Electron microscopy of the intestinal microflora of fish

Abstract

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.

Introduction

It is accepted that fish possess specific intestinal microbiota consisting of aerobic, facultative anaerobic, and obligate anaerobic bacteria, but the bacterial composition may change with age, nutritional status, and environmental conditions. The intestinal microbiota have been classified as autochthonous or indigenous (when they are able to colonise the host's gut epithelial surface) or as allochthonous or transient. In this context, light and electron microscopic examinations of gut samples are important tools for investigating the microbial ecology of the gastrointestinal (GI) tract ecosystem and determining the presence of autochthonous or allochthonous microbiota. Several studies on various fresh- and saltwater fish using transmission electron microscopy (TEM) and/or scanning electron microscopy (SEM) have demonstrated bacteria in the intestinal lumen and associated with the intestinal epithelium Lesel and Pointel, 1979, Austin and Al-Zahrani, 1988, Hansen and Olafsen, 1999, Hansen et al., 1992, Kuperman and Kuz'mina, 1994, Grisez et al., 1996, Bergh et al., 1997, Ringø and Olsen, 1999, Ringø et al., 2001, Ringø et al., 2002, Hellberg and Bjerkås, 2000, Lødemel et al., 2001. Furthermore, some studies have revealed translocation of bacterial cells by endocytosis in the GI tract of both fish larvae Hansen and Olafsen, 1999, Hansen et al., 1992, Grisez et al., 1996 and adult fish Ringø et al., 2001, Ringø et al., 2002. Uptake of intact bacterial antigens has also been detected Hansen and Olafsen, 1990, Olafsen and Hansen, 1992.

This review provides an overview of electron microscopical studies on gut-associated bacteria together with a critical evaluation of the results obtained so far. This is highly relevant as the digestive tract is a potential port of entry for pathogens Chair et al., 1994, Olsson, 1995, Olsson et al., 1996, Grisez et al., 1996, Romalde et al., 1996, Jöborn et al., 1997, Robertson et al., 2000, Lødemel et al., 2001. Prevention of disease requires knowledge about where the pathogens colonise within the GI tract and in which regions the highest frequencies of translocation occur. Finally, directions for further research are proposed. As this review focuses on electron microscopical studies on gut-associated bacteria in the fish intestine, readers with special interest in bacterial species colonising the GI tract of different fish species are referred to the comprehensive reviews on this topic by Cahill (1990), Ringø et al. (1995), Hansen and Olafsen (1999) and Ringø and Birkbeck (1999).