益生菌被視為相對友善且可作為水產養殖上替代抗生素抵抗病菌感染之免疫刺激物,本研究中由吳郭魚腸道中篩選一株分泌蛋白酶、澱粉酶、木聚醣酶與脂肪酶之菌株,該菌株經由16S rDNA序列與生化分析鑑定為巨大芽孢桿菌(Bacillus megaterium),於飼料中添加105、106 與107 CFU/g餵食吳郭魚8週以評估其對成長與先天免疫之功效。結果顯示吳郭魚每日餵食107 CFU/g之巨大芽孢桿菌(B. megaterium),與控制組校較可顯著改善體重增加、生長速率、飼料效益與存活率,並增加腸道中消化酵素如澱粉酶、纖維素酶、木聚糖酶與脂肪酶之活性。此外,吳郭魚頭腎白血球之吞噬活性、呼吸爆與血液中之溶菌酶活性均顯著高於控制組;而頭腎白血球之超氧化物岐化酶活性則顯著低於控制組。吳郭魚每日餵食107 CFU/g之巨大芽孢桿菌(B. megaterium),肝臟分子層次的營養代謝與生長相關基因,以及頭腎與脾臟之免疫相關基因表現,相對於控制組均顯著增加。上述結果說明每日餵食巨大芽孢桿菌(B. megaterium)可改善吳郭魚腸道消化酵素與免疫調節,具有作為吳郭魚養殖疾病控制應用之潛力。
Probiotics are considered ecofriendly alternatives to antibiotics and immunostimulants against pathogen infections in aquaculture. In the present study, a protease‐, amylase‐, xylanase‐ and cellulase producing isolate was isolated from the gut of Nile tilapia. The isolated was identified as Bacillus megaterium by 16S rDNA sequencing and biochemical analysis. The beneficial effects of Bacillus megaterium on growth and innate immunity in Nile tilapia were evaluated by feeding tilapia a basal diet or basal diet containing 105, 106 and 107 CFU/g for 8 weeks. In the present study, the dietary supplementation with 107 CFU/g of B. megaterium obtained improvement of growth performances including weight gain (WG), specific growth rate (SGR), feed efficiency (FE) and survival rate (SR) of Nile tilapia. The results showed that Nile tilapia fed with 107 CFU/g of B. megaterium exhibited significant increasing of digestive enzyme activities (such as amylase, cellulase, xylanase and lipase activities) compared to those of fish in control group. Moreover, the respiratory burst (RB) and phagocytic activity (PA) of head kidney leukocytes as well as the serum lysozyme activity of Nile tilapia fed with 107 CFU/g of B. megaterium were significantly increased compared to the control group. Meanwhile, superoxide dismutase (SOD) levels in the head kidney leukocytes of Nile tilapia fed with 107 CFU/g of B. megaterium were also significantly reduced than those in fish of control group. The molecular expression level of growth- and metabolism-, immune-related genes were significantly induced in Nile tilapia fed with 107 CFU/g of B. megaterium compared to those in fish of control group. The fish fed with B. megaterium displayed a lower cumulative mortality rate than control group subsequent challenge test with Streptococcus iniae. Therefore, it indicates that the administration of B. megaterium can improve digestive enzyme activity and enhance immune responses in Nile tilapia.