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The increase in marine diseases, particularly in economically important mollusks, is a growing concern. Among them, the Pacific oyster ( Crassostrea gigas ) production faces challenges from several diseases, such as the Pacific Oyster Mortality Syndrome (POMS) or vibriosis. The microbial education, which consists of exposing the host immune system to beneficial microorganisms during early life stages is a promising approach against diseases. This study explores the concept of microbial education using controlled and pathogen-free bacterial communities and assesses its protective effects against POMS and Vibrio aestuarianus infections, highlighting potential applications in oyster production. We demonstrate that it is possible to educate the oyster immune system by adding microorganisms during the larval stage. Adding culture based bacterial mixes to larvae protects only against the POMS disease while adding whole microbial communities from oyster donors protects against both POMS and vibriosis. The efficiency of immune protection depends both on oyster origin and on the composition of the bacterial mixes used for exposure. No preferential protection was observed when the oysters were stimulated with their sympatric strains. Furthermore, the added bacteria were not maintained into the oyster microbiota, but this bacterial addition induced long term changes in the microbiota composition and oyster immune gene expression. Our study reveals successful immune system education of oysters by introducing beneficial microorganisms during the larval stage. We improved the long-term resistance of oysters against critical diseases (POMS disease and Vibrio aestuarianus infections) highlighting the potential of microbial education in aquaculture.

Although interactions between microalgae and bacteria are observed in both natural environment and the laboratory, the modalities of coexistence of bacteria inside microalgae phycospheres in laboratory cultures are mostly unknown. Here, we focused on well-controlled cultures of the model green picoalga Ostreococcus tauri and the most abundant member of its phycosphere, Marinobacter algicola. The prevalence of M. algicola in O. tauri cultures raises questions about how this bacterium maintains itself under laboratory conditions in the microalga culture. The results showed that M. algicola did not promote O. tauri growth in the absence of vitamin B12 while M. algicola depended on O. tauri to grow in synthetic medium, most likely to obtain organic carbon sources provided by the microalgae. M. algicola grew on a range of lipids, including triacylglycerols that are known to be produced by O. tauri in culture during abiotic stress. Genomic screening revealed the absence of genes of two particular modes of quorum-sensing in Marinobacter genomes which refutes the idea that these bacterial communication systems operate in this genus. To date, the ‘opportunistic’ behaviour of M. algicola in the laboratory is limited to several phytoplanktonic species including Chlorophyta such as O. tauri. This would indicate a preferential occurrence of M. algicola in association with these specific microalgae under optimum laboratory conditions.

The interactions between bacteria and phytoplankton regulate many important biogeochemical reactions in the marine environment, including those in the global carbon, nitrogen, and sulfur cycles. At the microscopic level, it is now well established that important consortia of bacteria colonize the phycosphere, the immediate environment of phytoplankton cells. In this microscale environment, abundant bacterial cells are organized in a structured biofilm, and exchange information through the diffusion of small molecules called semiochemicals. Among these processes, quorum sensing plays a particular role as, when a sufficient abundance of cells is reached, it allows bacteria to coordinate their gene expression and physiology at the population level. In contrast, quorum quenching mechanisms are employed by many different types of microorganisms that limit the coordination of antagonistic bacteria. This review synthesizes quorum sensing and quorum quenching mechanisms evidenced to date in the phycosphere, emphasizing the implications that these signaling systems have for the regulation of bacterial communities and their activities. The diversity of chemical compounds involved in these processes is examined. We further review the bacterial functions regulated in the phycosphere by quorum sensing, which include biofilm formation, nutrient acquisition, and emission of algaecides. We also discuss quorum quenching compounds as antagonists of quorum sensing, their function in the phycosphere, and their potential biotechnological applications. Overall, the current state of the art demonstrates that quorum sensing and quorum quenching regulate a balance between a symbiotic and a parasitic way of life between bacteria and their phytoplankton host.

