Quorum sensing (QS) refers to the capacity of bacteria to monitor their population density and regulate gene expression accordingly: the QS-regulated processes deal with multicellular behaviors (e.g. growth and development of biofilm), horizontal gene transfer and host-microbe (symbiosis and pathogenesis) and microbe-microbe interactions. QS signaling requires the synthesis, exchange and perception of bacterial compounds, called autoinducers or QS signals (e.g. N-acylhomoserine lactones). The disruption of QS signaling, also termed quorum quenching (QQ), encompasses very diverse phenomena and mechanisms which are presented and discussed in this review. First, we surveyed the QS-signal diversity and QS-associated responses for a better understanding of the targets of the QQ phenomena that organisms have naturally evolved and are currently actively investigated in applied perspectives. Next the mechanisms, targets and molecular actors associated with QS interference are presented, with a special emphasis on the description of natural QQ enzymes and chemicals acting as QS inhibitors. Selected QQ paradigms are detailed to exemplify the mechanisms and biological roles of QS inhibition in microbe-microbe and host-microbe interactions. Finally, some QQ strategies are presented as promising tools in different fields such as medicine, aquaculture, crop production and anti-biofouling area.

1. In a rapidly changing world, ecology has the potential to move from empirical and conceptual stages to application and management issues. It is now possible to make large-scale predictions up to continental or global scales, ranging from the future distribution of biological diversity to changes in ecosystem functioning and services. With these recent developments, ecology has a historical opportunity to become a major actor in the development of a sustainable human society. With this opportunity, however, also comes an important responsibility in developing appropriate predictive models, correctly interpreting their outcomes and communicating their limitations. There is also a danger that predictions grow faster than our understanding of ecological systems, resulting in a gap between the scientists generating the predictions and stakeholders using them (conservation biologists, environmental managers, journalists, policymakers). 2. Here, we use the context provided by the current surge of ecological predictions on the future of biodiversity to clarify what prediction means, and to pinpoint the challenges that should be addressed in order to improve predictive ecological models and the way they are understood and used.3. Synthesis and applications. Ecologists face several challenges to ensure the healthy development of an operational predictive ecological science: (i) clarity on the distinction between explanatory and anticipatory predictions; (ii) developing new theories at the interface between explanatory and anticipatory predictions; (iii) open data to test and validate predictions; (iv) making predictions operational; and (v) developing a genuine ethics of prediction. Supporting InformationAdditional Supporting Information may be found in the online version of this article.Appendix S1. Characteristics of mechanistic and phenomenological models in ecology.Appendix S2. Non-exhaustive list, of international initiatives of the scientific community aiming for sharing ecological data.

The concentration of GABA increases rapidly in wounded plant tissues, but the implication of this GABA pulse for plant-bacteria interactions is not known. Here we reveal that GABA stimulated the inactivation of the N-(3-oxooctanoyl)homoserine lactone (OC8-HSL) quorum-sensing signal (or ''quormone'') by the Agrobacterium lactonase AttM. GABA induced the expression of the attKLM operon, which was correlated to a decrease in OC8-HSL concentration in Agrobacterium tumefaciens cultures. The Agrobacterium GABA transporter Bra was required for this GABA-signaling pathway. Furthermore, transgenic tobacco plants with elevated GABA levels were less sensitive to A. tumefaciens C58 infection than were wild-type plants. These findings indicate that plant GABA may modulate quorum sensing in A. tumefaciens, thereby affecting its virulence on plants. Whereas GABA is an essential cell-to-cell signal in eukaryotes, here we provide evidence of GABA acting as a signal between eukaryotes and pathogenic bacteria. The GABA signal represents a potential target for the development of a strategy to control the virulence of bacterial pathogens.phytopathology ͉ plant signal ͉ lactonase ͉ quorum quenching

A Gram-negative, non-sporulating, rod-shaped, motile bacterium, with a single polar flagellum, designated strain PsJN T , was isolated from surface-sterilized onion roots. This isolate proved to be a highly effective plant-beneficial bacterium, and was able to establish rhizosphere and endophytic populations associated with various plants. Seven related strains were recovered from Dutch soils. Based on 16S rRNA gene sequence data, strain PsJN T and the Dutch strains were identified as representing a member of the genus Burkholderia, as they were closely related to Burkholderia fungorum (98?7 %) and Burkholderia phenazinium (98?5 %). Analysis of whole-cell protein profiles and DNA-DNA hybridization experiments confirmed that all eight strains belonged to a single species. Strain PsJN T had a DNA G+C content of 61?0 mol%. Only low levels of DNA-DNA hybridization to closely related species were found. Qualitative and quantitative differences in fatty acid composition between strain PsJN T and closely related species were identified. The predominant fatty acids in strain PsJN T were 16 : 0, 18 : 1v7c and summed feature 3 (comprising 16 : 1v7c and/or iso-15 : 0 2-OH). Isolate PsJN T showed high 1-aminocyclopropane-1-carboxylate deaminase activity and is therefore able to lower the ethylene level in a developing or stressed plant. Production of the quorum-sensing signal Abbreviations: ACC, 1-aminocyclopropane-1-carboxylate; GFP, green fluorescent protein; NAHL, N-acyl-homoserine lactone.The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene sequences of B. phytofirmans strains PsJN T , G44-5 and G6-5 are AY497470, AY836218 and AY836219.A neighbour-joining tree showing the position of B. phytofirmans sp. nov. within the genus Burkholderia, a dendrogram derived from the protein patterns of the strains studied and cross-sections showing chickpea roots with strain PsJN T

