As a significant strategy for inhabitation, exopolysaccharides (EPS) are biopolymers that produced by microorganisms to protect the cells under harsh environmental conditions. They have attracted tremendous research interests due to their structural features, physicochemical properties, as well as the health promoting effects. Lactic acid bacteria (LAB) are significant producers of EPS which has been widely used in both food and pharmaceutical industries. Since the diverse and complex chemical structures of EPS, it is important to explore superior strain with improved yield, functional and bioactive properties of EPS. In this study, an EPS-producing strain S28 was successfully isolated from dried milk cake from Inner Mongolia. The production yield of EPS by strain S28 can reach up to (126.4±4.3) mg/L. Based on the physiological, biochemical and morphological observation, the strain S28 was preliminarily characterized to be a strain of LAB. Subsequently, 16S rDNA sequencing was conducted and the strain was characterized as Pediococcus pentosaceus S28. P. pentosaceus S28 can grow in abundance in acidic environment and the optical density (OD595) value of culture medium was increased to about 1.0 after 48 h incubation in MRS culture medium of pH 4.5. P. pentosaceus S28 also showed tolerance to bile salts which could survive after 3 h fermentation in MRS culture medium containing 0.3%-1.0% bile salts. About half of P. pentosaceus S28 was survived after digestion by simulated gastric fluid in vitro, and the survival rate after continuous simulated intestinal digestion was 35%. EPS produced by P. pentosaceus S28 was purified by TCA treatment, ethanol precipitation, and dialysis at a molecular weight cut-off of 8 000-14 000 Da. Gel chromatography analysis was conducted and the molecular weight of EPS was identified to be 9.82×105 Da. Based on the monosaccharide composition analysis by HPLC, mannose (32.38%), glucose (33.20%) and galactose (26.16%) were identified as the major monomers (>91% in total) of EPS. Besides, EPS also contained small amount of glucuronic acid (4.82%), xylose (0.76%), arabinose (0.90%), and fucose (1.78%). Infrared spectroscopy scans demonstrated that functional groups of carbonyl group, hydroxyl group, and glycosidic bonds exist in EPS. Eight strains of Lactobacillus were selected to test the in vitro prebiotic activities of EPS as the only carbon source in MRS medium. EPS showed strain specific prebiotic activity, and the growth of Lactobacillus delbrueckii subsp. bulgaricus NRRL B-548, Lactobacillus reuteri CICC 6132, Lactobacillus casei AS 1.62, and Lactobacillus casei subsp. casei NRRL B-1922 were significantly improved by EPS with the optical density (OD595) values were higher than that of the negative control. On the contrary, Lactobacillus brevis NRRL B-4527, Lactobacillus coryniformis subsp. coryniformis NRRL B-4391, Lactobacillus acidophilus NRRL B-4495, and Lactobacillus delbrueckii subsp. lactis AS 1.2625 couldn’t use EPS as the carbon source for growth with no significant changes of the optical density (OD595) as compared with the negative control. The isolation and identification of P. pentosaceus S28 provides the theoretical basis for the development and utilization of abundant microbial resources in traditional Chinese fermented foods. P. pentosaceus S28 is tolerant to acids, bile salts, and simulated gastrointestinal digestion, thus has great potential to be developed into probiotics. The prebiotic activity of EPS produced by P. pentosaceus S28 indicated that EPS could have great health beneficial effects through the modulation of gut microbiota. Findings gained in this study will introduce the potential utilization of P. pentosaceus S28 for EPS production and application in function foods.
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