Showing papers by "Tunis University published in 2003"

Genetic relationship in the 'Bacillus cereus group' by rep-PCR fingerprinting and sequencing of a Bacillus anthracis-specific rep-PCR fragment.

Abstract: Aims: To evaluate the genetic relationship in the Bacillus cereus group by rep-PCR fingerprinting. Methods and Results: A collection of 112 strains of the six species of the B. cereus group was analysed by rep-PCR fingerprinting using the BOX-A1R primer. A relative genetic distinctness was found among the species. Cluster analysis of the rep-PCR patterns showed clusters of B. thuringiensis strains quite separate from those of B. cereus strains. The B. anthracis strains represented an independent lineage in a B. cereus cluster. The B. mycoides, B. pseudomycoides and B. weihenstephanensis strains were clustered into three groups at some distance from the other species. Comparison of sequences of AC-390, a typical B. anthracis rep-PCR fragment, from 27 strains of B. anthracis, B. cereus, B. thuringiensis and B. weihenstephanensis, representative of different clusters identified by rep-PCR fingerprinting, confirmed that B. anthracis diverges from its related species. Conclusions: The genetic relationship deduced from the rep-PCR patterns indicates a relatively clear separation of the six species, suggesting that they can indeed be considered as separate units. Significance and Impact of the Study: rep-PCR fingerprinting can make a contribution in the clarification of the genetic relationships between the species of the B. cereus group.

Thermal optima of photosynthetic functions and thermostability of photochemistry in cork oak seedlings.

Abstract: Temperature effects on photosynthesis were studied in seedlings of evergreen Mediterranean cork oak (Quercus suber L.). Responses to changes in temperature and the temperature optima of maximal carboxylation rate (V(cmax)) and maximal light-driven electron flux (J(max)) were estimated from gas exchange measurements and a leaf-level photosynthesis model. The estimated temperature optima were approximately 34 and 33 degrees C for V(cmax) and J(max), respectively, which fall within the lower range of temperature optima previously observed in deciduous tree species. The thermostability of the photosynthetic apparatus was estimated according to the temperature at which basal chlorophyll a fluorescence begins to increase (T(c)). The T(c) was highly variable, increasing from 42 to 51 degrees C when ambient temperature rose from 10 to 40 degrees C, and increasing from 44 to 54 degrees C with decreasing soil water availability while net CO(2) assimilation rate dropped to almost zero. When a heat shock was imposed, an additional small increase in T(c) was observed in drought-stressed and control seedlings. Maximal T(c) values following heat shock were about 56 degrees C, which, to our knowledge, are the highest values that have been observed in tree species. In conclusion, the intrinsic temperature responses of cork oak did not differ from those of other species (similar T(c) under ambient temperature and water availability, and relatively low thermal optima for photosynthetic capacity in seedlings grown at cool temperatures). However, the large ability of cork oak to acclimate to drought and elevated temperature may be an important factor in the tolerance of this evergreen Mediterranean species to summer drought and high temperatures.

Morphological and Phylogenetic Studies of Marine Myxobolus spp. from Mullet in Ichkeul Lake, Tunisia

Abstract: Six species of the genus Myxobolus (Myxozoa) from the marine environment were collected from two species of mullet (Mugil cephalus and Liza ramada) in Ichkeul Lake, Tunisia. Four of these species were described previously (Myxobolus bizerti, Myxobolus ichkeulensis, Myxobolus spinacurvatura, and Myxobolus episquamalis) and two (Myxobolus exiguus and Myxobolus muelleri) are redescribed. The small subunit ribosomal (18S rDNA) sequences of these six myxozoans were obtained and compared to traditional criteria used in the identification and taxonomy of myxozoan species (such as spore morphology, host specificity, and tissue tropism). A distance analysis of 1,600-1,700 base pairs of the 18S rDNA of the six species indicates that they formed a monophyletic group separate from Myxobolus spp. found as parasites of freshwater fish. The sequence analyses also confirm that these morphologically different Myxobolus spp. that infect mullet represent different species. Lastly, M. exiguus and M. muelleri, which were found in the same host, exhibit clear differences in spore morphology but sequencing of two different regions of the 18S rDNA show they are closely related. These results demonstrate the utility of DNA sequence data in providing more detailed relationships among the Myxobolus species based upon existing morphological taxonomic approaches. We suggest that future descriptions of Myxobolus spp. provide both careful spore descriptions as part of the traditional criteria but also 18S rDNA sequence data that will aid in situations where morphological details may be absent or misleading.

