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

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 (10g/kg XOS or GOS). These diets did not include Cr2O3 (-Cr2O3 diets). Three similar additional diets were formulated to include 5g/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 5g/kg Cr2O3 does not interfere with gilthead sea bream (Sparus aurata) gut microbiota and gut morphology, suggesting that a dietary incorporation level of 5g/kg Cr2O3 can safely be used as inert marker in digestibility studies.