Isolation and in-vitro and in-vivo characterisation of a mutant of Pseudomonas aeruginosa PAO1 that exhibited a reduced postantibiotic effect in response to imipenem

The postantibiotic effect (PAE) is the persistent inhibition of bacterial growth after a brief exposure to an antibiotic. Most β-lactams do not induce a PAE for Gram-negative bacteria, but PAEs have been reported for carbapenems and penems. This study investigated the effect of sequential doses of imipenem on the PAE for Pseudomonas aeruginosa and Escherichia coli cultures in a chemostat. The PAE for the bacterial population did not change even after six successive exposures to imipenem. Nevertheless, screening of colonies isolated after repeated drug exposure identified a single P. aeruginosa mutant whose imipenem PAE was shortened, although the MIC was unchanged. The PAEs for the parent and mutant were studied in vitro in batch culture by monitoring: (i) viable counts; (ii) electrical impedance of the culture medium; (iii) incorporation of radiolabelled N-acetyl-D-glucosamine and (iv) cell volume changes. PAEs for the parent and mutant were found to be significantly different by all in-vitro methods used. Moreover, the median cell volume in antibiotic-exposed cultures remained much smaller and less heterogeneous than in the control cultures, even though both cultures were growing at the same rate. The mutant was found to have a reduced expression of a 52 kDa outer membrane protein. These observations suggest that factors in addition to suppression of bacterial growth should be considered when studying the PAE. The PAEs of imipenem for the parent and mutant were studied in a thigh infection model in leucopenic mice. Similar PAEs were observed in vivo for both parent and mutant in one experiment and no PAEs for either organism were found in a second experiment. This study showed that although the PAE is a stable in-vitro phenomenon, the lack of correlation between the in-vitro and in-vivo results warrants caution in attributing clinical significance to the PAE of imipene

Methodology for studying strain inhomogeneities in polycrystalline thin films during in situ thermal loading using coherent x-ray diffraction

International audienceCoherent x-ray diffraction is used to investigate the mechanical properties of a single grain within a polycrystalline thin film in situ during a thermal cycle. Both the experimental approach and finite element simulation are described. Coherent diffraction from a single grain has been monitored in situ at different temperatures. This experiment offers unique perspectives for the study of the mechanical properties of nano-objects

Native amine dehydrogenases can catalyze the direct reduction of carbonyl compounds to alcohols in the absence of ammonia

Native amine dehydrogenases (nat-AmDHs) catalyze the (S)-stereoselective reductive amination of various ketones and aldehydes in the presence of high concentrations of ammonia. Based on the structure of CfusAmDH from Cystobacter fuscus complexed with Nicotinamide adenine dinucleotide phosphate (NADP+) and cyclohexylamine, we previously hypothesized a mechanism involving the attack at the electrophilic carbon of the carbonyl by ammonia followed by delivery of the hydride from the reduced nicotinamide cofactor on the re-face of the prochiral ketone. The direct reduction of carbonyl substrates into the corresponding alcohols requires a similar active site architecture and was previously reported as a minor side reaction of some native amine dehydrogenases and variants. Here we describe the ketoreductase (KRED) activity of a set of native amine dehydrogenases and variants, which proved to be significant in the absence of ammonia in the reaction medium but negligible in its presence. Conducting this study on a large set of substrates revealed the heterogeneity of this secondary ketoreductase activity, which was dependent upon the enzyme/substrate pairs considered. In silico docking experiments permitted the identification of some relationships between ketoreductase activity and the structural features of the enzymes. Kinetic studies of MsmeAmDH highlighted the superior performance of this native amine dehydrogenases as a ketoreductase but also its very low activity towards the reverse reaction of alcohol oxidation

«La relation de limitation et d’exception dans le français d’aujourd’hui : excepté, sauf et hormis comme pivots d’une relation algébrique »

