Investigation of contact-induced near-surface materials transformations using nanomechanical testing.

Mechanical surface treatments, such as shot peening – burnishing – deep rolling, are known for their efficiency to improve resistance to abrasive wear and local fatigue crack propagation. They are based on repeated contact loadings that create large plastic strains in the near-surface leading to compressive residual stress field and local grain refinement (Tribologically Tranformed Surfaces, Fig1). A significant gradient of mechanical properties over 100 µm is usually observed. This paper aims to present a methodology based on nanomechanical testing –i.e. micropillar compression, nanoindentation - and EBSD measurements to explain microstructure changes induced by such treatments. This methodology is applied to various cases ranging from severe shot peening (Fig1) to sliding friction contacts (Fig2). Please click Additional Files below to see the full abstract

Strukturrevision einer weit verbreiteten marinen Sulfonolipidklasse basierend auf deren Isolierung und Totalsynthese

Bakterien der marinen Roseobacter-Gruppe spielen eine wichtige Rolle in globalen biogeochemischen Prozessen. Prominente Vetreter dieser Gruppe produzieren schwefelhaltige Aminolipide (SAL), die für die Bildung von Biofilmen und die Besiedlung von Meeresoberflächen von entscheidender Bedeutung sind. Obwohl Genome Mining-Ansätze und massenspektrometrische Studien homotaurinhaltige Strukturen für eine Gruppe von SALs postulierten, blieben deren relative und absolute Strukturen bisher unbekannt, was biochemische und funktionelle Untersuchungen behinderte. In dieser Studie konnten wir die absoluten Strukturen durch eine Kombination von analytischen Techniken, Isolierungs- und Abbauexperimenten sowie Totalsynthese bestimmen. Im Gegensatz zu vorherigen Strukturvorschlägen sind die hier untersuchten Aminolipide durch eine ungewöhnliche N,O-acylierte Cysteinolsäure Kopfgruppe gekennzeichnet, weshalb wir die Substanzklasse Cysteinolide genannt haben. Durch gezielte Netzwerk-basierende metabolomische Studien konnten wir zudem die Verteilung und strukturelle Vielfalt von Cysteinoliden in verschiedenee Vertretern der bakteriellen Roseobacter-Gruppe kartieren. Insgesamt konnten in dieser Studie 14 verschiedene Aminolipide, einschließlich der in dieser Studie isolierten Cysteinolide, synthetisiert werden. Der Vergleich der erhaltenen analytischen Daten ermöglichte tiefergehende strukturelle Einblicke in die Charakteristika diese Substanzgruppe, welche für Studien zum bakteriellen Sulfonolipid-Stoffwechsel und zu biogeochemischen Nährstoffkreislauf in den Ozeanen von großer Bedeutung sein werden

A novel class of sulfur-containing aminolipids widespread in marine roseobacters

Marine roseobacter group bacteria are numerically abundant and ecologically important players in ocean ecosystems. These bacteria are capable of modifying their membrane lipid composition in response to environmental change. Remarkably, a variety of lipids are produced in these bacteria, including phosphorus-containing glycerophospholipids and several amino acid-containing aminolipids such as ornithine lipids and glutamine lipids. Here, we present the identification and characterization of a novel sulfur-containing aminolipid (SAL) in roseobacters. Using high resolution accurate mass spectrometry, a SAL was found in the lipid extract of Ruegeria pomeroyi DSS-3 and Phaeobacter inhibens DSM 17395. Using comparative genomics, transposon mutagenesis and targeted gene knockout, we identified a gene encoding a putative lyso-lipid acyltransferase, designated salA, which is essential for the biosynthesis of this SAL. Multiple sequence analysis and structural modeling suggest that SalA is a novel member of the lysophosphatidic acid acyltransferase (LPAAT) family, the prototype of which is the PlsC acyltransferase responsible for the biosynthesis of the phospholipid phosphatidic acid. SAL appears to play a key role in biofilm formation in roseobacters. salA is widely distributed in Tara Oceans metagenomes and actively expressed in Tara Oceans metatranscriptomes. Our results raise the importance of sulfur-containing membrane aminolipids in marine bacteria

Marine Bacteria Display Different Escape Mechanisms When Facing Their Protozoan Predators

