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. .

Aim: To assess Pediococcus acidilactici as a dietary supplement for on‐growing red tilapia (Oreochromis niloticus). Methods and Results: Tilapia were fed either a control diet or control diet supplemented with Ped. acidilactici at 10⁷ CFU g⁻¹ for 32 days. Ped. acidilactici colonized the intestinal tract and significantly affected the intestinal microbial communities. PCR‐DGGE revealed direct antagonism of gastric Ped. acidilactici with an endogenous uncultured bacterium during a period of reverting to nonsupplemented feeding. Light microscopy revealed that gut integrity and leucocyte levels were unaffected by Ped. acidilactici; however, blood leucocyte levels and serum lysozyme activity were elevated after 14‐days' feeding. No significant improvements in growth performance were observed at the end of the trial (day 32), but survival was significantly higher in the probiotic group. Conclusions: The study demonstrates that oral supplementation of Ped. acidilactici modulates intestinal bacterial communities in on‐growing red tilapia and also stimulates some aspects of the nonspecific immune response. Significance and Impact of the study: To our knowledge this is the first study assessing the effects of probiotics on the gut microbiota of tilapia using culture‐independent methods. Such methods are crucial to understand the mechanisms which underpin and mediate host benefits.