Feeding gilthead sea bream with increasing dietary bacterial single cell protein level: Implication on growth, plasma biochemistry, gut histology, and gut microbiota

Bacterial single cell protein (SCP) is considered a promising circular protein ingredient for aquafeed, due to the high protein content, and for the possibilities to grow them on different substrates such as organic waste thus leading to low environmental footprint and affordable production costs. Their use as raw material has been assessed in several farmed species, however, research on Mediterranean ones is still scarce. Hence, a study was undertaken to evaluate growth, plasma biochemistry, gut histology and gut microbiota (GM) response of gilthead sea bream (Sparus aurata) fed diets with increasing levels of bacterial SCP in comparison to a control without SCP. Three isonitrogenous and isolipidic extruded diets (44% protein; 19% lipid) were formulated with different bacterial SCP (derived from Corinebacterium glutamicum) level (10% SCP, SPC10; 15% SCP, SCP15; 20% SCP, SCP20) to replace vegetable protein ingredients (total replacement of soy protein concentrate and partial replacement of corn gluten), while a control diet (CTRL) was formulated without SCP. Fish groups of 45 individuals (initial weight: 75 g) were fed to visual satiation over 108 days. At the end of the trial there were no significant differences on growth, feed intake, feed conversion rate, protein efficiency ratio and protein apparent digestibility. Most of plasma parameters were found to be equal for all treatments, except for those related to nucleic acids molecules degradation such as phosphorus and urea which were higher in SCP10 and SCP20 compared to CTRL, respectively. No morphological alterations were found in the intestines of any fish analysed. Different responses of the overall GM structure in relation to the bacterial SCP inclusion level were detected. Specifically, SCP exerted a positive effect on GM internal diversity which increased at increasing dietary SCP inclusion level. In addition, SCP inclusion lead to increase in the abundance of Bacillus spp., taxa which can potentially support nutrition, immune system, and disease resistance. In conclusion, it seems feasible to include up to 20% of SCP dietary level for gilthead sea bream without compromising growth, feed efficiency and health parameters.