FISHERIES RESEARCH INSTITUTE,MOA,TAIWAN

This study was devoted to screening of levan-producing bacteria from brackish water tilapia culture pound. Five strains with levan-producing capability were isolated. To investigate the sucrose utilization of the isolates, the yield amount and average molecular weight of produced levan were examined. In addition, the structural characteristics of the levans were analyzed by partial acid hydrolysis and nuclear magnetic resonance (NMR) spectroscopy. During the 168 h incubation period in the medium containing 12% sucrose at 28℃, strains M1, M3, and M6 produced 20.35-23.01 mg/mL levan after 48 h incubation, while the sucrose in the medium has been used in the range of 75-84%. The amount of levan produced by strain M11 was 19.81 mg/mL after 96 h incubation, and its sucrose utilizations rate only reached 64%. Strain T1 from 24 h to 48 h incubation had produced levan 12.71 mg/mL and 29.15 mg/mL, respectively. After 48 h and till 120 h, the amount of produced levan is keeping evenly to 29 mg/mL, as its sucrose utilization rate was 92%. The amount of levan then decreased gradually with prolonged incubation time after 120 h incubation. The main average molecular weight of levans produced by the isolates M1, M3, M6, M11, and T1 were 10.79, 10.78, 10.31, 1.78 and 14.71 KDa, respectively. The hydrolysates of derived from acid and heating treatments of the levan were mainly fructose. The ¹H NMR and ¹³C NMR spectra of the products of the 5 isolates were very similar to peak positions for levan produced. Based on obtain data, the fructans produced by the 5 isolates were found to be the levan containing β(2-6)-linked backbone.