L’identification, chez divers organismes, d’enzymes de dégradation des N-acyl homosérineslactones (NAHLs) impliquées dans la signalisation QS pose la question de leurs rôles dans lesinteractions bactéries-eucaryotes. Dans une première partie, une synthèse bibliographique analyse lesconnaissances acquises sur ces enzymes dégradant les NAHLs. Dans une seconde partie, la croissancedes bactéries dégradant les signaux NAHLs a été stimulée par l’application de g-caprolactone (GCL)dans la rhizosphère de plants de pommes de terre à des fins de phytoprotection. L’effet de cetraitement sur la diversité des communautés bactériennes rhizosphériques a été évalué en combinantdifférentes approches d’écologie microbienne moléculaire comme la DGGE, le pyroséquençaged’amplicons rrs, et la métagénomique fonctionnelle. Cette dernière approche appliquée à une banquede 30 000 clones environ a conduit à l’identification d’un gène qsdB codant la dégradation des signauxNAHL. Ce travail révèle ainsi l’existence d’une nouvelle classe d’enzymes de dégradation des NAHLsappartenant à la famille des enzymes possédant une signature amidase (AS) dont des membres sontpar ailleurs impliqués dans la dégradation de composés xénobiotiques. Dans une troisième partie, unsystème expérimental a été développé afin de mesurer le transfert conjugatif du plasmide de virulenceTi (tumor inducing) chez des dérivés du pathogène Agrobacterium tumefaciens, appelés «tricheurs»,incapables de produire des signaux NAHLs mais utilisateurs de ceux produits par les autres bactéries.Ce modèle a permis de montrer l’effet modérateur de lactonases dégradant les NAHLs exprimées chezdes agrobactéries produisant les NAHLs, chez des bactéries réceptrices du plasmide Ti, ou des planteshôtes des agrobactéries sur le transfert conjugatif initié par les tricheurs. L’ensemble de ce travailrévèle à la fois une nouvelle famille d’enzymes impliquées dans la dégradation des NAHLs, ainsiqu’un nouveau rôle de ces enzymes dans la modulation des flux de gènes entre bactériesphytopathogènes en interaction avec une plante hôte. / Identification of bacterial and eukaryotic enzymes that degrade N-acyl homoserine lactones(NAHLs) involved in QS signaling raises the question of their roles in bacteria-eucaryotesinteractions. In a first part of this study, a bibliographic report analyzes the current data on thoseNAHL-degrading enzymes. In a second part, the growth of NAHL-degrading bacteria was stimulatedby g-caprolactone (GCL) amendment in potato rhizosphere to protect this plant against the soft-rotpathogen Pectobacterium. The effect of the GCL treatment on rhizospheric bacterial communities wasevaluated by a combination of different molecular microbial ecology techniques such as DGGE,pyrosequencing and functional metagenomic. This last approach was applied to generate ametagenomic library of ca. 30,000 clones and lead to the identification of the qsdB gene that encodesNAHL degradation, This work revealed the occurrence of a novel class of NAHL-degrading enzymesthat belong to the amidase signature (AS) family, some members of which being involved inxenobiotic compound degradation. In a third part, an experimental system was developed to measurethe conjugative transfer of Ti plasmid in various strains of the pathogen Agrobacterium tumefaciens,including “cheaters”, i.e. bacteria unable to produce NAHL signals but capable to use signals producedby other bacteria. Using this model; variations of the plasmid transfer of cheaters were measured whenNAHL-degrading lactonases were expressed in agrobacteria that produce NAHL signals, in recipientbacteria of Ti plasmid, or in agrobacterial host plant. Taken together, thesis experiments revealed anovel class of enzymes involved in NAHL-degradation and a new role for thoses enzymes in themodulation of gene transfer between pathogenic bacteria interacting with host plants.

The purpose of this study was to investigate how anonymous online players treat each other in the game environment in Counter Strike (CS GO), where the way they communicate can affect other people's gaming experience through cheating or "griefing". We have also sought to examine how issues of anonymity affected player's experiences. Data was collected through the use of qualitative methods, such as semi-structured interviews and non-participant observations. Ten respondents participated in the interviews, five women and five men who had played Counter Strike for at least two years. The questions which they were asked related to attitudes, communication, gaming experience, trust, behavior, cheaters and griefers inCounter Strike. The results were analyzed with the help of previous research and theory related to the problem of our study. The results showed that the anonymous players responded to each other positively or negatively depending on various factors such as performance, gender, experience, communication and trust. Positive attitudes gave a positive gaming experience that increased the teamwork and changed the anonymity to friendship, while negative attitudes gave negative gameplay experience that made players lose the desire to play for awhile, because they had high expectations of others. In order to improve Counter Strike for anonymous players, we recommend that the game company improves its security, by blocking cheaters and griefers faster than they do at present, by changing blocking of a cheater from only a blocked account to a whole computersystem, and by making the game more expensive. We also recommend applying region or country functions when they people play anonymously to avoid linguistic misunderstandings and to ease communication by ensuring that players from the same country play with eachother and speak a common language. / Syftet med denna uppsats är att undersöka hur anonyma online-spelare bemöter varandra i spelmiljön som omger Counter Strike (CS GO), där det kan uppstå ett visst kommunikativt beteende som kan påverka andras spelupplevelse, såsom att fuska eller "griefa". Vi ville även undersöka hur man kan lösa de svårigheter som anonyma spelare upplevde när de spelade med varandra. Undersökningen använder kvalitativa metoder där data insamlades med hjälpav semistrukturerade intervjuer. Respondenterna som deltog i intervjuerna var totalt tio personer, fem tjejer och fem killar som hade spelat Counter Strike i minst två år. Frågorna som ställdes berörde bemötande, kommunikation, spelupplevelse, tillit, beteende, fuskare och griefers i Counter strike. Resultatet analyserades med hjälp av relevant tidigare forskning och teori. Resultaten visar att anonyma spelare bemötte varandra positivt eller negativt beroende på olika faktorer, såsom prestation, kön, erfarenhet, kommunikation och tillit. Positiv bemötandegav en positiv spelupplevelse som ökade lagsamarbetet och ändrade anonymitet till vänskap, medan negativ bemötande gav negativ spelupplevelse som gjorde att man tappade lust att spela ett tag, eftersom man hade höga förväntningar på andra. För att kunna förbättra Counter Strike för anonyma spelare rekommenderar vi att spelutvecklarna förbättrar sitt säkerhetssystem, genom att blockera fuskare och griefers snabbare än vanligt, förändra blockeringen av fuskare till att omfatta både konto och datorsystem, samt göra spelet dyrare. Vi rekommenderar även att tillämpa en region- eller landfunktion när man ska spela med fyra andra anonyma, för att förhindra språkmissförstånd samt underlätta kommunikation med varandra när man hamnar med spelare från samma land och talar ett gemensamt språk.