Bacterial translocation and pathogenesis in the digestive tract of larvae and fry

The digestive tract of fish is essentially a muscular tube lined by a mucous membrane of columnar epithelia] cells that exhibit regional variation in structure and function. In the last two decades, our understanding of translocation of bacteria in the fish gut has increased, and electron microscopy has contributed to this knowledge. Translocation is the movement of viable and non-viable bacteria and bacterial products (such as endotoxin) from the intestinal lumen through the epithelial mucosa to infect otherwise sterile tissues. This paper provides an overview of the gastrointestinal tract of larvae and fry from an electron microscopical perspective focussing on factors (mucus, interactions between probiotics and pathogens, and stress) affecting translocation of bacteria (transcellular and paracellular), cellular damage (specific attack on tight junctions and desmosomes caused by pathogenic bacteria) and pathogenesis. Furthermore, the paper will present information on the occurrence and translocation of pathogenic bacteria in the gastrointestinal tract of larvae, fry and adult fish. Currently, it is generally accepted that the major routes of infection in fish are through the skin, gills and gastrointestinal tract. As the gastrointestinal tract is a potential port of entry for pathogens, the use of electron microscopy in studies on microbiota in fish digestive tract is a valuable method for increasing our understanding of the mechanisms involved in entry of the pathogens. Possible mechanisms involved in translocation of bacteria in fish gastrointestinal tract, autochthonous (indigenous) and pathogenic, are discussed. (C) 2007 Elsevier B.V All rights reserved.