Cultivable Bacteria In The Hexi Corridor Desert And Their Anti-radiation Activity And Mechanisms

Using culture-dependent and 16S rRNA gene sequencing methods,97 bacterial strains were isolated and identifieed from hypolithic and peripheral soils collected from the desert of Hexi Corridor.The method of simulating ultraviolet radiation in the natural environment was used to measure the isolates anti-radiation activity,and the strains with the most radiation tolerance were screened.The extracellular polysaccharides?EPS?of the most radiation-resistant strain was extracted and purified to characterize the structure and the factors affecting EPS production.Furthermore,we measured the antioxidant activity of EPS to scavenge the free radicals in vitro.We want to reveal whether the EPS play a role in resisting radiation in the desert bacteria.The main results are as follows.1.We isolated a total of 310 bacterial strains from ten soil samples collected in typical gravel Gobi Desert in the west of Hexi Corridor.The analysis of 16S rRNA gene sequencing aligned the strains to 97 bacteria which belong to three major bacterial phyla,Actinobacteria,Firmicutes,and Proteobacteria,and 34 genera.Among them,66 and 31 bacterial species were identified from the hypolithic and peripheral samples,respectively.Arthrobacter,Kocuria,Pseudomonas,Agrococcus,Planomicrobium,Streptomyces,and Bacilus are common to both types of samples.2.The activity of antiradiation of the 97 strains was measured.The result indicated that these bacterial strains displayed a certain degree of anti-radiation activity.Of those,4 strains,S3-2,S6-11,S8-7,and S9-8,which showed the hightest survival rate,were selected for further study.To measure whether the metabolites have antiradiation activity,we detected the protection effect of the fermentation bruth on E.coli exposure to UV radiation.The result showed that the survival rate of E.coli exposure to UV radiation was increased by 22.33%by the use of the fermentation bruth of strain S6-11.The antiradiation activity of strain S6-11 was5.59 times relative to E.coli.16S rRNA gene sequence analysis showed 97.95%similarity of S6-11 with Arthrobacter enclensis and it is assigned as Arthrobacter enclensis S6-11.3.To determine the factors that affect the EPS production of A.enclensis S6-11,the culture conditions were optimized using the single factor test.The optimal conditions that affected the growth of strain S6-11 were:temperature 30°C,pH 7,inoculation amount 5%,and shaker speed 160 rpm,and the medium compositions were 50 g/L of sucrose,8 g/L of KNO₃,and 6 g/L of KH₂PO₄.The EPS was extracted by condensing the fermentation bruth to 1/3 of the original volume and then adding 3 times of ethanol at pH 6.4.The EPS of A.enclensis S6-11 was purifided using the column chromatography and UPLC technologies.The phenol-sulphate colorimetry method assayed the EPS content of12.57 mg/mL.The ultraviolet absorption method determined 2.7%of protein content in the EPS.The sulfuric acid-carbazole method assayed the content of 9.43%uronic acid.The purified EPS exhibited the characteristic adsorption peak of polysaccharides which consists of glucuronic acid,glucose,and mannose arabinose with a molar ratio of 0.20:0.94:0.66:0.61.The EPS could scavenge 68.1%,50.86%,and 40.08%of DPPH,O₂^-,and–OH,respectively,indicating the great antioxidation capacity of the EPS secreted by strain S6-11.Our experimental results imply that EPS play a role in bacterial antiradiation and this may be one of the mechanisms for survival of bacteria in the Hexi Corridor desert habitats with strong solar radiation.