Tilapia is recognised to adapt well to different production systems. However, despite its ability to cope well with different stress factors and environments, growth performance and health status can be negatively affected under intensive culture.

Developing nutritional tools to support gut health can be a valuable strategy towards reducing pathogen impact and antibiotic utilisation.

The objective of this study was to evaluate the effect of yeast probiotic supplementation in different practical diets on growth performance and gut health of Nile tilapia (Oreochromis niloticus).
Two basal diets were formulated, one with higher fishmeal (10%) and lower soybean meal (14%) content (diet SBM14) and the other with lower fishmeal (5%) and higher soybean meal (28%) content (diet SBM28).

Those basal diets were supplemented or not with 1 g/kg diet of yeast probiotic Actisaf^® (final concentration of 107 CFU/g of feed). All diets were isoproteic, isolipidic and isoenergetic.
Nile tilapia fingerlings with an initial body weight of 0.7g were hand-fed with those diets to apparent satiation, in quadruplicates, for 8 weeks. Growth performance was monitored throughout, and at the end of the trial, two fish per tank were sampled for gut health evaluation.

Midgut microvilli morphology was analysed through transmission electron microscope scanning electron microscopy examination. mRNA expression levels of several inflammation-related cytokines as well as hsp70 were assessed in the hindgut by real-time RT-PCR. An analysis of autochthonous and allochthonous microbiota of the hindgut was also conducted through 16S rRNA gene pyrosequencing.
Yeast probiotic supplementation in both SBM14 and SBM28 diets enhanced fish growth performance and increased midgut microvilli length, potentially resulting in an increased nutrient absorption capacity.

Results of gene transcript expression analyses revealed a down-regulation of hsp70, il1β and tgfβ in the hindgut of fish from SBM14 group when supplemented Actisaf^®, indicating a reduced inflammation status and alleviation of cellular stress. The diversity of gut autochthonous microbiota was not affected by yeast supplementation, but allochthonous microbiota was enriched in Lactococcus spp in both supplemented groups.

In conclusion, these results showed that supplementing practical diets with Actisaf^® globally improves gut health status of Nile tilapia, resulting in better growth performances.