Intensive ¢sh production worldwide has increased the risk of infectious diseases. However, before any infection can be established, pathogens must penetrate the primary barrier. In ¢sh, the three major routes of infection are the skin, gills and gastrointestinal (GI) tract. The GI tract is essentially a muscular tube lined by a mucous membrane of columnar epithelial cells that exhibit a regional variation in structure and function. In the last two decades, our understanding of the endocytosis and translocation of bacteria across this mucosa, and the sorts of cell damage caused by pathogenic bacteria, has increased. Electron microscopy has made a valuable contribution to this knowledge. In the ¢sh-farming industry, severe economic losses are caused by furunculosis (agent, Aeromonas salmonicida spp. salmonicida) and vibriosis [agent, Vibrio (Listonella) anguillarum]. This article provides an overview of the GI tract of ¢sh from an electron microscopical perspective focusing on cellular damage (speci¢c attack on tight junctions and desmosomes) caused by pathogenic bacteria, and interactions between the 'good' intestinal bacteria [e.g. lactic acid bacteria (LAB)] and pathogens. Using di¡erent in vitro methods, several studies have demonstrated that co-incubation of Atlantic salmon (Salmo salar L.) foregut (proximal intestine) with LAB and pathogens can have bene¢cial e¡ects, the cell damage caused by the pathogens being prevented, to some extent, by the LAB. However, there is uncertainty over whether or not similar e¡ects are observed in other species such as Atlantic cod (Gadus morhua L.). When discussing cellular damage in the GI tract of ¢sh caused by pathogenic bacteria, several important questions arise including: (1) Do di¡erent pathogenic bacteria use di¡erent mechanisms to infect the gut? (2) Does the gradual development of the GI tract from larva to adult a¡ect infection? (3) Are there di¡erent infection patterns between di¡erent ¢sh species? The present article addresses these and other questions. Ã Intestinal sac.wAs only 13 autochthonous bacterial strains were isolated one can not draw a general conclusion.zUssing chamber.PC, pyloric cacea; FG, foregut; MG, midgut; HG, hindgut; HGC, hindgut chamber; NI, not investigated.Lactic acid bacteria vs. pathogens in the gastrointestinal tract of ¢sh E RingÖ et al.
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