In recent years, use of supercritical-fluid chromatography (SFC) with CO2 as the mobile phase has been expanding in the research laboratory and industry since it is considered to be a green analytical method. This technique offers numerous advantages, such as good separation and sensitive detection, short analysis times, and stability of analytes. In this study, a method for quantification of N-acyl homoserine lactones (AHLs), signaling molecules responsible for cell-to-cell communication initially discovered in bacteria, by SFC coupled with high-resolution mass spectrometry (HRMS) was developed. The SFC conditions and MS ionization settings were optimized to obtain the best separation and greatest sensitivity. The optimal analysis conditions allowed quantification of up to 30 AHLs in a single run within 16 min with excellent linearity (R2 > 0.998) and sensitivity (picogram level). This method was then applied to study AHL production by one Gram-negative endophytic bacterium, Paraburkholderia sp. BSNB-0670. Nineteen known AHLs were detected, and nine abundant HSLs were quantified. To further investigate the production of uncommon AHLs, a molecular networking approach was applied on the basis of the SFC–HRMS/MS data. This led to additional identification of four unknown AHLs annotated as N-3-hydroxydodecanoylol homoserine lactone, N-3-hydroxydodecadienoyl homoserine lactone, and N-3-oxododecenoyl homoserine lactones (two isomers).

Bacteria play a crucial role in marine biogeochemistry by releasing, consuming and transforming organic matter. Far from being isolated entities, bacteria are involved in numerous cell–cell interactions. Among such interactions, quorum sensing (QS) allows bacteria to operate in unison, synchronizing their actions through chemical communication. This review aims to explore and synthesize our current knowledge of the involvement of QS in the regulation of bacterial processes that ultimately impact marine biogeochemical cycles. We first describe the principles of QS communication and the renewed interest in its study in marine environments. Second, we highlight that the microniches where QS is most likely to occur due to their high bacterial densities are also hotspots of bacterially mediated biogeochemical transformations. Many bacterial groups colonizing these microniches harbor various QS systems. Thereafter, we review relevant QS-regulated bacterial processes in marine environments, building on research performed in both complex marine assemblages and isolated marine bacteria. QS pathways have been shown to directly regulate organic matter degradation, carbon allocation and nutrient acquisition but also to structure the community composition by mediating colonization processes and microbial interactions. Finally, we discuss current limitations and future perspectives to better characterize the link between QS expression and the bacterial mediation of biogeochemical cycles. The picture drawn by this review highlights QS as one of the pivotal mechanisms impacting microbial composition and functions in the oceans, paving the way for future research to better constrain its impact on marine biogeochemical cycles.

Quorum sensing (QS), a cell-to-cell communication system involved in the synchronization of bacterial behavior in a cell-density-dependent manner has been shown to control phenotypes such as luminescence, virulence, and biofilm formation. The marine strain, Shewanella woodyi MS32 has been identified as a luminous bacterium. Very little information is known on this bacterium, in particular if its luminescence and biofilm formation are controlled by QS. In this study, we have demonstrated that S. woodyi MS32 emits luminescence in planktonic and sessile conditions. The putative QS regulatory genes homologous to luxI and luxR identified in the S. woodyi MS32 genome, named swoI and swoR, are divergently transcribed and are not genetically linked to the lux operon in contrast with its closest parent Shewanella hanedai and with Aliivibrio fischeri. Interestingly, the phylogenetic analysis based on the SwoI and SwoR sequences shows that a separate horizontal gene transfer (HGT) occurred for the regulatory genes and for the lux operon. Functional analyses demonstrate that the swoI and swoR mutants were non-luminescent. Expression of lux genes was impaired in the QS regulatory mutants. N-octanoyl-L-homoserine lactone (C₈-HSL) identified using liquid chromatography mass spectrometry in the wild-type strain (but not in ΔswoI) can induce S. woodyi luminescence. No significant difference has been detected between the wild-type and mutants on adhesion and biofilm formation in the conditions tested. Therefore, we have demonstrated that the luxCDABEG genes of S. woodyi MS32 are involved in luminescence emission and that the swoR/swoI genes, originated from a separate HGT, regulate luminescence through C₈-HSL production.

Clownfishes and sea anemones form an intriguing long-term association, but the mechanism underlying this symbiosis is not well understood. Since clownfishes seem to cover themselves with sea anemone mucus, we investigated the microbiomes of the two partners to search for possible shifts in their compositions. We used a 16S rRNA gene sequencing strategy to study the dynamics of the microbiota during the association between the clownfish Amphiprion ocellaris and its host Heteractis magnifica under laboratory conditions. The experiment conducted in aquaria revealed that both clownfish and sea anemone mucus had specific signatures compared to artificial sea water. The microbiomes of both species were highly dynamic during the initiation of the symbiosis and for up to seven days after contact. Three families of bacteria ( Haliangiaceae, Pseudoalteromonadacae, Saprospiracae ) were shared between the two organisms after symbiosis. Once the symbiosis had been formed, the clownfishes and sea anemone then shared some communities of their mucus microbiota. This study paves the way for further investigations to determine if similar microbial signatures exist in natural environments, whether such microbial sharing can be beneficial for both organisms, and whether the microbiota is implicated in the mechanisms that protect the clownfish from sea anemone stinging.

