Organisme unicellulaire | EPFL Graph Search

vignette|Quatre espèces de volvocales, des algues unicellulaires. Les organismes unicellulaires sont des organismes composés d'une seule cellule dont les diverses fonctions vitales sont assurées par des organites hautement spécialisés, à la différence des organismes multicellulaires. Cette appellation regroupe deux catégories : les organismes procaryotes (sans noyau) et eucaryotes (avec noyau). Parmi les procaryotes, tous sont unicellulaires et on retrouve les bactéries et les archées. Parmi les eucaryotes, beaucoup sont pluricellulaires mais on retrouve aussi des unicellulaires comme les protozoaires, les algues unicellulaires ou les levures. Les organismes unicellulaires sont considérés comme la plus ancienne forme de vie, ayant émergé il y a environ d'années. Les virus en revanche n'ont pas une structure cellulaire et leur qualité « d'organisme » est controversée. En règle générale, les organismes unicellulaires sont microscopiques et sont présents dans tous les milieux et sur tou

Natural competence of the pathogen Acinetobacter baumannii: an elusive phenomenon

Nina Vesel

Natural competence for transformation is an important driver of horizontal DNA exchange between different organisms. This can result in accumulation of dangerous genetic features, such as antibiotic resistance genes, in a single organism. One example of an organism that frequently acquires antibiotic resistance genes is Acinetobacter baumannii, a Gram-negative opportunistic pathogen that has recently become problematic in hospital settings. Even though natural competence for transformation was demonstrated for some A. baumannii isolates, its competence regulon, the exact mechanism of DNA uptake, and the contribution of transformation to the acquisition of antibiotic resistance genes were largely unexplored at the beginning of this thesis. The aim of this thesis was therefore to better characterize optimal conditions of competence, the underlying competence regulon, and the necessary set of machinery proteins. Additionally, we aimed to identify the limiting factors that oppose transformation-mediated exchange of DNA in a small selection of strains. Here, we first addressed the general aspects of natural transformation, such as the competence window and the DNA uptake machinery. Our results showed that transformation is growth phase-dependent in A. baumannii and correlated with type IV pili (T4P) production. We demonstrated that T4P are essential for transformation and surface-associated motility but are only produced in a subfraction of the bacterial population. Furthermore, T4P production and assembly were under control of the PilSR two component system and the chemotaxis-like Pil-Chp system, respectively. These two regulatory systems were essential for competence development in A. baumannii, which is in contrast to what was observed for its close relative A. baylyi. We also investigated the reasons for non-transformability of certain A. baumannii isolates. We showed that comM interruption as well as the diversity of PilA protein sequences did not explain the strains' non-transformability. Instead, decreased expression of certain pilus genes was associated to the absence of transformability, which was also reflected in the protein level of the major component of T4P - PilA. Since the previously identified regulators could not explain the reduced transcript levels of the respective pilus genes, a transposon sequencing screen was performed to identify novel transformation-relevant genes. By comparison of transcriptional profiles of such genes between the transformable and non-transformable strains, we identified possible candidates that might explain non-transformability. Lastly, we explored whether the epigenome can influence A. baumannii's transformability. Our results showed that the source of transforming DNA has a significant effect on transformation of certain, yet not all tested strains. Specifically, for strain A118 the transformation levels were significantly decreased when non-self DNA was used as the donor DNA. Consequently, we identified a A118-specific restriction modification (RM) system that methylated specific DNA sequences in this strain and fostered the discrimination of self versus foreign DNA. Collectively, the findings of this thesis deciphered several important aspects of natural transformation in A. baumannii, which might ultimately help to better understand the spread of antibiotic resistance genes in this organism.

EPFL2022

Marine and freshwater micropearls: biomineralization producing strontium-rich amorphous calcium carbonate inclusions is widespread in the genus Tetraselmis (Chlorophyta)

Michaël Bensimon, Montserrat Filella

Unicellular algae play important roles in the biogeochemical cycles of numerous elements, particularly through the biomineralization capacity of certain species (e.g., coccolithophores greatly contributing to the "organic carbon pump" of the oceans), and unidentified actors of these cycles are still being discovered. This is the case of the unicellular alga Tetraselmis cordiformis (Chlorophyta) that was recently discovered to form intracellular mineral inclusions, called micropearls, which had been previously overlooked. These intracellular inclusions of hydrated amorphous calcium carbonates (ACCs) were first described in Lake Geneva (Switzerland) and are the result of a novel biomineralization process. The genus Tetraselmis includes more than 30 species that have been widely studied since the description of the type species in 1878. The present study shows that many other Tetraselmis species share this biomineralization capacity: 10 species out of the 12 tested contained micropearls, including T. chui, T. convolutae, T. levis, T. subcordiformis, T. suecica and T. tetrathele. Our results indicate that micropearls are not randomly distributed inside the Tetraselmis cells but are located preferentially under the plasma membrane and seem to form a definite pattern, which differs among species. In Tetraselmis cells, the biomineralization process seems to systematically start with a rod-shaped nucleus and results in an enrichment of the micropearls in Sr over Ca (the Sr/Ca ratio is more than 200 times higher in the micropearls than in the surrounding water or growth medium). This concentrating capacity varies among species and may be of interest for possible bioremediation techniques regarding radioactive Sr-90 water pollution. The Tetraselmis species forming micropearls live in various habitats, indicating that this novel biomineralization process takes place in different environments (marine, brackish and freshwater) and is therefore a widespread phenomenon.

COPERNICUS GESELLSCHAFT MBH2018

Influence of Temperature, VFA and pH on the inactivation of pathogens in anaerobic digestion of faecel sludge

Livia Jost

On-site sanitation technologies such as septic tanks are widely used in low- and middle-income countries. A comprehensive management of the accumulating faecal sludge (FS) is however missing in most contexts and uncontrolled disposal of FS results in serious public and environmental health risks. Given the high pathogen and organic matter content of FS it is urgent to find an appropriate method to treat FS. One promising, but little investigated technology to treat FS is anaerobic digestion (AD). The goal of the present thesis was to evaluate the influence of temperature, VFA and pH on the inactivation of pathogens in AD of synthetic FS. Ascaris suum eggs and bacteriophages MS2 and T4 were selected as model organisms for parasitic, respectively viral pathogens. At a temperature of 50°C, Ascaris suum eggs were inactivated after 6 h, independent of the pH and the VFA concentration. Bacteriophages MS2 and T4 were however not fully inactivated after 6 h at 50°C, their initial concentration was reduced by almost 99 % for T4 but only by 50 % for MS2. The pH had an effect on bacteriophages T4 but not on MS2. VFA concentration did not have an influence on the bacteriophages inactivation at the tested temperatures. Viruses and not parasites seem to be the critical organisms to consider in thermophilic AD (TAD) at 50°C of FS. Future research could therefore focus on the inactivation of human viruses in TAD as bacteriophages are conservative indicators for inactivation of human viruses.

2015