Screening Of Exopolysaccharide-producing Lactic Acid Bacteria And Molecular Characteristic Analysis Of Exopolysaccharide

Lactic acid bacteria(LAB)with a generally regarded as safe(GRAS)status are known to synthesize EPSs which are positive contribution against the adverse environments.LAB EPS not only has good rheological properties,but also has probiotic effects,such as anti-oxidation,anti-tumor and so on.Screened EPS-producing strain from china-fermented food which harbored a wide species diversity of wild LAB and then obtained EPS with a specific function,which is a significant research.In this study,two EPS-producing strains named B-2 and L-7 were isolated from the pineapple fermentation broth and home-made sausages respectively by combining with the slimy colonies and polysaccharide production.They were identified as Leuconostoc citreum and Lactobacillus sake via colony morphology,physiological and biochemical experiments and 16s rDNA sequencing,respectively.Leu.citreum B-2 and Lb.sake L-7 were cultured in MRS-S medium at 30?for 48 h,respectively.The crude EPS were prepared by removing thallus and proteins,followed by ethanol precipitation and dialysis.The retentate was further purified using gel column chromatography.The collection was lyophilized and weighted.The EPS yeild achieved to28.3 g/L and 5.3 g/L,respectively.The results of UV-visible spectroscopy showed that the EPSs were absent of protein and nucleic acids.According to gas chromatography(GC)analysis,the monosaccharide component of B-2 EPS was glucose.The result of high performance size exclusion chromatography(HPSEC)analysis showed that the molecular weight of B-2 EPS was 3.77×10~6 Da.The structural characterization of EPS determined by fourier-transform infrared(FT-IR)and nuclear magnetic resonance(NMR)spectroscopy.The EPS produced by Leu.citreum B-2was composed of?-(1?6)linked D-glucopyranose units existing in the main chain with?-(1?3)branching and?-(1?2)branching.By the same methods,L-7 EPS was linear glucan with?-(1?6)glucosidic bond and the molecular weight of L-7 EPS was 1×10~7Da.The surface structure of EPS was obseved by scanning electron microscopy(SEM).The SEM of the B-2 EPS appeared irregular sheets with glittering surface and compact structure.The water solubility index(WIS)and water holding capacity(WHC)of B-2EPS were 80%and 150%,respectively.Equally,the SEM of the L-7 EPS appeared hollow tubular structure.The WIS and WHC were 96%and 272%,respectively.In summary,the characteristic of two EPSs showed a good application prospect.