EELS study of interfaces in magnetoresistive LSMO/STO/LSMO tunnel junctions

Abstract: A magnetic tunnel junction consists of two ferromagnetic conducting electrodes separated by an insulating thin layer The performance of such a system strikingly depends on the last conducting atomic layers in contact with the insulator Consequently, the present paper reports a nanoscale electron energy loss spectroscopy (EELS) study, which has been performed across a couple of La066Sr033MnO3,/SrTiO3/La066Sr033MnO3 tunnel junctions with different barrier thickness es (15 nm and 5 nm respectively) It aims at determining not only the chemical composition in the interface areas, but also the effect of the neighbouring atoms on their electronic structure Using recent improvements in the STEM-EELS data acquisition and processing techniques (systematic use of spectrum-line and spectrum-image modes, multivariate statistical analysis, 2D energy deconvolution schemes, etc), the local chemical information is better extracted with shorter acquisition times, while the large increase of the data set contributes to validate the results Within the accuracy level of these measurements, the elemental composition of the different phases remains stable up to the interfaces with no evidence of extra doping Furthermore, weak changes on the Mn-2p edge fine structures (weak shift to lower energy loss values and extra splitting on the top of the Mn L3 line are observed on all the interfaces They are interpreted as a consequence of a slight reduction of the local Mn valence likely accompanied by a strain induced change in local symmetry The discussion is focussed on all spectral changes identified at a (sub)nanometer scale and their potential effects on the degradation of magnetic and transport properties measured, close to room temperature, at a macroscopic level

Nitrogen Metabolism in Tomato Plants Under Cadmium Stress

Abstract: Young seedlings of tomato (Lycopersicon esculentum Mill.) grown on nitric medium and treated with different cadmium (Cd) concentrations (0–50 µM) were used. Results obtained show that Cd remains predominantly located in the roots, which then seem to play the role of trap‐organs. Increasing Cd concentration in the medium leads particularly to a decrease in NO3 − accumulation, together with decrease in the activities of nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS). On the other hand, stimulations were observed for ammonium accumulation, anabolitic activity of glutamate dehydrogenase (GDH) and protease activity. Contrarily, decreases were obtained for soluble protein contents and catabolitic activity of GDH. These results were discussed in relation to the hypothesis attributing to the mitochondrial enzyme NADH‐dependent GDH, an important role in the plant defense processes, in maintaining the glutamate pool in conditions where the need for NH4 + detoxification is in...

Genetic diversity in Tunisian perennial ryegrass revealed by ISSR markers

Abstract: The genetic diversity in Tunisian perennial ryegrass (Lolium perenne) was examined by the help of inter-simple sequence repeats (ISSR). Starting from eighteen accessions, a large number of polymorphic ISSR markers were currently generated using appropriate primers (a total of 136, which average of 12.6 polymorphic bands/primer). These markers were considered to estimate the genetic distance among accessions and to draw phylogenetic trees. Our data provide evidence of a high degree of genetic diversity in Tunisian ryegrass. In addition, both cultivars and wild types present a high degree of divergence suggesting a complex domestication process in this crop. Moreover, spontaneous populations of Tunisian ryegrass have been identified as important ecotypes that are suitable in selection programs to improve grasslands.

An optical soliton pair among absorbing three-level atoms

Abstract: The motion of a pair of solitons propagating through an absorbing three-level system in the lambda configuration is analysed. In the course of this analytic analysis, the existence condition for solitary wave pairs is derived. This condition allows for two possible velocities of the soliton pair.

