Production of bimodal molecular weight levan by a Lactobacillus reuteri isolate from fish gut

An exopolysaccharide (EPS) synthesizing potentially probiotic Gram-positive bacterial strain was isolated from fish (Tor putitora) gut, and its EPS was structurally characterized. The isolate, designated as FW2, was identified as Lactobacillus reuteri through 16S rRNA gene sequencing and phylogenetic analysis. This isolate produces fructan-type EPS using sucrose as a substrate. Based on ¹³C-NMR spectroscopy, methylation analysis and monosaccharide composition, the EPS was identified as a linear levan polymer with fructose as main constituent linked via β(2 → 6) linkages. Based on molecular weight (MW) distribution, two groups of levan were found to be produced by the isolate FW2: one with high MW (4.6 × 10⁶ Da) and the other having much lower MW (1.2 × 10⁴ Da). The isolate yielded about 14 g/L levan under optimized culturing parameters including aeration conditions, pH, temperature and substrate concentration. The obtained bimodal molecular weight linear levan is the first of its type to be synthesized by a L. reuteri isolate from fish gut. Bimodal molecular weight prebiotic levan together with the probiotic potential of the producing strain would provide a new promising synbiotic combination for use in aqua culture.