Free-living amoeba are members of microbial communities such as biofilms in terrestrial, fresh, and marine habitats. Although they are known to live in close association with bacteria in many ecosystems such as biofilms, they are considered to be major bacterial predators in many ecosystems. Little is known on the relationship between protozoa and marine bacteria in microbial communities, more precisely on how bacteria are able survive in environmental niches where these bacterial grazers also live. The objective of this work is to study the interaction between the axenized ubiquitous amoeba Acanthamoeba castellanii and four marine bacteria isolated from immersed biofilm, in order to evaluate if they would be all grazed upon by amoeba or if they would be able to survive in the presence of their predator. At a low bacteria-to-amoeba ratio, we show that each bacterium is phagocytized and follows a singular intracellular path within this host cell, which appears to delay or to prevent bacterial digestion. In particular, one of the bacteria was found in the amoeba nucleolar compartment whereas another strain was expelled from the amoeba in vesicles. We then looked at the fate of the bacteria grown in a higher bacteria-to-amoeba ratio, as a preformed mono- or multi-species biofilm in the presence of A. castellanii. We show that all biofilms were subjected to detachment from the surface in the presence of the amoeba or its supernatant. Overall, these results show that bacteria, when facing the same predator, exhibit a variety of escape mechanisms at the cellular and population level, when we could have expected a simple bacterial grazing. Therefore, this study unravels new insights into the survival of environmental bacteria when facing predators that they could encounter in the same microbial communities

Diversité des interactions microbiennes au sein de l'environnement marin : De biofilms multi-spécifiques à multi-organismes

The formation of multi-organisms biofilms is poorly studied especially with marine organisms. This work first showed that strains harvested in biofilms from the Mediterranean Sea displayed a heterogeneity in their biofilm formation abilities and a diversity of matrix compounds. The study of multi-species biofilms revealed antagonistic and beneficial effects of some strains on the biofilm development of their partners. The interactions between amoebae and marine bacteria inoculated at a low ratio showed that all the strains tested were phagocytosed by Acanthamoeba castellanii. However, different mechanisms of escapement from their predators have been unraveled, such as a bacterial localization within the cell nucleolus or within fecal pellets expelled from amoebae. However, when the amoebae were added to a preformed bacterial monospecies or multispecies biofilms, a majority of bacteria detached from the surface. The amoebae supernatant induced also a bacterial detachment on two of the bacteria in monospecies biofilms, as well as morphological changes of these bacteria, suggesting that amoeba chemical cues are secreted and detected by the bacteria. Therefore, although a simple grazing of non-pathogenic marine bacteria by amoebae could have been expected, a diversity of bacterial behaviors was unraveled giving an idea on the diversity of interaction mechanisms that may exist in the marine environment.La formation de biofilm multi-organismes est rarement étudiée en particulier avec des organismes marins. Ce travail a d’abord mis en évidence une hétérogénéité des capacités de formation du biofilm et de la matrice de bactéries isolées de Méditerranée. L’étude de biofilms multi-espèces a permis de révéler des effets antagonistes et bénéfiques de certaines souches sur le développement du biofilm de leurs partenaires. Les interactions entre amibes et bactéries marines inoculées à un faible ratio ont ensuite montré que toutes les souches testées ont été phagocytées par Acanthamoeba castellanii. Cependant, différents mécanismes d’échappement à leur prédateur ont été mis en évidence tels qu’une localisation intranucléolaire des bactéries ou au sein de pelotes fécales expulsées de l’amibe. En revanche, lorsque les amibes ont été ajoutées sur des biofilms monospécifiques ou multi-spécifiques préalablement formés, une majorité de bactéries a fini par se détacher. Le surnageant amibien a également induit un détachement chez deux des bactéries de leur biofilm monospécifique, ainsi que des modifications phénotypiques chez ces mêmes souches, suggérant que des composés amibiens sont secrétés et détectés par les bactéries. Par conséquent, alors qu’on aurait pu s’attendre à un simple broutage des bactéries marines par les amibes, une diversité de comportements bactériens a été mis en évidence donnant une idée sur la diversité des mécanismes d’interaction pouvant exister dans l’environnement marin

Diversity of microbial interactions in the marine environment : from multi-specific to multi-organisms biofilms

