesch-Nyhan syndrome (complete HGPRT deficiency) and that of the Kelley-Seegmiller syndrome (partial HGPRT deficiency). The clinical and biochemical features of the HGPRT deficiency are not completely known. METHODS: A series of 12 patients, 8 with the Lesch-Nyhan syndrome and 4 with the Kelley-Seegmiller syndrome are described. The plasma and urine concentrations of hypoxanthine, xanthine and uric acid were compared with those obtained in 20 normal subjects and 41 patients with primary gout. The molecular defect which determines the deficient HGPRT activity was studied in one patient with the Kelley-Seegmiller syndrome. RESULTS: The 8 patients with the Lesch-Nyhan syndrome presented choreoathetosis, corticospinal motor system dysfunction, mental retardation and signs of self mutilation. The neurologic manifestations of the patients with the Kelley-Seegmiller syndrome were very heterogeneous: two patients had psychomotor retardation with spastic movement, one was mentally retarded with generalized dystonia and one patient only had gout with no neurologic manifestations. The erythrocytic HGPRT activity ranged between 0.28 and < 0.01 nmol/h and mg of hemoglobin in all the patients. Plasma and urine purine concentrations were very high, being greater than those in normal subjects and patients with gout (p < 0.01). A mutation was identified in exon 3 (substitution of guanine with thymine) conditioning the substitution of the normal glycine amino acid by valine (HGPRT Madrid) on molecular study. CONCLUSIONS: The hypoxanthine-guanine phosphoribosyltransferase deficiency has a heterogeneous clinical expression. The activity of this enzyme in erythrocytes and the results of the metabolism of the purines do not allow prediction of the severity of the clinical manifestations.

Adult↗
PubMed1994

[Early bacterial infections in 103 patients treated with bone marrow transplantation].

PURPOSE: To analyse bacterial infections in the period immediately following bone-marrow transplantation. PATIENTS AND METHODS: A retrospective study of 174 febrile episodes appearing on 103 patients treated with bone-marrow transplantation in 1990 and 1991 was carried out, special attention being paid to the bacterial infections. RESULTS: Virtually all patients (100/103) had at least one febrile episode, and its infectious character was documented in 54% of the instances. Gram-positive germs were most commonly present, 85% of the bacteria isolated, and coagulase-negative staphylococci, especially St epidermidis, predominated (60%). Different species of streptococci, mostly of the viridans group, were isolated in 22% of the blood cultures attained in the first febrile episodes. The mortality due to infection in the series as a whole was 4.8%. CONCLUSIONS: Infections by gram-positive germs, especially coagulase-negative staphylococci, are commonly found among the patients subjected to bone-marrow transplantation. Increased streptococci infections, mostly of the viridans group, are also appreciated. These facts, along with decreased number of gram-negative infections, must be born in mind when designing initial antimicrobial coverage for these patients.

Adolescent↗
PubMed1994

[Cavitated lung pneumonia due to meningococcus].

Neisseria meningitidis is a common germ of the respiratory pathways which seldom produces pulmonary infection. Some established cases of pneumonia by meningococcus and, exceptionally, of complications like the empyema, have been described. However, just one case of cavitated pneumonia by N. meningitidis has been described in the medical literature. We present a case of pneumonia with cavitation produced by N. meningitidis, which was diagnosed through fibrobronchoscopy with transbronchial biopsy. This observation is of very high clinical interest, because N. meningitidis is a pathogenic agent which is not taken into account for the differential diagnosis of cavitated pneumonias.

Humans↗
PubMed1993

Hereditary nephropathy associated with hyperuricemia and gout.

BACKGROUND: The clinical characteristics of hereditary nephropathy associated with hyperuricemia or gout have not been fully described, and the pathogenetic role of increased serum urate concentration is controversial. METHODS: We examined the clinical characteristics of 14 patients and purine metabolism of seven patients, while they were on a purine-restricted diet, in two families with hereditary nephropathy associated with asymptomatic hyperuricemia or gout. Results of plasma and urinary purine measurements were compared with those obtained in 25 patients with gout and renal insufficiency and in 25 normal subjects. Eight subjects in both families were followed up for a mean of 44 months. Allopurinol was given to all patients and enalapril maleate to hypertensive subjects. RESULTS: All patients had some combination of hyperuricemia, gout, renal insufficiency, arterial hypertension, and reduced kidney size. Decreased glomerular filtration rate was proportional to the decreased renal plasma flow. Renal vascular resistance was markedly increased in the patients with diminished renal plasma flow. All patients with familial nephropathy showed diminished urinary uric acid, hypoxanthine, and xanthine excretion rates. Purine under-excretion was more severe in affected patients with familial nephropathy than in patients with gout and renal insufficiency. Kidney biopsy specimens from three patients with familial nephropathy showed tubulointerstitial lesions and ischemic changes in glomeruli but no uric acid crystals. The kidney uric acid content was normal. Allopurinol treatment normalized serum urate levels, but serum creatinine concentrations increased and creatinine clearance decreased in all patients with familial nephropathy. One patient with gout only at initial evaluation developed renal failure during the follow-up period. CONCLUSIONS: Increased serum urate concentrations in hereditary nephropathy associated with hyperuricemia and gout are due to severe impairment of uric acid excretion. Hyperuricemia does not appear, however, to be of pathogenetic relevance and may be a consequence of a primary disruption of renal hemodynamics.

Adolescent↗
PubMed1991

Female gout. Clinical spectrum and uric acid metabolism.

We reviewed the clinical features and uric acid metabolism in 37 female patients with gout. In 32 female patients (86%), gout was diagnosed after menopause. Among the five premenopausal patients, four had renal insufficiency and one had superactivity of phosphoribosylpyrophosphate synthetase. More than 50% of the female patients had osteoarthritis, hypertension, or renal insufficiency or were treated with diuretics. Comparison with 220 male patients with gout showed that female patients developed gout significantly later, more frequently had associated diseases, and more often were receiving diuretics, whereas significantly more male than female patients had alcoholism. The articular features of gout were similar in both groups. However, the prevalence of tophi was higher and its localization different in female than in male patients. Female patients with gout had a higher mean serum urate concentration and a lower mean urinary uric acid excretion than did male patients with gout. These differences were significant and independent of the effects of age, renal insufficiency, alcoholism, or previous diuretic intake. Renal underexcretion of uric acid appears to be more severe in female than in male patients with gout.

Adult↗