Chapter 10 Pathogenesis and the gastrointestinal tract of growing fish

This chapter presents information on (1) adhesion of bacteria to mucosal surfaces, (2) protection against bacterial adhesion, (3) bacterial translocation, (4) invasion of host cells, (5) effect of diet in disease resistance, and (6) data obtained from endothermic animals, which may have relevance to pathogenesis of fish. A number of bacteria cause pathology in the gut of fish and this can be a route of systemic infection in many instances, comparable to that of invasive enteropathogens of mammals. To adhere successfully, colonize, and produce disease, the pathogen must overcome the host defence system. Adhesion of bacteria to surfaces such as epithelial cells involves different types of interaction, depending on the distance separating the bacteria from the surface. The indigenous intestinal flora is prevented from gaining access to other sites in the body by a single epithelial cell layer on the mucosa. The mechanisms that help the bacteria to translocate from the gut to appear in other organs are an important phenomenon in the pathogenesis of opportunistic infections by indigenous intestinal bacteria. Entry into host cells is a specialized strategy for survival and multiplication utilized by a number of pathogens that can exploit existing eukaryotic internalization pathways. Methods including the use of expressed markers such as green fluorescent protein and laser confocal microscopy will provide more definitive analysis of pathways of invasion by pathogens taken up via the gastrointestinal (GI) tract.