A gene involved in N-acyl homoserine lactone (N-AHSL) degradation was identified by screening a genomic library of Rhodococcus erythropolis strain W2. This gene, named qsdA (for quorum-sensing signal degradation), encodes an N-AHSL lactonase unrelated to the two previously characterized N-AHSL-degrading enzymes, i.e., the lactonase AiiA and the amidohydrolase AiiD. QsdA is related to phosphotriesterases and constitutes the reference of a novel class of N-AHSL degradation enzymes. It confers the ability to inactivate N-AHSLs with an acyl chain ranging from C 6 to C 14 , with or without substitution at carbon 3. Screening of a collection of 15 Rhodococcus strains and strains closely related to this genus clearly highlighted the relationship between the ability to degrade N-AHSLs and the presence of the qsdA gene in Rhodococcus. Bacteria harboring the qsdA gene interfere very efficiently with quorum-sensing-regulated functions, demonstrating that qsdA is a valuable tool for developing quorum-quenching procedures.

Bacteria degrading the quorum-sensing (QS) signal molecule N-hexanoylhomoserine lactone were isolated from a tobacco rhizosphere. Twenty-five isolates degrading this homoserine lactone fell into six groups according to their genomic REP-PCR and rrs PCR-RFLP profiles. Representative strains from each group were identified as members of the genera Pseudomonas, Comamonas, Variovorax and Rhodococcus. All these isolates degraded N-acylhomoserine lactones other than the hexanoic acid derivative, albeit with different specificity and kinetics. One of these isolates, Rhodococcus erythropolis strain W2, was used to quench QS-regulated functions of other microbes. In vitro, W2 strongly interfered with violacein production by Chromobacterium violaceum, and transfer of pathogenicity in Agrobacterium tumefaciens. In planta, R. erythropolis W2 markedly reduced the pathogenicity of Pectobacterium carotovorum subsp. carotovorum in potato tubers. These series of results reveal the diversity of the QSinterfering bacteria in the rhizosphere and demonstrate the validity of targeting QS signal molecules to control pathogens with natural bacterial isolates.

The Agrobacterium tumefaciens C58 genome contains three putative N-acyl homoserine lactone (acyl-HSL) hydrolases, which are closely related to the lactonase AiiA of Bacillus. When expressed in Escherichia coli, two of the putative acyl-HSL hydrolases, AttM and AiiB, conferred the ability to degrade acyl-HSLs on the host. In Erwinia strain 6276, the lactonases reduced the endogenous acyl-HSL level and the bacterial virulence in planta.N-Acyl homoserine lactones (acyl-HSLs) are diffusible signal molecules used by many gram-negative bacteria for a form of cell-to-cell communication termed quorum sensing (QS) (8). When a critical concentration of these molecules is present in the environment, i.e., when a critical cell density is reached, the acyl-HSLs bind an intracellular protein that acts as a transcriptional regulator of several genes and operons. QS regulates diverse functions, including the expression of virulence factors in several pathogenic bacteria, such as Pseudomonas aeruginosa, Agrobacterium tumefaciens, and Erwinia carotovora (Pectobacterium carotovorum) (16). Consequently, any physical or biological factors that alter the normal accumulation of acyl-HSLs may affect the virulence of such bacteria and could provide novel tools for their biological control (7). Such an approach was successfully used by Dong et al. (4): these researchers identified a lactonase enzyme in Bacillus sp. strain 204B1 that inactivates acyl-HSLs by opening the lactone ring (3). The corresponding gene, aiiA from strain 204B1 (aiiA 204B1 ), was cloned, characterized, and expressed in plants. The resulting lactonase activity in these transgenic plants sufficed to decrease their susceptibility to infection by virulent Erwinia (4). Another gene, attM, was identified by Tn5 mutagenesis in A. tumefaciens (18). The deduced amino acid sequence of attM shows similarities with the amino acid sequences of the AiiA lactonases that are present in the Bacillus species (5, 10).We investigated the distribution of genes homologous to the published Bacillus sp. strain 204B1 aiiA sequence (3) among the sequenced bacterial genomes available on the National Center for Biotechnology Information (NCBI) database. Using the Blastp program (http://www3.ncbi.nlm.nih.gov/BLAST/), 13 amino acid sequences deduced from open reading frames (ORFs) with higher identity scores to AiiA 240B1 and belonging to eubacterial species were retained. The Bacillus sequences already identified as lactonases were excluded from this in silico search. Among these 13 ORFs, the previously identified AttM lactonase of Agrobacterium (Ag.tu.gi16119365) exhibited the best identity score (32%). In addition to this protein, two distinct putative AiiA homologues were identified in the A. tumefaciens C58 genome. To facilitate the following discussion, we termed them AiiB (Ag.tu.gi16119885) and AiiC (Ag.tu .gi17938672). While the gene encoding AttM and the aiiC locus are located on the pAt plasmid, the third locus, aiiB, lies on the pTi plasmid. The other AiiA-related ORFs (listed below) ...

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