It is well known that healthy gut microbiota is essential to promote host health and well‐being. The intestinal microbiota of endothermic animals as well as fish are classified as autochthonous or indigenous, when they are able to colonize the host's epithelial surface or are associated with the microvilli, or as allochthonous or transient (associated with digesta or are present in the lumen). Furthermore, the gut microbiota of aquatic animals is more fluidic than that of terrestrial vertebrates and is highly sensitive to dietary changes. In fish, it is demonstrated that [a] dietary form (live feeds or pelleted diets), [b] dietary lipid (lipid levels, lipid sources and polyunsaturated fatty acids), [c] protein sources (soybean meal, krill meal and other meal products), [d] functional glycomic ingredients (chitin and cellulose), [e] nutraceuticals (probiotics, prebiotics, synbiotics and immunostimulants), [f] antibiotics, [g] dietary iron and [h] chromic oxide affect the gut microbiota. Furthermore, some information is available on bacterial colonization of the gut enterocyte surface as a result of dietary manipulation which indicates that changes in indigenous microbial populations may have repercussion on secondary host–microbe interactions. The effect of dietary components on the gut microbiota is important to investigate, as the gastrointestinal tract has been suggested as one of the major routes of infection in fish. Possible interactions between dietary components and the protective microbiota colonizing the digestive tract are discussed.

A study was conducted to investigate the effect of mannan oligosaccharide (MOS) on the gut microbiota and intestinal morphology of rainbow trout under commercial farming conditions. Juvenile (mean initial BW 38.2 ± 1.7 g) and subadult (111.7 ± 11.6 g) trout were fed 2 dietary treatments for 111 and 58 d, respectively. The control treatment consisted of a standard commercial diet, and the MOS treatment consisted of the control diet supplemented with 0.2% MOS. Morphology of the anterior and the posterior intestine was examined with light and electron microscopy. Light microscopy demonstrated increased gut absorptive surface area in the subadult MOS group. Additionally, electron microscopy revealed an increase in microvilli length and density in the subadult MOS group compared with the control (P < 0.05). However, no significant improvements were detected in the juvenile group. Culture-based evaluation of the intestinal microbiota showed that MOS significantly reduced (P < 0.05) the viable intestinal bacterial populations (by approximately 2 log scales in all cases). Levels of Aeromonas/Vibrio spp. were significantly decreased (P < 0.05) in the juvenile MOS group (9% of the total microbiota) compared with the juvenile control group (37%). Additionally, analysis of microbial communities was conducted using denaturing gradient gel electrophoresis of PCR-amplified 16S rDNA. The denaturing gradient gel electrophoresis fingerprinting revealed an alteration of bacterial populations; analysis of similarity, similarity percentages, and nonmetric multidimensional scaling analysis showed that MOS reduced species richness and increased similarity of bacterial populations found within the subadult and juvenile groups. The current study shows that MOS modulates intestinal microbial communities, which subsequently improve gut morphology and epithelial brush border. Copyright © 2009. .

The effect of dietary probiotics (Bacillus subtilis, Bacillus licheniformis and Enterococcus faecium) used singularly and synergistically on the growth performance, intestinal microbiota and health status of rainbow trout (Oncorhynchus mykiss Walbaum) were assessed after 10 weeks feeding on supplemented diets. No significant improvements of weight gain or specific growth rate were observed in the probiotic fed groups. However, a significant improvement of feed conversion ratio was observed in the group fed E. faecium. High levels of probiotic species were observed in the posterior gastrointestinal tract as transient digesta‐associated populations and potentially resident mucosal populations. Bacillus subtilis and B. licheniformis levels accounted for 36% of the total culturable microbial population adhered to the mucosa and 62% in the digesta. E. faecium levels accounted for 45% of the mucosal population and 89% of the population in the digesta. An increase of serum lysozyme activity was observed in the fish fed diets containing the Bacillus probionts and elevated leukocyte levels were observed in fish fed diets containing Bacillus + E. faecium synergistically. The results of the current study demonstrate potential for B. subtilis, B. licheniformis and E. faecium to improve feed utilization, modulate intestinal microbiota and the health status of rainbow trout.