The potency of agarolytic bacteria as producer of exogenous agarase enzyme to improve growth of abalone juvenile (Haliotis asinina Linn. 1758)

Abstract

There is a great potential of using probiotics in aquaculture to increase growth rates and improve nutritional status of cultured animals. The aims of this research were to isolate and select agarolytic bacteria, to identify isolates based on physiological, biochemical and molecular properties, to analyze agarase activity of the isolates, to evaluate colonization ability of agarolytic isolates in the gastrointestinal tract of abalone, and to analyze the potency of Alg3.1-Abn1.2 bacterial strains as exogenous agarase producer to improve growth of abalone. Selection of agarolytic bacteria as probiotic was based on criteria of their antibiotics resistance, degree of agar liquefaction, and growth rate. Dinitrosalysilic acid was used to measure in vitro and in situ agarase activity. Viable cell count was used to determine whether the probionts could colonize gastrointestinal tract of abalone. Growth performance of gnotobiotic and normal abalone were evaluated and compared with abalone fed cake-Gracilaria supplemented with probiotic strains. The result of isolation can be achieved 14 agarolytic bacteria. Seven of them showed antibiotic resistance. Based on their growth rate, degree of agar liquefaction and qualitative agarolytic activity were carried out three isolate that potential as exogenous agarase producers, i.e. Alg4.2, Alg3.1 and Abn1.2 isolates. These isolates were identified as Vibrio. The ability of Alg3.1 strain to degrade agar was higher than the others, whereas the highest starch degradation was achieved by Abn1.2 strain. All isolates showed high capacity to degrade casein. Generally, the degree of carbohydrate and protein hydrolysis were increased up to 48 h of incubation. In vitro agarase activity of mix culture Alg3.1-Abn1.2 (0.593 nkat/mL) was higher than that of single culture of Alg3.1 (0.489 nkat/mL), Alg4.2 (0.423 nkat/mL) and Abn1.2 (0.411 nkat/mL). So the mix culture was chosen as candidate of probiotic. Abalone fed diet supplemented with mix culture of Alg3.1-Abn1.2 strains exhibited higher growth rate compared to that of abalone fed standard diet under laboratory conditions. The increasing of biomass of gnotobiotic and normal abalone supplemented with Alg3.1-Abn1.2 strains were higher 32.22 and 11.09% respectively than that of gnotobiotic and normal abalone fed standard diet. The improving of gaining daily shell length of gnotobiotic and normal abalone supplemented with probiotics were higher 24.17 and 21.88% than that of abalone fed unsupplemented diet. The efficiency and the feed conversion of abalone which supplemented with Alg3.1-Abn1.2 strains was better than that of control abalone. Application of Alg3.1-Abn1.2 strains as diet increased the activity of agarase and amylase exogen in the abalone digestive tract. The number of culturable cell reisolated from abalone fed probiotic-supplemented cake for 14 days was 106- 107 cfu/g.