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.

The microbiome plays an important role in lipid metabolism but how the introduction of probiotic communities affects host lipid metabolism is poorly understood. Using a multidisciplinary approach we addressed this knowledge gap using the zebrafish model by coupling high-throughput sequencing with biochemical, molecular and morphological analysis to evaluate the changes in the intestine. Analysis of bacterial 16S libraries revealed thatLactobacillus rhamnosuswas able to modulate the gut microbiome of zebrafish larvae, elevating the abundance of Firmicutes sequences and reducing the abundance of Actinobacteria. The gut microbiome changes modulated host lipid processing by inducing transcriptional down-regulation of genes involved in cholesterol and triglycerides metabolism (fit2, agpat4, dgat2, mgll,hnf4α, scapandcck) concomitantly decreasing total body cholesterol and triglyceride content and increasing fatty acid levels.L. rhamnosustreatment also increased microvilli and enterocyte lengths and decreased lipid droplet size in the intestinal epithelium. These changes resulted in elevated zebrafish larval growth. This integrated system investigation demonstrates probiotic modulation of the gut microbiome, highlights a novel gene network involved in lipid metabolism, provides an insight into how the microbiome regulates molecules involved in lipid metabolism and reveals a new potential role forL. rhamnosusin the treatment of lipid disorders.