Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal indivuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research

New film-forming poly(urethane-amide-imide) block copolymers: Influence of soft block on membrane properties for the purification of a fuel octane enhancer by pervaporation

A new family of eight poly(urethane-amide-imide) (PUAI) block copolymers with the same hard block and different soft blocks were synthesized in two steps from a dianhydride monomer containing amide functions (4,4′-methylene- bis(trimellitic anhydride-N-phenylamide)) and α,ω-dihydroxy telechelic oligomers which varied in both chemical structure (polyethers: PEG, PTMG, PPG; polyester: PCL) and molar weight (MW ≅ 600 or 1000 g/mol). The PUAI were obtained in high yields (ranging from 81 to 98 wt%) and with reduced viscosities which varied from 0.36 to 0.84 dL/g (for C = 1 mg/mL in DMF at 25°C). Their characterization by FTIR and 1H NMR fully confirmed their chemical structure. Their solubility was typically limited to a few wt/vol% even in strong apolar diprotic solvents like DMF and NMP. This particular feature showed the very strong physical cross-linking of their very stiff hard block and enabled to cast membranes capable of withstanding exposure to many common organic solvents. Systematic permeability experiments showed that the PUAI membranes could be used to separate the azeotropic mixture EtOH (20 wt%)/ETBE very easily, with interesting prospects for the purification of ETBE (a fuel octane enhancer used instead of lead derivatives in the European Community). An analysis in terms of structure-property relationships pointed out that the soft block molar weight and polarity were two key parameters for the optimization of selective permeability. The best compromise was obtained with the soft block PEG1000. The corresponding polymer led to performances so far outstanding for polyamideimides with a very high flux of more than 1.1 kg/h m2 for a normalized thickness of 5 μm at 50°C and a selectivity α = 22.7 in the high range for this kind of separation. © 2004 Elsevier Ltd. All rights reserved

Morphology and dynamical behaviour of grafted copolymers

International audienceThe interactions between the main chain and lateral chains in grafted polymers play an important role on their dynamics. This is a complex issue, since several length scales are to be taken into account, from the local intramacromolecular interactions between linked moieties up to intermolecular interactions in phase separated systems. Such phenomena are investigated on the basis of new copolymers obtained by controlled radical polymerization. The effect of two parameters are studied separately, namely (i) the number of grafted chains on the main chain (cellulose acetate) and (ii) the length of grafted moieties (poly(methyl(diethylene glycol)methacrylate)). The dynamics of molecular motions was assessed by Low Frequency Mechanical Spectroscopy and Broad Band Dielectric Spectroscopy. The complex shear and dielectric moduli were measured on thin films (60m thickness) as a function of frequency and temperature. The sensitivity of these relaxation spectroscopy techniques, as compared to calorimetry measurements, allowed to study the effects of (i) the chemical structure of the copolymers and (ii) the microstructure at the nanoscale, as investigated by Small Angle X-ray Scattering. It is shownthat the main parameter is the soft grafted moities content: as expected the molecular mobility increases as the content of soft grafted chains increases. In addition, it is shown that this molecular mobility is drastically depressed by thermal treatment

Heterotrophic protists as a trophic link between picocyanobacteria and the pearl oyster Pinctada margaritifera in the Takapoto lagoon (Tuamotu Archipelago, French Polynesia)

International audiencePearl oysters are farmed in oligotrophic tropical atoll lagoons where planktonic communitiesare dominated by production from cyanobacteria smaller than 2 μm. Paradoxically, the pearloyster Pinctada margaritifera only retains particles larger than 2 μm. In this study, we assess the relativecontribution of hetero/mixotrophic microbiota to the available planktonic resource. In TakapotoAtoll, picocyanobacteria are the dominant biomass (20 μg C l–1). The carbon biomass of ciliates anddinoflagellates ranges from 1 to 24 and 0.5 to 5 μg C l–1 respectively, with a mean of 6 μg C l–1 for ciliatesand 2 μg C l–1 for dinoflagellates. The possible retention by P. margaritifera on a natural protistsuspension was investigated. Due to its high clearance rates (ca 20 l h–1 g–1) the pearl oyster retained85 μg C h–1 g–1 from ciliates and 65 μg C h–1 g–1 from dinoflagellates. Conversely, cyanobacteria werenot efficiently retained by the bivalve and did not efficiently contribute to its diet. From our experiments,we concluded that hetero/mixotrophic protists rapidly and efficiently process the picoplanktonicresource towards filter-feeders, particularly pearl oysters