Optimization and physicochemical characterization of low molecular levan from Enterococcus faecium MC-5 having potential biological activities

Abstract

In the present study, Enterococcus faecium MC-5 isolated from the fish gut was cultured in the MRS broth to produce exopolysaccharide (EPS). The maximum EPS production was optimized using Response Surface Methodology (RSM) with three different factors which include sucrose, ammonium nitrate and pH. RSM result predicted that maximum EPS (16.48 g/L) production could be attained by supplementing 30 g/L sucrose and 15 g/L ammonium nitrate in the MRS broth with the initial pH of 6.407. The EPS produced was subjected to purification by column chromatography techniques. Monosaccharide analysis by HPLC confirmed the presence of fructose monomer. The purified EPS MC-5 showed a molecular weight of 24 kDa. The purified EPS was further characterized by FT-IR and NMR which confirmed the presence of β-(2,6) fructofuranoside units and suggested that the EPS MC-5 is a levan-type polysaccharide. Congo red analysis suggested that the EPS MC-5 possesses a triple-helix structure whereas the SEM images showed that the EPS has a porous and globular structure. The EPS was found to be semi-crystalline through XRD analysis. Thermogravimetric analysis showed EPS MC-5 possesses higher thermal stability having a degradation temperature of 338.13 °C. The EPS MC-5 exhibited excellent antioxidant activities and showed promising antibiofilm activity against Listeria monocytogenes, Salmonella enterica, Salmonella typhi and Vibrio anguillarum. The present study is the first report of levan-type EPS production by Enterococcus faecium sp., which has been characterized for its physicochemical and biological properties. These properties associated with EPS MC-5 suggested that it can serve as a potential ingredient or supplement in the food system with significant health benefits.