| Summary: | The gill and gastrointestinal tract are primary entry routes for pathogens. The symbiotic microbiota are essential to the health, nutrition and disease of fish. Though the intestinal microbiota of Nile tilapia (<i>Oreochromis niloticus</i>) has been extensively studied, information on the mucosa-associated microbiota of this species, especially the gill and gastrointestinal mucosa-associated microbiota, is lacking. This study aimed to characterize the gill and gastrointestinal mucosa- and digesta-associated microbiota, as well as the intestinal metabolite profiles in the New Genetically Improved Farmed Tilapia (NEW GIFT) strain of farmed adult Nile tilapia by high-throughput sequencing and gas chromatography/mass spectrometry metabolomics. The diversity, structure, composition, and predicted function of gastrointestinal microbiota were significantly different across gastrointestinal regions and sample types (Welch <i>t</i>-test; <i>p</i> < 0.05). By comparing the mucosa- and digesta-associated microbiota, linear discriminant analysis (LDA) effect size (LEfSe) analysis revealed that <i>Pelomonas</i>, <i>Ralstonia</i><i>pickettii</i>, Comamonadaceae, and <i>Staphylococcus</i> were significantly enriched in the mucosa-associated microbiota, whereas many bacterial taxa were significantly enriched in the digesta-associated microbiota, including Chitinophagaceae, <i>Cetobacterium</i>, <i>Candidatus</i><i>Competibacter</i>, <i>Methyloparacoccus</i>, and chloroplast (LDA score > 3.5). Furthermore, <i>Undibacterium</i>, <i>Escherichia</i>–<i>Shigella</i>, <i>Paeniclostridium</i>, and <i>Cetobacterium</i> were dominant in the intestinal contents and mucosae, whereas <i>Sphingomonas</i><i>aquatilis</i> and <i>Roseomonas</i><i>gilardii</i> were commonly found in the gill and stomach mucosae. The Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt2) analysis revealed that the predictive function of digesta-associated microbiota significantly differed from that of mucosa-associated microbiota (R = 0.8152, <i>p</i> = 0.0001). In addition, our results showed a significant interdependence between specific intestinal microbes and metabolites. Notably, the relative abundance values of several potentially beneficial microbes, including <i>Undibacterium</i>, <i>Crenothrix</i>, and <i>Cetobacterium</i>, were positively correlated with most intestinal metabolites, whereas the relative abundance values of some potential opportunistic pathogens, including <i>Acinetobacter</i>, <i>Mycobacterium</i>, <i>Escherichia</i>–<i>Shigella</i>, <i>Paeniclostridium</i>, <i>Aeromonas</i>, and <i>Clostridium</i><i>sensu</i><i>stricto</i> 1, were negatively correlated with most intestinal metabolites. This study revealed the characteristics of gill and gastrointestinal mucosa-associated and digesta-associated microbiota of farmed Nile tilapia and identified a close correlation between intestinal microbes and metabolites. The results serve as a basis for the effective application of targeted probiotics or prebiotics in the diet to regulate the nutrition and health of farmed tilapia.
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