Morphology and biofilm studies of the intestines of the Nile tilapia (Oreochromis niloticus)

Tilapia, a warm fresh-water herbivorous fish, forms a major component of the species used in tropical and subtropical aquaculture, mainly due to its fast growth, efficient use of natural feeds, resistance to diseases, and tolerance to a variety of environmental conditions.

The study investigated the morphology and mucus histochemistry of the intestines of Nile tilapia (Oreochromis niloticus), the biofilm on the microvillar surface of the intestines, and interaction of these biofilms with a microbe prevalent in tilapia habitats, Aeromonas hydrophila. The Nile tilapia intestines were processed for routine histology and electron microscopy. The use of antimucus antibodies in the fixative preserved the biofilm for light and electron microscopic evaluation. The relationship between time, location, numbers of A. hydrophila, and the biofilm was then investigated using an immunofluorescent test.

The intestines of Nile tilapia reflect adaptations for a herbivorous diet, in their length, and presence of mucosal foldings. The thin walled intestines are compacted in the abdominal cavity in complex convolutions bound by the mesentery and mesenteric fat. There are no regional distinctions besides progressive caudal tapering. The Nile tilapia intestines have no structural demarcation between the lamina propria and the submucosa, which borders the bi-layer muscularis and are wrapped in the serosal layer. The luminal surface of the intestines of Nile tilapia, maintained in both flow-through and recirculating fish holding facilities is lined with an adherent biofilm. The biofilm appeared in several forms and, was associated with a mucin matrix and numerous microbial morphotypes. Exposure to A. hydrophila revealed that it is associated with the biofilm, but did not establish autochthonity.

The preservation of the biofilm provides an opportunity to discern the intestinal microbial ecology and dynamics, and its role in herbivorous digestion, gut immunity and the relationship to the microbial population in the aquatic environment.