Screening And Identification Of Exopolysaccharide Producing Strains In Dunhuang Saline-alkali Soil And Study On Their Polysaccharide Activities

As an active substance produced by bacterial metabolism,bacterial exopoly-saccharide has the advantages of short growth cycle,high yield,unique physical and chemical properties,thus has become one of the hotspots of current research.There are many kinds of microorganisms capable of producing exopolysaccharides,but only a few them have been put into industrial production.Therefore,the development of novel microbial exopolysaccharides with application value is an urgent problem to be solved in the world.Because the exopolysaccharides produced by microorganisms in extreme environments are more functional and diverse,the Hexi Corridor can be further explored as a potential bacterial source for producing exopolysaccharide microorganisms with application value.In order to develop exopolysaccharide that can be put into industrial production,thelaboratoryisolatedandscreenedastrainofhigh-yield exopoly-saccharide from the saline-alkali soil in Dunhuang area,determined the optimal medium composition,its exopolysaccharide was extracted and purified and studied the activities of the exopolysaccharide.The main research contents and results are as follows:?1?In this study,18 strains of exopolysaccharides were isolated from Dunhuang saline-alkali soil by dilution coating method.A high-yield exopolysaccharide strain II4-01 was screened by phenol sulfuric acid method.It was identified as Bacillus licheniformis bycombiningphysiologicalandbiochemicalcharacteristics,morphological characteristics and 16S rDNA molecular biological identification method;?2?The optimal medium is depended on single-factor optimized medium test and orthogonal test.The results showed that yiled of Bacillus licheniformis II4-01 was in 25 g/L sucrose,25 g/L soybean powder,0.5 g/L MgSO₄,2 g/L Na₂HPO₄·12H₂O,1g/L NaH₂PO₄·2H₂O,the fermentation yield of the fermentation medium at pH 7.5 was the highest,which reached 8.594 g/L.The fermentation medium of strain II4-01 is optimized to increase the production of exopolysaccharide by 10.16 times.?3?The exopolysaccharide of strain II4-01 was extracted and purified by alcohol precipitation,Sevag method and dialysis method.The pure polysaccharide samples were detected by UV and IR.The UV detection results showed that the polysaccharide did not show characteristic absorption peaks at 260 nm and 280 nm,which could exclude the influence of protein and nucleic acid.It is believed that the main component of the product is polysaccharide;The infrared spectroscopy shows that the stretching vibration peak of the polysaccharide conforms to the characteristic peak of the sugar,and further proves that the extract is a polysaccharide substance;?4?The antioxidant of the exopolysaccharide showed that the crude and pure polysaccharide had certain scavenging ability to DPPH·free radical and·OH radical and H₂O₂.Among them,polysaccharide has the strongest ability to scavenge·OH radicals.When the concentration of crude polysaccharide is 8 mg/mL,the clearance rate can reach 90.42%;when the concentration of pure polysaccharide is 8 mg/mL,the clearance rate can reach 61.55%.Therefore,the exopolysaccharide has the potential to be developed into a natural antioxidant;?5?The hygroscopicity and moisturizing activity of the polysaccharide were determined by the humidity control method of the dryer.It showed that the pure polysaccharide had the strongest moisture absorption capacity under the environment of humidity of 43%and 81%.when the RH was 43%,the moisture absorption capacity is pure polysaccharide>xanthan gum>crude polysaccharide>chitosan.Meanwhile,when the RH was 81%,themoisture absorption capacity is pure polysaccharide>crude poly-saccharide>xanthan gum>chitosan;In saturated Na₂CO₃and dry silica gel,pure polysaccharides have the strongest moisturizing ability.Under saturated Na₂CO₃ environment,the moisturizing ability of pure polysaccharides is not obvious compared with chitosan and crude polysaccharides.However,in dry silica environment,the moisturizing activity of pure polysaccharides is obviously superior to chitosan,xanthan gum and crude polysaccharide,so it is feasible to develop pure polysaccharide as a natural moisturizer;?6?The flocculation activity of fermentation broth and polysaccharide on kaolin and Yellow River water was determined.The results showed that the flocculating activity of fermentation broth on kaolin was strong,which could reach 74.72%.The flocculation rate of crude polysaccharide and pure polysaccharide to kaolin was16.07%and 19.42%,respectively.The flocculation activity of the sample on the Yellow River water showed that the flocculating activities of the fermentation broth,crude polysaccharide and pure polysaccharide were 24.05%,26.17%and 28.99%,respectively.It indicates that the flocculation effect of the polysaccharide is not significant.