Early pathogenesis of Yersinia ruckeri infections in rainbow trout (Oncorhynchus mykiss, Walbaum)

Yersinia ruckeri is the causative agent of yersiniosis or enteric redmouth disease (ERM) and causes significant losses in salmonid aquaculture worldwide. Although infection with this agent has been reported in other fish species, salmonids and especially rainbow trout are most susceptible to ERM. Vaccination of fish with a commercial Y. ruckeri bacterin provides good levels of protection, however, disease outbreaks do occur from time to time. The understanding of how this pathogen causes disease is crucial in order to develop new preventive and therapeutic approaches. The aim of this thesis was to study the early pathogenesis of Y. ruckeri infections, with emphasis on the identification of the portal of entry and the investigation of different traits in vitro that have been associated with bacterial virulence. The General Introduction gives an overview of the micro-organism, Y. ruckeri, and the disease it causes. From this review it is clear that only little information is available about the pathogenesis of Y. ruckeri. Only recently, the mechanisms by which the pathogen causes disease have started to be unravelled. In the first experimental study, the portal of entry of Y. ruckeri and its tissue distribution at different time intervals were investigated using an immersion infection model in rainbow trout. This model strongly mimics the natural way of infection and therefore, is suitable to study infectious diseases in fish. Additional experimental infections were carried out to determine whether a different tissue distribution is related to differences in virulence. Bacteriological and histological examination revealed the presence of high numbers of bacteria in the gills immediately after infection. Many Y. ruckeri were found to be attached to the gill mucus and some were detected within the gill capillaries between 0 and 2.5 h post infection (p.i.). This indicates that bacteria first adhere to gill mucus and thereafter invade the branchial vasculature leading to colonization of the internal organs. These findings show that the gills are an important portal of entry. Nevertheless, only a virulent strain was able to survive and multiply in the host, causing septicaemia and death several days after infection whereas exposure to avirulent strains resulted in a complete removal of bacteria from the internal organs. This could indicate that immune evasion is a major virulence property of Y. ruckeri. In the second experimental study, the early interactions of virulent and avirulent Y. ruckeri strains with the gill and gut tissue of rainbow trout were determined using a standardized gill and gut perfusion model. For this purpose, perfused gill and gut explants were inoculated for 1 h with Y. ruckeri and thereafter, processed for bacteriological, histological and immunohistochemical examination for demonstration of bacterial adherence. For the first time, both perfusion models were also used to study bacterial invasion. Therefore, the perfusion fluid at the efferent gill arch during gill perfusion and the organ bath fluid following gut perfusion were examined bacteriologically. A non-invasive Escherichia coli strain was included as negative control for invasion. In both the gill and gut perfusion models, virulent and avirulent Y. ruckeri strains highly adhered to gill and gut tissue and significant differences were not observed. Bacteria were seen in the mucus, associated with the epithelium and invaded in the lamina propria of the gills. Y. ruckeri was also observed in the crypts, attached to the villi and within the epithelial layer of the gut. Few bacteria were also detected in the intestinal lamina propria mucosae. Our results showed that virulent and avirulent Y. ruckeri invaded the gill and gut whereas a non-invasive E. coli strain was not isolated from the perfusion fluid and organ bath fluid in the gill and gut perfusion model, respectively. These findings indicate that Y. ruckeri is able to colonize and invade the gills and intestine of rainbow trout. Significant differences in adherence and invasive capacity were not observed between virulent and avirulent strains. In the third experimental study, different traits that have been associated with bacterial virulence were compared between virulent and avirulent Y. ruckeri strains, including adhesion to mucus and cell lines, invasion and intracellular survival in cell lines and serum resistance. The adhesion and invasion mechanisms were further characterized for selected Y. ruckeri strains. The virulent isolates highly adhered to isolated gill and gut mucus of rainbow trout, whereas the majority of the avirulent strains demonstrated significantly lower adhesion. A decrease in adherence capability following bacterial treatment with sodium metaperiodate and proteolytic enzymes suggested that carbohydrates as well as proteins play a role and consequently, lectins may be involved in the adhesion. All Y. ruckeri strains were able to adhere to and invade CHSE-214, FHM and R1 cells. One avirulent strain showed high adherence and invasive capacity in the three cell lines, whereas the virulent strains were moderately adhesive and invasive. The internalization of several isolates was inhibited by colchicine and cytochalasin-D, suggesting the involvement of microtubules and microfilaments. Monodansylcadaverine never inhibited Y. ruckeri internalization and therefore, the formation of clathrin-coated pits in receptor-mediated endocytosis may not play a role. For all strains, intracellular survival assays showed a decrease of viable bacteria in the cells 6 h after inoculation. This suggests that Y. ruckeri is not able to multiply or survive inside cultured cells or remains intracellularly for only a short period. Analysis of the susceptibility to the bactericidal effect of rainbow trout serum demonstrated that virulent strains were serum resistant, whereas avirulent strains were generally serum sensitive. Taken together, adherence to mucus and resistance to the bactericidal effect of serum seems to be correlated with in vivo virulence. In the General Discussion, the results of the three experimental studies are discussed and hypothetical virulence mechanisms are proposed in relation to the known virulence factors of Y. ruckeri. Finally, perspectives are given on how these findings may contribute to the prevention of ERM. In conclusion, the gills and gut were identified as possible important portals of entry for Y. ruckeri in rainbow trout. This pathogen is shown to be able to adhere to and invade in non-phagocytic cells. The adherence of the pathogen seems to involve bacterial lectins that bind specific carbohydrate structures on host surfaces. Invasion appears to be a microtubule and microfilament dependent process. Virulence of Y. ruckeri isolates is probably a complex combination of adhesion and colonization capacity, resistance to the bactericidal effect of serum and other factors which still needs to be determined

