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.

Plants accumulate free L-proline (Pro) in response to abiotic stresses (drought and salinity) and presence of bacterial pathogens, including the tumor-inducing bacterium Agrobacterium tumefaciens. However, the function of Pro accumulation in hostpathogen interaction is still unclear. Here, we demonstrated that Pro antagonizes plant GABA-defense in the A. tumefaciens C58-induced tumor by interfering with the import of GABA and consequently the GABA-induced degradation of the bacterial quorumsensing signal, 3-oxo-octanoylhomoserine lactone. We identified a bacterial receptor Atu2422, which is implicated in the uptake of GABA and Pro, suggesting that Pro acts as a natural antagonist of GABA-signaling. The Atu2422 amino acid sequence contains a Venus flytrap domain that is required for trapping GABA in human GABA B receptors. A constructed atu2422 mutant was more virulent than the wild type bacterium; moreover, transgenic plants with a low level of Pro exhibited less severe tumor symptoms than did their wild-type parents, revealing a crucial role for Venus flytrap GABA-receptor and relative abundance of GABA and Pro in hostpathogen interaction.plant defense ͉ virulence ͉ plant tumor ͉ lactonase

The phytopathogen Agrobacterium tumefaciens C58 expresses two lactonases, AttM and AiiB. We showed that expression of the aiiB gene was controlled by agrocinopines A and B and required the agrocinopine-ABC transporter Acc, but was not affected by the level of quorum-sensing (QS) signal 3-oxo-octanoylhomoserine lactone (OC8-HSL). In the presence of agrocinopines, a constructed aiiB mutant accumulated OC8-HSL at a level 10-fold higher than that of the wild-type strain, and showed an exacerbated expression of a key QS-regulated function, conjugation of Ti plasmid (in vitro and in planta), as well as an increase of the number of emerging tumors on the host plant. The expression and acyl-HSL-degrading activity of AttM were evident in the presence of wounded tissues; however, in unwounded plant tumors, the QS-regulated functions were weakly affected in an attM mutant. By contrast, we observed that attM conferred a selective advantage in the course of colonization of plant tumors. Finally, polymerase chain reaction survey of genes attM and aiiB showed that they were not strictly conserved in the genus Agrobacterium. This work proved that the lactonases AttM and AiiB are regulated by different plant signals and are implicated in different functions in the course of the A. tumefaciens C58-host interaction.

Because of their random nature, rates of locomotion of polymorphonuclear neu trophils (PMN) can only be measured by direct, visual techniques that will take into account the multidirectional pathway which they follow. Few such studies have been performed, and data relating the effects of temperature and pH to the rate of locomotion of PMN observed by direct, visual techniques are incomplete and were performed several decades ago with elementary methods (1-3). As a result, Qlo values available for this biological process are fragmentary (4).The purpose of the present study was to obtain more extensive data on the subject with up-to-date techniques to determine to what extent temperature and pH affect, over a wide range, the rate of locomotion of PMN. Such measurements could also be compared with those performed by the indirect technique using the migration of leukocytes through Millipore filters (5).Methods. A special system ( Fig. 1) was designed to study PMN under conditions of a constant microenvironment. This system comprises two parts: a series of equilibrating tonometers (Eschweiler) and a microobservation chamber in communication with the latter. The equilibrating tonometers (volume: 15 ml) are immersed in a constant temperature bath. They are connected through humidifiers to gas mixtures of known concentrations of 0 2 , N2 and C02. The observation chamber is made of a glass slide and a cover slip sealed with resin and connected with two polyethylene tubes (0.5-mm i.d.). One of

SummaryFor some crops, the only possible approach to gain a specific trait requires genome modification. The development of virus‐resistant transgenic plants based on the pathogen‐derived resistance strategy has been a success story for over three decades. However, potential risks associated with the technology, such as horizontal gene transfer (HGT) of any part of the transgene to an existing gene pool, have been raised. Here, we report no evidence of any undesirable impacts of genetically modified (GM) grapevine rootstock on its biotic environment. Using state of the art metagenomics, we analysed two compartments in depth, the targeted Grapevine fanleaf virus (GFLV) populations and nontargeted root‐associated microbiota. Our results reveal no statistically significant differences in the genetic diversity of bacteria that can be linked to the GM trait. In addition, no novel virus or bacteria recombinants of biosafety concern can be associated with transgenic grapevine rootstocks cultivated in commercial vineyard soil under greenhouse conditions for over 6 years.

N-Acylhomoserine lactone (AHL)-mediated quorum-sensing (QS) regulates virulence functions in plant and animal pathogens such as Agrobacterium tumefaciens and Pseudomonas aeruginosa. A chemolibrary of more than 3500 compounds was screened using two bacterial AHL-biosensors to identify QS-inhibitors (QSIs). The purity and structure of 15 QSIs selected through this screening were verified using HPLC MS/MS tools and their activity tested on the A. tumefaciens and P. aeruginosa bacterial models. The IC50 value of the identified QSIs ranged from 2.5 to 90 µg/ml, values that are in the same range as those reported for the previously identified QSI 4-nitropyridine-N-oxide (IC50 24 µg/ml). Under the tested culture conditions, most of the identified QSIs did not exhibit bacteriostatic or bactericidal activities. One third of the tested QSIs, including the plant compound hordenine and the human sexual hormone estrone, decreased the frequency of the QS-regulated horizontal transfer of the tumor-inducing (Ti) plasmid in A. tumefaciens. Hordenine, estrone as well as its structural relatives estriol and estradiol, also decreased AHL accumulation and the expression of six QS-regulated genes (lasI, lasR, lasB, rhlI, rhlR, and rhlA) in cultures of the opportunist pathogen P. aeruginosa. Moreover, the ectopic expression of the AHL-receptors RhlR and LasR of P. aeruginosa in E. coli showed that their gene-regulatory activity was affected by the QSIs. Finally, modeling of the structural interactions between the human hormones and AHL-receptors LasR of P. aeruginosa and TraR of A. tumefaciens confirmed the competitive binding capability of the human sexual hormones. This work indicates potential interferences between bacterial and eukaryotic hormonal communications.

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Quorum-sensing (QS) signals of the N-acylhomoserine lactone (NAHL) class are cleaved by quorum-quenching enzymes, collectively named NAHLases. Here, functional metagenomics allowed the discovery of a novel bacterial NAHLase in a rhizosphere that was treated with γ-caprolactone. As revealed by rrs-DGGE and rrs-pyrosequencing, this treatment increased the percentage of the NAHL-degrading bacteria and strongly biased the structure of the bacterial community, among which Azospirillum dominated. Among the 29 760 fosmids of the metagenomic library, a single one was detected that expressed the qsdB gene conferring NAHL-degradation upon E. coli and decreased QS-regulated virulence in Pectobacterium. Phylogenetic analysis of the 34 orfs of the fosmid suggested that it would belong to an unknown Proteobacterium - probably a γ-proteobacterium. qPCR quantification of the NAHLase-encoding genes attM, qsdA, and qsdB revealed their higher abundance in the γ-caprolactone-treated rhizosphere as compared to an untreated control. The purified QsdB enzyme exhibited amidase activity. QsdB is the first amidase signature (AS) family member exhibiting NAHLase-activity. Point mutations in the AS-family catalytic triad K-S-S abolished the NAHLase activity of QsdB. This study extends the diversity of NAHLases and highlights a common phylogenic origin of AS-family enzymes involved in the degradation of natural compounds, such as NAHLs, and xenobiotics, such as nylon and linuron.