Effects of dietary supplementation with microbial phytase on the growth, bone minerals, antioxidant status, innate immunity and disease resistance of African catfish fed on high soybean meal-based diets

The microbial phytase, derived from Buttiauxella gaviniae, Yersinia mollarettiv and Hafnia spp., is proven to be safe for avian and porcine feeding and promotes their overall growth performance. Here, we have evaluated microbial phytase's effects on the growth, bone mineral content, antioxidant status, immune responses and the resistance of African catfish (Clarias gariepinus) fed with high soybean meal-based diets against Aeromonas hydrophila infection. Five isonitrogenous diets (40% protein) were supplemented with different levels of microbial phytase ranging from 0 as a control to 250, 500, 750 and 1000 FTU/kg diet. African catfish (n = 300; 8.5 +/- 0.3 g) were allocated in 15 50-L tanks (in triplicates) and were fed on the prepared tested diets for 12 weeks. After the end of the feeding period of 12 weeks, 10 fish from each replicate was intraperitoneally infected with A. hydrophila (0.5 x 10(5) CFU/ml) and monitored for 14 days. Dietary phytase levels linearly and quadratically improved the growth performance of African catfish and stimulated feed intake. Bone levels of calcium, phosphorus, magnesium and zinc were also positively modulated in phytase-fed fish, especially at 750-1000 FTU/kg diet. Similarly, counts of red and white blood cells as well as haemoglobin, packed cells volume, platelets, lymphocytes and heterocytes were significantly modulated in all fish fed with phytase-supplemented diets. Higher levels of serum total protein, albumin and globulin were also observed in fish fed with a 750-1000 FTU/kg diet of phytase. Conversely, aspartate and alanine aminotransferase activities were lower in fish fed with a 750-1000 FTU/kg diet of phytase, compared to those fed the control diet. Moreover, antioxidant enzymes (superoxide dismutase, catalase and glutathione peroxidase), reduced glutathione levels and immune responses (lysozyme, respiratory burst, protease and alkaline phosphatase activities) biomarkers were linearly and quadratically elevated, while malondialdehyde values were linearly and quadratically decreased in fish groups fed with phytase-based diets. After administering A. hydrophila, 60.0% of the fish fed the control diet perished, while no mortalities were observed in fish fed with 750-1000 FTU/kg diets. Taken together, the current study reveals that dietary phytase could improve the growth performance, blood profile, bone mineralization, antioxidant activities, immunity and overall protection of African catfish against A. hydrophila infection. Dietary phytase may be efficiently used in the feeding of African catfish to enhance their overall performance and mitigate health conditions with optimum level of 900 FTU/kg diet.