This study investigated the effects of microbiota-directed fibers (MDFs) – α-mannans and β-mannan – as feed supplements in Atlantic salmon. Multi-omic analysis (16S rRNA gene profiling, metatranscriptomics, targeted metabolomics, and salmon organ transcriptomics) revealed negligible effects on host gene expression, gut microbiome structure, and function at a 0.2% dietary inclusion. A higher dose (4%) of β-mannan significantly shifted microbiome composition but lacked biologically relevant effects on salmon. Only *Burkholderia-Caballeronia-Paraburkholderia* showed consistent abundance shifts. The study highlights the need for high-resolution microbiome characterization for designing effective MDF strategies and identifies potential endogenous lactic acid bacteria as targets for future interventions.

Shashank Gupta, Arturo Vera-Ponce de León, Miyako Kodama, Matthias Hoetzinger, Cecilie G. Clausen, Louisa Pless, Ana R. A. Verissimo, Bruno Stengel, Virginia Calabugi, Renate Kvingedal, Stanko Skugor, Bjørge Westereng, Thomas Nelson Harvey, Anna Nordborg, Stefan Bertilsson, Morten T. Limborg, Turid Mørkøre, Simen R. Sandve, Phillip B. Pope, Torgeir R. Hvidsten, Sabina Leanti La Rosa