L'emploi des marqueurs d'accord dans les débats télévisés néerlandophones et francophones : à la recherche d'un ethos communicatif 'belge' perdu?

Cette étude s’inscrit dans un ensemble de recherches cherchant à dégager le style conversationnel, voire l' « ethos communicatif » (Kerbrat-Orecchioni 2002) des communautés néerlandophone et francophone de Belgique, dans une approche comparative et interculturelle de l’analyse de la conversation (à la française). Elle porte plus particulièrement sur le marquage de l’accord. A partir de l’analyse de deux séries de débats télévisés comparables, l’étude fait ressortir des différences significatives qui montrent que les francophones ont tendance à marquer l’accord de manière plus emphatique, que ce soit par l’usage de marqueurs témoignant d’un engagement plus fort ou formellement renforcés. S’il est vrai que les néerlandophones utilisent plus de marqueurs d’accord, ceux-ci constituent des marques plus faibles, fonctionnant comme de simples régulateurs/accusés de réception ou, encore, comme des signes d’accord provisoire, typiques d’un style conversationnel consensuel. Ces résultats rattachent les francophones de Belgique à l’éthos communicatif des Français caractérisé par un style plus engagé (« high involvement style », e.a. Stadler 2006), voire plus confrontationnel (Béal 2010), alors que les néerlandophones s’alignent plutôt sur le style conversationnel anglo-saxon

In vitro markers for virulence in Yersinia ruckeri

In this study, different traits that have been associated with bacterial virulence were studied in Yersinia ruckeri. Two isolates that had been shown to cause disease and mortality in experimentally infected rainbow trout were compared with five avirulent isolates. Both virulent isolates showed high adhesion to gill and intestinal mucus of rainbow trout, whereas the majority of non-virulent strains demonstrated significantly lower adhesion. A decrease in adherence capability following bacterial treatment with sodium metaperiodate and proteolytic enzymes suggested the involvement of carbohydrates and proteins. All strains were able to adhere to and invade CHSE-214, FHM and R1 cells. One non-virulent strain was highly adhesive and invasive in the three cell lines, whereas the virulent strains showed moderate adhesive and invasive capacity. The internalization of several isolates was inhibited by colchicine and cytochalasin-D, suggesting that microtubules and microfilaments play a role. For all strains, intracellular survival assays showed a decrease of viable bacteria in the cells 6h after inoculation, suggesting that Y. ruckeri is not able to multiply or survive inside cultured cells. Analysis of the susceptibility to the bactericidal effect of rainbow trout serum demonstrated that virulent Y. ruckeri strains were serum resistant, whereas non-virulent strains were generally serum sensitive

