Electron microscopy of the intestinal microflora of fish

The role, and even the existence, of stable indigenous microbiota in fish was not accepted until the 1970s. In the last decade, our understanding of adhesion and translocation of bacteria in the fish gut has increased, and electron microscopy has contributed significantly to this knowledge. This review summarises the information available on gut-associated bacteria and on the translocation of bacteria in fish gastrointestinal tract. In several studies on various fresh- and saltwater fish, bacteria in the intestinal lumen and epithelium-associated bacteria have been demonstrated by using transmission electron microscopy and/or scanning electron microscopy. Some of these studies have demonstrated translocation of bacterial cells by endocytosis in the gastrointestinal tract of larvae and adult fish as well as uptake of intact bacterial antigens. Endocytosis of bacteria in the digestive tract is highly relevant as the gastrointestinal tract is a potential port of entry for pathogens. (C) 2003 Published by Elsevier B.V.