Molecular Epidemiology and Characterization of Plasmid-Encoded β-Lactamases Produced by Tunisian Clinical Isolates of Salmonella enterica Serotype Mbandaka Resistant to Broad-Spectrum Cephalosporins

Abstract: We studied 31 clinical isolates of Salmonella enterica serotype Mbandaka resistant to broad-spectrum cephalosporins and recovered in Tunisia over a 5-year period. The transferability of this resistance was demonstrated by conjugation experiments. Thirty of the 31 isolates were positive in the double-disk synergy test. By isoelectric focusing analysis, all of the isolates were found to produce a band of β-lactamase activity with a pI of 5.9. Three of these isolates produced an additional band with a pI of 7.6. PCR and DNA sequencing identified these β-lactamases as TEM-4 and SHV-2a, respectively. The remaining isolate, highly resistant to ceftazidime but susceptible to cefepime, produced a β-lactamase that focused at pI 7.8. No synergy was detected by the double-disk synergy test. Sequence analysis of the bla gene amplified by PCR showed that the plasmid-mediated AmpC-type enzyme was ACC-1a. Fingerprinting analysis by repetitive-element PCR and enterobacterial repeat intergenic consensus-PCR suggested that 29 of the 31 Salmonella serotype Mbandaka isolates belonged to the same clonal population.

Directed Evidential Networks with Conditional Belief Functions

Abstract: The main question addressed in this paper is how to represent belief functions independencies by graphical model. Directed evidential networks (DEVNs) with conditional belief functions are then proposed. These networks are directed acyclic graphs (DAGs) similar to Bayesian networks but instead of using probability functions, we use belief functions. Directed evidential network with conditional belief functions has the advantage of providing an appropriate representation of the knowledge that can be produced as conditional relationships.

Short-range order in liquid N-methylformamide HCONHCH3, as studied by x-ray scattering and density functional theory calculations

Abstract: The local order in liquid N-methylformamide has been studied by using x-ray diffraction and a density functional theory (DFT). Experimental data were analyzed to yield the total structure function SM(q) and the intermolecular pair correlation function gL(r). DFT calculations, using, namely, the standard TZV basis set, were performed to study the relative stability of the two possible isomers (cis and trans) and to examine some possible intermolecular arrangements in the liquid state. X-ray measurements can be interpreted in term of cyclic trimers of cis form molecules where each monomer can establish two N–H⋯O hydrogen bonds that are described.

Synthesis, structure and lithium intercalation reaction in LiMoVO6 brannerite-type materials

Abstract: The brannerite-type vanadate LiMoVO6 was prepared by different synthetic methods including the solid-state reaction of oxides and wet chemistry techniques in aqueous solution. The physicochemical and electrochemical properties of the LiMoVO6 products were extensively investigated. Thermal (TG-DTA) analyses and XRD data showed that powders with a quasi-layered structure (C2/m space group) were obtained at temperatures below 400 °C by the acidification reaction of aqueous solutions. The local structure was characterized by Raman scattering, FTIR and 7Li solid state MAS NMR. The discharge–charge profiles were evaluated in rechargeable Li/LiClO4 + PC/LiMoVO6 cells and chemical diffusion coefficients of Li+ ions were estimated. Lithium insertion in Li1 + xMoVO6 positive electrodes fired at 550 °C exhibited an initial capacity retention of ≈200 mA h g−1 which declines rapidly upon charge–discharge cycling. Upon cycling, incremental capacity and Raman features show the instability of the brannerite lattice with the appearance of the Li2MoO4 product.

Fukutin-related protein gene mutated in the original kindred limb-girdle MD 2I

Abstract: The authors mapped an autosomal recessive form of limb-girdle MD on chromosome 19q13.3 (LGMD2I), further narrowed down the candidate region to 1.1 Mb, and identified one new homozygous mutation in the fukutin-related protein (FKRP) gene on patients of the original Tunisian family. Immunohistochemical and immunoblot analysis showed abnormal expression of alpha-dystroglycan and laminin-alpha2 supporting the hypothesis that FKRP has a role in the interaction between the extracellular matrix components.