L’analyse des emplois prépositionnels et des emplois conjonctifs d’ “excepté”, de “sauf” et d’ “hormis” permet d’envisager les trois prépositions/conjonctions comme le pivot d’un binôme, comme la plaque tournante d’une structure bipolaire. Placées au milieu du binôme, ces prépositions sont forcées par leur sémantisme originaire dûment métaphorisé de jouer le rôle de marqueurs d’inconséquence systématique entre l’élément se trouvant à leur gauche et celui qui se trouve à leur droite. L’opposition qui surgit entre les deux éléments n’est donc pas une incompatibilité naturelle, intrinsèque, mais extrinsèque, induite. Dans la plupart des cas (emplois limitatifs), cette opposition prend la forme d’un rapport entre une « classe » et le « membre (soustrait) de la classe », ou bien entre un « tout » et une « partie » ; dans d’autres (emplois exceptifs), cette opposition se manifeste au contraire comme une attaque de front portée par un « tout » à un autre « tout ». De plus, l’inconséquence induite mise en place par la préposition/conjonction paraît, en principe, tout à fait insurmontable. Dans l’assertion « les écureuils vivent partout, sauf en Australie » (que l’on peut expliciter par « Les écureuils vivent partout, sauf [qu’ils ne vivent pas] en Australie »), la préposition semble en effet capable d’impliquer le prédicat principal avec signe inverti, et de bâtir sur une telle implication une sorte de sous énoncé qui, à la rigueur, est totalement inconséquent avec celui qui le précède (si « les écureuils ne vivent pas en Australie », le fait qu’ils « vivent partout » est faux). Néanmoins, l’analyse montre qu’alors que certaines de ces oppositions peuvent enfin être dépassées, d’autres ne le peuvent pas. C’est, respectivement, le cas des relations limitatives et des relations exceptives. La relation limitative, impliquant le rapport « tout » - « partie », permet de résoudre le conflit dans les termes d’une somme algébrique entre deux sous énoncés pourvus de différent poids informatif et de signe contraire. Les valeurs numériques des termes de la somme étant déséquilibrées, le résultat est toujours autre que zéro. La relation exceptive, au contraire, qui n’implique pas le rapport « tout » - « partie », n’est pas capable de résoudre le conflit entre deux sous énoncés pourvus du même poids informatif et en même temps de signe contraire : les valeurs numériques des termes de la somme étant symétriques et égales, le résultat sera toujours équivalent à zéro

Mathematical modelling of slow pyrolysis of a particle of treated wood waste

International audienc

Déshydratation des boues activées en cellule de filtration-compression : écart par rapport à la théorie classique

International audienceSludge dewatering is classically represented by a (t/V, V) curve with FCC tests, allowing to determine the Specific Resistance of Filtration (SRF) by using Ruth's equation. Many tests in FCC with varying initial sludge concentrations as well as volume suspension dewatered or pressure showed that the SRF was not constant along filtration phase. It implied that filtration theory is not suitable for highly compressible material. In expression phase, sludge dewatering is function of the applied pressure and the historical material. Thus, porosity throughout the cake is dependent of the pressure and some relaxation times.La déshydratation des boues est classiquement représentée par une courbe dans un diagramme (t/V, V) en utilisant des tests en cellule de filtration-compression, ce qui permet de déterminer la résistance spécifique de filtration à l'aide de l'équation de Ruth. En faisant varier la concentration ou le volume utilisés ou les pressions appliquées, nous montrons que la résistance spécifique de filtration n'est pas constante pendant la phase de filtration. Ce résultat implique que la théorie de la filtraiton n'est pas applicable aux matériaux compressibles tels que les boues. De plus, pendant la phase de compression, la cinétique de déshydratation dépend à la fois de la pression appliquée et de l'histoire du matériau. Aussi, la porosité dépend à la fois de la pression mais aussi des caractéristiques rhéologiques, notamment les temps de relaxation

Activated sludge dewatering in a filtration compression cell: deviations in comparison to the classical theory

International audienc

Effet des cations sur la déshydratation des boues résiduaires activées

International audienceEven after mechanical dewatering, the residual water within activated sludge remains high. Due to its complex structure, this material is usually extremely compressible and known to be difficult to dewater. The ability of sludge to dewater depends on its biological nature, its composition and also the type of treatment it comes from. Indeed, changes in ionic strength and in ionic composition of sludge affect the stability of structural properties of activated sludge and thus the dewatering efficiency. Some authors have highlighted correlations between the ionic strength of sludge and the ability to dewater evaluated by parameters such as CST (Capillary Suction Time) or SRF (Specific Resistance of Filtration) [1-3]. However, those parameters are often limited to fully characterise sludge dewatering. It is usually accepted that the best lab-scale device to investigate sludge behaviour during mechanical dewatering is the Filtration-Compression Cell (FCC). It enables to describe the two successive steps of dewatering: filtration and expression respectively. The purpose of this work was to study the effect of the type and the content of cations on the flocs structure and on the ability of the sludge to be dewatered. In parallel, physicochemical parameters (turbidity, surface charges content, hydrophobicity, EPS content etc.) were analysed to better understand the sludge behaviour during dewatering. Experimental results have shown that the highest the salinity, the slowest the dewatering kinetics but the driest the sludge is. Moreover, the turbidity of the water phase and the surface charges increase with the cations content. These observations can be explained by a change of ionic strength which tends to reduce flocs cohesion and thus lead to flocs disintegration and fine particles release. Due to the presence of fine particles into the bulk water the filter medium and the filter cake tend to clog, which limits the filtrate flowrate namely during the filtration stage. It seems that organic macromolecules such as EPS were also released from the flocs, leading to a decrease of the amount of the water entrapped within flocs which results to an improvement of the sludge dewatering. Moreover, the sludge behaviour during the expression stage seems to depend in part on the change of the flocs structure. The filter cake was more compact when salt was added which tends to limit its compression