Effects of Dietary Rosemary ( <i>Rosmarinus officinalis</i> ) Leaf Powder and <i>Bacillus subtilis</i> on the Growth Performance, Digestive Enzymes, Antioxidant and Immune-Related Gene Expression, and Resistance to <i>Aeromonas hydrophila</i> in Comm

Abstract Due to increasing demand about the application of natural feed additives in aquaculture, the efficacy of separate and the combination of rosemary ( Rosmarinus officinalis ) and Bacillus subtilis on the growth performance, blood indices, innate immunity, antioxidant defense, digestive enzymes activity, the expression levels of genes with immunity modulation, as well as resistance against Aeromonas hydrophila , in common carp, Cyprinus carpio is reported. Fish (44.63±0.04 g) were divided into four treatments including the T0 (control treatment) fed with basal diet, T1 fed with rosemary at the rate of 3%, T2 fed with 1.5×10 7 CFU g–1 B. subtilis , and T3 fed with a combination of rosemary at the rate of 3% and 1.5×10 7 CFU g–1 B. subtilis. After 56 days enriched feed with B. subtilis and combination of rosemary and B. subtilis caused significant improvement in weight gain (WG %), feed conversion ratio (FCR), hematological and biochemical indices, the innate immune responses including total Ig level, lysozyme and ACH50 activities as well as the chymotrypsin, pepsin, protease, and amylase activities correlated to the control group (P&lt;0.05). Additionally, the serum CAT and GPX activities were significantly enhanced in the T3 group (P&lt;0.05). The mRNA expression of CAT and LYS genes significantly increased in T2 and T3 contrary to T0 (P&lt;0.05). Fish challenged with A. hydrophila had variable survival and the best survival was in the T3. In summary, the combination of the rosemary and B. subtilis enhanced immune parameters and the ability of fish to cope with A. hydrophila as a fish pathogen. These findings indicate the potential of these substances as a promising feed additive.