Openings and closings in tourist offices in Belgium, France and the Netherlands.: A relational analysis of their structural properties

This paper deals with openings and closings in 400 service encounters in tourist offices situated in Belgium’s two main language communities, Flanders and Wallonia, in the north of France and the south of the Netherlands. On the basis of a detailed, bottom-up quantitative analysis of the structural properties of the openings and closings, we draw part of the interactional profiles of the tourist office encounters. Differences between the four regions are shown to be related to the degree of volubility and involvement of the interactants and to the degree of ritualisation and efficiency of the opening and the closing section

L’autopromotion en contraste : une analyse sémantico-pragmatique des résumés LinkedIn publiés en France et aux Etats-Unis

Cette contribution est vouée à la description cross-culturelle des stratégies d’autopromotion utilisées par des professionnels de la communication français et américains dans leur résumé publié sur LinkedIn. En mettant à contribution un corpus de quelques 200 résumés, l’analyse porte, plus précisément, sur la sélection des stratégies sémantico-pragmatiques destinées à présenter l’auteur comme un professionnel compétent. S’inscrivant dans le cadre théorique de la politesse, l’étude met au jour quelques différences qui se laissent rattacher à des différences plus générales situées sur le plan du style de communication. Ainsi, certaines caractéristiques des résumés français pointent vers un style de communication davantage orienté sur l’implicite, alors que les résumés américains présentent plus de traits d’un style de communication explicite. Par ailleurs, il s’est avéré aussi que les modes d’autopromotion français et américain semblent reposer (partiellement) sur des « constructions de soi » différentes : si les résumés américains manifestent plus de caractéristiques d’une construction de soi indépendante, les résumés français présentent plus de traits d’une construction de soi interdépendante.This contribution presents a cross-cultural analysis of self-praise strategies used by French and American communication professionals in their LinkedIn summary. Based on a corpus of some 200 summaries, the analysis focuses on the selection of the semantico-pragmatic strategies aimed at presenting the author as a skilful professional. Within the theoretical framework of politeness, the study reveals some differences that can be linked to more general differences in communication style. Some characteristics of French summaries point to a more implicit communication style, while American summaries have more features of an explicit communication style. Moreover, the data tend to show that the French and American modes of self-promotion are (partially) based on different "self-construals": whereas American summaries show more characteristics of an independent self-construal, French summaries show more of the features of an interdependent self-building

‎« Comme » devant l’attribut de l’objet : une approche ‎constructionnelle

Cet article examine les structures à attribut de l’objet en comme et sans comme dans le cadre théorique de la grammaire constructionnelle. Il évalue, plus précisément, deux solutions qui ont été envisagées dans ce cadre pour rendre compte du problème de l’existence de structures minimalement différentes : la première analyse chacune des structures comme des constructions différentes ; la deuxième consiste à les considérer comme des instanciations (ou “allostructions”) d’une seule construction. Au terme d’une étude lexicale, sémantique, catégorielle et informationnelle des structures attributives concernées, c’est l’analyse en termes d’allostructions qui l’emporte, la structure sans comme étant la variante non marquée et celle avec comme la variante marquée.This paper examines resultative and depictive object complement structures with and without comme (’as’) within the framework of construction grammar. It compares and evaluates, more specifically, two solutions which have been proposed to tackle the existence of slightly different structures : the first analyzes each structure as a distinct construction, whereas the second solution consists in analyzing them as instantiations (“allostructions”) of one single construction. Our lexical, semantic, syntactic and informational account of the object complement structures is supportive of the second solution, the structure without comme (’as’) being the unmarked and the comme structure the marked form