Since the discovery of quorum sensing (QS) in the 1970s, many studies have demonstrated that Vibrio species coordinate activities such as biofilm formation, virulence, pathogenesis, and bioluminescence, through a large group of molecules called N-acyl homoserine lactones (AHLs). However, despite the extensive knowledge on the involved molecules and the biological processes controlled by QS in a few selected Vibrio strains, less is known about the overall diversity of AHLs produced by a broader range of environmental strains. To investigate the prevalence of QS capability of Vibrio environmental strains we analyzed 87 Vibrio spp. strains from the Banyuls Bacterial Culture Collection (WDCM911) for their ability to produce AHLs. This screening was based on three biosensors, which cover a large spectrum of AHLs, and revealed that only 9% of the screened isolates produced AHLs in the defined experimental conditions. Among these AHL-producing strains, Vibrio tasmaniensis LGP32 is a well-known pathogen of bivalves. We further analyzed the diversity of AHLs produced by this strain using a sensitive bioguided UHPLC-HRMS/MS approach (Ultra-High-Performance Liquid Chromatography followed by High-Resolution tandem Mass Spectrometry) and we identified C10-HSL, OH-C12-HSL, oxo-C12-HSL and C14:1-HSL as QS molecules. This is the first report that documents the production of AHL by Vibrio tasmaniensis LGP32.

Intestinal dysbiosis is a key feature in the pathogenesis of inflammatory bowel disease (IBD). Acyl-homoserine lactones (AHL) are bacterial quorum-sensing metabolites that may play a role in the changes in host cells-gut microbiota interaction observed during IBD. The objective of our study was to investigate the presence and expression of AHL synthases and receptor genes in the human gut ecosystem during IBD. We used an in silico approach, applied to the Inflammatory Bowel Disease Multi’omics Database comprising bacterial metagenomic and metatranscriptomic data from stools of patients with Crohn’s disease (CD) (n = 50), ulcerative colitis (UC) (n = 27) and non-IBD controls (n = 26). No known putative AHL synthase gene was identified; however, several putative luxR receptors were observed. Regarding the expression of these receptor genes, the luxR gene from Bacteroides dorei was under-expressed in IBD patients (p = 0.02) compared to non-IBD patients, especially in CD patients (p = 0.02). In the dysbiosis situation, one luxR receptor gene from Bacteroides fragilis appeared to be over-expressed (p = 0.04) compared to that of non-dysbiotic patients. Targeting LuxR receptors of bacterial quorum sensing might represent a new approach to modulate the gut microbiota in IBD.

N -Acyl homoserine lactone (AHL)-mediated Quorum sensing (QS) is one of the most studied social behavior among Proteobacteria . However, despite the current knowledge on QS-associated phenotypes such as bioluminescence, biofilm formation, or pathogenesis, the characterization of environmental factors driving QS in realistic ecological settings remains scarce. We investigated the dynamics of AHL and AHL-producing Vibrio among 840 isolates  collected fortnightly from the Salses-Leucate Mediterranean lagoon in spring and summer 2015 and 2016. Vibrio isolates were characterized by gyrB gene sequencing, Enterobacterial repetitive intergenic consensus polymerase chain reaction, and genome sequencing, and AHL production was investigated by a biosensors-based UHPLC–HRMS/MS approach. Our results revealed, for the first time, a succession of V. mediterranei isolates with different AHL production phenotypes over time and this dynamics was observed in a single genotype (average genomic nucleotide identity >99.9). A multivariate DistLM analysis revealed that 83.4% of the temporal variation of V. mediterranei QS phenotypes was explained by environmental variables. Overall, our results suggest that isolates of a single genotype are able to change their QS phenotypes in response to environmental conditions, highlighting the phenotypic plasticity of bacterial communication in the environment.