Gut microbiota and gut morphology of gilthead sea bream (<italic>Sparus aurata</italic>) juveniles are not affected by chromic oxide as digestibility marker.

Abstract: The present work aimed to evaluate whether the use of chromic oxide (Cr₂O₃) as dietary inert marker in fish digestibility studies interferes with gut microbial community modulation and gut morphology. To assess the effects of Cr₂O₃ under potential diverse microbiota populations, dietary Cr₂O₃ was tested using challenging plant feedstuffs (PF)‐based diets supplemented or not with prebiotics, as prebiotics are expected to modify gut microbiota populations. For that purpose, three diets were formulated to include circa 20:80 fish meal and PF as protein sources, without (CTR) or with prebiotic supplementation (10 g/kg XOS or GOS). These diets did not include Cr₂O₃ (−Cr₂O₃ diets). Three similar additional diets were formulated to include 5 g/kg Cr₂O₃ (+Cr₂O₃ diets). Cr₂O₃ effects on gut microbiota were assessed for the first time in the allochthonous (digesta) and autochthonous (mucosa) community by a culture‐independent molecular approach, using denaturing gradient gel electrophoresis (DGGE). No differences in gut bacterial profiles (number of operational taxonomic units, microbiota richness, diversity and similarity indices) were observed between dietary treatments. No significant alterations in submucosa layer structure, enterocytes and eosinophilic granular cells structure, goblet cells and leucocytes quantity were detected in the distal intestine among diets. In conclusion, data indicate that dietary inclusion of 5 g/kg Cr₂O₃ does not interfere with gilthead sea bream (<italic>Sparus aurata</italic>) gut microbiota and gut morphology, suggesting that a dietary incorporation level of 5 g/kg Cr₂O₃ can safely be used as inert marker in digestibility studies. [ABSTRACT FROM AUTHOR]