Stochastic Production Frontier and Technical Inefficiency: A Sensitivity Analysis

Abstract: The present paper focuses attention on the sensitivity of technical inefficiency to most commonly used one‐sided distributions of the inefficiency error term, namely the truncated normal, the half‐normal, and the exponential distributions. A generalized version of the half‐normal, which does not embody the zero‐mean restriction, is also explored. For each distribution, the likelihood function and the counterpart of the estimator of technical efficiency are explicitly stated (Jondrow, J., Lovell, C. A. K., Materov, I. S., Schmidt, P. ([1982]), On estimation of technical inefficiency in the stochastic frontier production function model, J. Econometrics19:233–238). Based on our panel data set, related to Tunisian manufacturing firms over the period 1983–1993, formal tests lead to a strong rejection of the zero‐mean restriction embodied in the half normal distribution. Our main conclusion is that the degree of measured inefficiency is very sensitive to the postulated assumptions about the distributio...

Ferroelectric versus relaxor behaviour in Na0.5Bi4.5Ti4O15-BaBi4Ti4O15 solid solutions

Abstract: Sodium-bismuth titanate Na0.5Bi4.5Ti4O15 (NBT) and barium bismuth titanate BaBi4Ti4O15 (BBT) and their solid solutions were prepared by solid state reaction of the constituent oxides. The room temperature structural characteristics were determined by X-ray diffraction and the dielectric properties investigated in a wide range of temperatures (20–750°C) and frequencies (1 kHz–1 MHz). As the barium content increases, the unit cell change from orthorhombic to pseudo-tetragonal, the paraelectric to ferroelectric phase transition decreases from 655°C to 400–420°C and a relaxor-like behaviour is observed for high barium contents. Depending on the composition, dielectric data were fitted using both classical (low Ba) and modified Curie-Weiss law for relaxors (high Ba).

Estimates on the Green Function and Existence of Positive Solutions of Nonlinear Singular Elliptic Equations

Abstract: We establish a 3G-Theorem for the Green's function for an unbounded regular domain D in ℝn(n ≥ 3), with compact boundary. We exploit this result to introduce a new class of potentials K(D) that properly contains the classical Kato class . Next, we study the existence and the uniqueness of a positive continuous solution u in of the following nonlinear singular elliptic problem where φ is a nonnegative Borel measurable function in D × (0, ∞), that belongs to a convex cone which contains, in particular, all functions φ(x, t) = q(x)t-σ, σ ≥ 0 with q ∈ K(D). We give also some estimates on the solution u.

Towards the Verification and Validation of Online Learning Adaptive Systems

Abstract: Online Adaptive Systems in general, and learning neural nets in particular cannot be validated using traditional verification and validation techniques, because they evolve over time, and past learning data influences their behavior. In this paper we discuss a framework for reasoning about online adaptive systems, and see how this framework can be used to perform V\&V on such systems.

Positive Solutions for a Singular Nonlinear Problem on a Bounded Domain in R 2

Abstract: For a bounded regular Jordan domain Ω in R2, we introduce and study a new class of functions K(Ω) related on its Green function G. We exploit the properties of this class to prove the existence and the uniqueness of a positive solution for the singular nonlinear elliptic equation Δu+ϕ(x,u)=0, in D′(Ω), with u=0 on ∂Ω and u∈C―(Ω), where ϕ is a nonnegative Borel measurable function in Ω×(0,∞) that belongs to a convex cone which contains, in particular, all functions ϕ(x,t)=q(x)t−γ,γ>0 with nonnegative functions q∈K(Ω). Some estimates on the solution are also given.

Structural changes induced by NaCl in companion and transfer cells of Medicago sativa blades.

