Cell-bound exopolysaccharides from an axenic culture of the intertidal mudflat Navicula phyllepta diatom affect biofilm formation by benthic bacteria

International audienceAt low tide, intertidal mudflat biofilms cover large surfaces and are mainly responsible for the high productivity of these marine areas. In the European Atlantic coast, such biofilms are mainly composed of diatoms, especially Navicula phyllepta, bacteria, and microbial extracellular polymeric substances (EPS). To better understand interactions occurring between microorganisms, we first axenized a N. phyllepta culture with a new and simple protocol. Colloidal and bound EPS secreted by diatom cells during the exponential growth and the stationary phase were then harvested, and we tested their effects on the in vitro formation of biofilms by three marine bacteria. The latter had been isolated from a French Atlantic intertidal mudflat and were previously selected for their strong in vitro biofilm-forming ability. They belong to the Flavobacterium, Roseobacter, and Shewanella genera. Navicula phyllepta-bound EPS synthesized during the stationary phase specifically inhibited the biofilm formation by the Flavobacterium sp. strain, whereas they stimulated biofilm development by the two other strains. The EPS acted in all cases during the first stages of the biofilm establishment. Saccharidic molecules were found to be responsible for these activities. This is the first report on marine bacterial antibiofilm saccharides of microalgal origin. This work points out the complexity of the benthic natural biofilms with specific microalgae/bacteria interactions and underlines the possibility to use axenic diatoms as a source of bioactive compounds

On the bacterial communities associated with the corrosion product layer during the early stages of marine corrosion of carbon steel

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Biofilms en milieu marin. Exemple des vasières intertidales et des structures métalliques portuaires.

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Mechanism of Bactericidal Activity of Microcin L in Escherichia coli and Salmonella enterica▿

For the first time, the mechanism of action of microcin L (MccL) was investigated in live bacteria. MccL is a gene-encoded peptide produced by Escherichia coli LR05 that exhibits a strong antibacterial activity against related Enterobacteriaceae, including Salmonella enterica serovars Typhimurium and Enteritidis. We first subcloned the MccL genetic system to remove the sequences not involved in MccL production. We then optimized the MccL purification procedure to obtain large amounts of purified microcin to investigate its antimicrobial and membrane properties. We showed that MccL did not induce outer membrane permeabilization, which indicated that MccL did not use this way to kill the sensitive cell or to enter into it. Using a set of E. coli and Salmonella enterica mutants lacking iron-siderophore receptors, we demonstrated that the MccL uptake required the outer membrane receptor Cir. Moreover, the MccL bactericidal activity was shown to depend on the TonB protein that transduces the proton-motive force of the cytoplasmic membrane to transport iron-siderophore complexes across the outer membrane. Using carbonyl cyanide 3-chlorophenylhydrazone, which is known to fully dissipate the proton-motive force, we proved that the proton-motive force was required for the bactericidal activity of MccL on E. coli. In addition, we showed that a primary target of MccL could be the cytoplasmic membrane: a high level of MccL disrupted the inner membrane potential of E. coli cells. However, no permeabilization of the membrane was detected

Biofilms in a marine environment: example of intertidal mudflats and metallic port structures

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La biocalcification bactérienne en milieu marin et ses applications

La précipitation du carbonate de calcium (CaCO3) biologiquement induite en milieu marin joue un rôle important dans le cycle biogéochimique du carbone. Cette biocalcification est gouvernée par quatre facteurs clés : le taux de carbone inorganique dissous dont dépend le taux de carbonates (CO32−) dans le système, le taux d’ions calciques (Ca2+), le pH et la disponibilité des sites de nucléation c’est-à-dire des zones de cristallisation primaire de la phase solide du minéral. Les bactéries impliquées dans la biocalcification marine vont alors agir sur un ou plusieurs de ces facteurs. Ce processus naturel, qui se produit dans divers contextes géologiques, peut être imité afin de développer un certain nombre de technologies permettant la séquestration des métaux lourds, la protection des métaux contre la corrosion, la restauration et le renforcement de matériaux préexistants et la consolidation de matériaux granulaires. Cette étude passe en revue les différentes activités métaboliques microbiennes menant à la précipitation du CaCO3 ainsi que leurs applications potentielles en milieu marin

Influence of Organic Matter/Bacteria on the Formation and Transformation of Sulfate Green Rust

International audienceThe corrosion processes of carbon steel immersed in natural seawater are influenced by microorganisms due to important biological activity. An analysis of the corrosion product layers formed on carbon steel coupons in natural or artificial seawater revealed that sulfate green rust GR(SO 4 2−) was favored in natural environments. In this paper, the role of organic matter/bacteria on the formation and transformation of this compound are addressed. GR(SO 4 2−) was precipitated from Fe(II) and Fe(III) salts in the presence of various marine bacterial species not involved in the redox cycle of Fe or S. Abiotic experiments were performed for comparison, first without any organic species and then with sodium acetate added as a small organic ion. The obtained aqueous suspensions were aged at room temperature for 1 week. The number of bacteria (CFU/mL) was followed over time and the solid phases were characterized by XRD. Whatever the fate of the bacteria (no activity, or activity and growth), the formation of GR(SO 4 2−) was favored and its transformation to magnetite completely inhibited. This effect is attributed to the adsorption of organic molecules on the lateral sides of the GR(SO 4 2−) crystals. A similar effect, though less important, was observed with acetate