La formation de biofilm multi-organismes est rarement étudiée en particulier avec des organismes marins. Ce travail a d’abord mis en évidence une hétérogénéité des capacités de formation du biofilm et de la matrice de bactéries isolées de Méditerranée. L’étude de biofilms multi-espèces a permis de révéler des effets antagonistes et bénéfiques de certaines souches sur le développement du biofilm de leurs partenaires. Les interactions entre amibes et bactéries marines inoculées à un faible ratio ont ensuite montré que toutes les souches testées ont été phagocytées par Acanthamoeba castellanii. Cependant, différents mécanismes d’échappement à leur prédateur ont été mis en évidence tels qu’une localisation intranucléolaire des bactéries ou au sein de pelotes fécales expulsées de l’amibe. En revanche, lorsque les amibes ont été ajoutées sur des biofilms monospécifiques ou multi-spécifiques préalablement formés, une majorité de bactéries a fini par se détacher. Le surnageant amibien a également induit un détachement chez deux des bactéries de leur biofilm monospécifique, ainsi que des modifications phénotypiques chez ces mêmes souches, suggérant que des composés amibiens sont secrétés et détectés par les bactéries. Par conséquent, alors qu’on aurait pu s’attendre à un simple broutage des bactéries marines par les amibes, une diversité de comportements bactériens a été mis en évidence donnant une idée sur la diversité des mécanismes d’interaction pouvant exister dans l’environnement marin.The formation of multi-organisms biofilms is poorly studied especially with marine organisms. This work first showed that strains harvested in biofilms from the Mediterranean Sea displayed a heterogeneity in their biofilm formation abilities and a diversity of matrix compounds. The study of multi-species biofilms revealed antagonistic and beneficial effects of some strains on the biofilm development of their partners. The interactions between amoebae and marine bacteria inoculated at a low ratio showed that all the strains tested were phagocytosed by Acanthamoeba castellanii. However, different mechanisms of escapement from their predators have been unraveled, such as a bacterial localization within the cell nucleolus or within fecal pellets expelled from amoebae. However, when the amoebae were added to a preformed bacterial monospecies or multispecies biofilms, a majority of bacteria detached from the surface. The amoebae supernatant induced also a bacterial detachment on two of the bacteria in monospecies biofilms, as well as morphological changes of these bacteria, suggesting that amoeba chemical cues are secreted and detected by the bacteria. Therefore, although a simple grazing of non-pathogenic marine bacteria by amoebae could have been expected, a diversity of bacterial behaviors was unraveled giving an idea on the diversity of interaction mechanisms that may exist in the marine environment

Multispecies Biofilm Development of Marine Bacteria Implies Complex Relationships Through Competition and Synergy and Modification of Matrix Components

International audienc

Trade-offs of lipid remodelling in a marine predator-prey interaction in response to phosphorus limitation

Phosphorus (P) is a key nutrient limiting bacterial growth and primary production in the oceans. Unsurprisingly, marine microbes have evolved sophisticated strategies to adapt to P limitation, one of which involves the remodeling of membrane lipids by replacing phospholipids with non-P-containing surrogate lipids. This strategy is adopted by both cosmopolitan marine phytoplankton and heterotrophic bacteria and serves to reduce the cellular P quota. However, little, if anything, is known of the biological consequences of lipid remodeling. Here, using the marine bacterium Phaeobacter sp. MED193 and the ciliate Uronema marinum as a model, we sought to assess the effect of remodeling on bacteria–protist interactions. We discovered an important trade-off between either escape from ingestion or resistance to digestion. Thus, Phaeobacter grown under P-replete conditions was readily ingested by Uronema, but not easily digested, supporting only limited predator growth. In contrast, following membrane lipid remodeling in response to P depletion, Phaeobacter was less likely to be captured by Uronema, thanks to the reduced expression of mannosylated glycoconjugates. However, once ingested, membrane-remodeled cells were unable to prevent phagosome acidification, became more susceptible to digestion, and, as such, allowed rapid growth of the ciliate predator. This trade-off between adapting to a P-limited environment and susceptibility to protist grazing suggests the more efficient removal of low-P prey that potentially has important implications for the functioning of the marine microbial food web in terms of trophic energy transfer and nutrient export efficiency

The Proteobacterial Methanotroph Methylosinus trichosporiumOB3b Remodels Membrane Lipids in Response to Phosphate Limitation

Methane is a potent greenhouse gas in the atmosphere, and its concentration has continued to increase in recent decades. Aerobic methanotrophs, bacteria that use methane as the sole carbon source, are an important biological sink for methane, and they are widely distributed in the natural environment. However, relatively little is known on how methanotroph activity is regulated by nutrients, particularly phosphorus (P). P is the principal nutrient constraining plant and microbial productivity in many ecosystems, ranging from agricultural land to the open ocean. Using a model methanotrophic bacterium, Methylosinus trichosporium OB3b, we demonstrate here that this bacterium can produce P-free glycolipids to replace membrane phospholipids in response to P limitation. The formation of the glycolipid monoglucuronic acid diacylglycerol requires plcP-agt genes since the plcP-agt mutant is unable to produce this glycolipid. This plcP-agt-mediated lipid remodeling pathway appears to be important for M. trichosporium OB3b to cope with P stress, and the mutant grew significantly slower under P limitation. Interestingly, comparative genomics analysis shows that the ability to perform lipid remodeling appears to be a conserved trait in proteobacterial methanotrophs; indeed, plcP is found in all proteobacterial methanotroph genomes, and plcP transcripts from methanotrophs are readily detectable in metatranscriptomics data sets. Together, our study provides new insights into the adaptation to P limitation in this ecologically important group of bacteria