Abstract: Medicago sativa var. Gabes is a perennial glycophyte that develops new shoots even in high salinity (150 mM NaCl). In the upper exporting leaves, K(+) is high and Na(+) is low by comparison with the lower leaves, where Na(+) accumulation induces chlorosis after 4 weeks of NaCl treatment. By secondary ion mass spectroscopy, a low Na(+)/K(+) ratio was detected in the phloem complex of blade veins in these lower leaves. By transmission electron microscopy, the ultrastructural features were observed in the phloem complex. In the upper leaves of both control and NaCl-treated plants, companion cells in minor veins were found to be transfer cells. These cells may well be involved in the intravenous recycling of ions and in Na(+) flowing out of exporting leaves. Under the effect of NaCl, companion cells in the main veins develop transfer cell features, which may favor the rate of assimilate transport from exporting leaves toward meristems, allowing the positive balance necessary for the survival in salt conditions. These features no longer assist the lower leaves when transfer cells are necrotized in both minor and main veins of NaCl-treated plants. As transfer cells are the only degenerating phloem constituent, our observations emphasize their role in controlling nutrient (in particular, Na(+)) fluxes associated with the stress response.

Acidic properties of a clay prepared from the reaction of zirconyl chloride solution containing sulfate ions with montmorillonite

Abstract: Sulfated zirconium oxyhydroxide clays were prepared by adding ammonium sulfate to the intercalation solution. The main parameter controlled in this study was the SO4:Zr ratio in solution. The characterization of the catalysts, their acidic properties and catalytic activities were examined by N-2-BET adsorption, chemical analysis, adsorption-desorption of basic molecules and conversion of n-hexane and isopropanol. Two different types of SO4-Zr polycation binding were proposed and related with the acidic properties of these solids. To enhance the acidity of the sulfated zirconium oxyhydroxide clay, the SO4:Zr molar ratio must be higher than 0.125. The higher this ratio, the higher is the activity of these solids. It appears that polymerised entities are more active than individual sulfated polycations. Isopropanol dehydration to propene as well as isomerization of n-hexane seem to be related to the number and strength of Bronsted acid sites. (C) 2003 Elsevier B.V. All rights reserved.

On the existence of positive solutions for a class of semilinear elliptic equations

Abstract: We prove some existence and nonexistence results for the semilinear elliptic equation Δu = q(x)f(u) on Ω ⊆ Rn (n ≥ 2) where u is required to blow up on the boundary of Ω and f is a nonnegative function which is assumed to be Lipschitz continuous and bounded away from zero on each interval [e, ∞) and have at worst linear growthIn particular, we extend some results already obtained in the case where f(u)=uγ, 0 > γ ≤ 1

Structural and electrochemical properties of LiCoO 2 and LiAl y Co 1−y O 2 (y=0.1 and 0.2) oxides: A comparative study of electrodes prepared by the citrate precursor route

Abstract: We present the characterization and electrode behavior of LiCoO2 and Al-doped LiAlyCo1−yO2 (y=0.1 and 0.2) oxides prepared by wet-chemical method from the citrate precursor route. We study the phase evolution as a function of the aluminum substitution and the modification on the intercalation and deintercalation of Li ions. Characterization methods include XRD, SEM, and FTIR. X-ray diffraction patterns show that samples belong to the LiCoO2-LiAlO2 solid solution and have the layered α-NaFeO2 structure. FT-infrared vibrational spectroscopy indicates the slight modification in the local structure related to the short-range environment of oxygen coordination around the cations in oxide lattices. The frequencies and relative intensities of the bands are sensitive to the covalency of the (Al, Co)O2 slabs. The overall electrochemical capacity of the LiAlyCo1−yO2 oxides have been reduced due to the sp metal substitution, however, a more stable charge-discharge cycling performances have been observed when electrodes are charged to 4.3 V as compared to the performances of the native oxide. Differences and similarities between LiCoO2 and Al-substituted oxides are discussed therefrom.