Screening And Identification Of LAB With High Biofilm Production And Study On The Secretion Conditions Of EPS

Lactic acid bacteria(LAB)in the state of biofilm can effectively resist adverse external environment,so as to better play the probiotic characteristics.Extracellular polysaccharide(EPS),as the main component of biofilm,can improve the texture,flavor and viscosity of the product,so it has attracted much attention in food processing.In this study,10 strains of LAB with high production of biofilm were screenedto explore thepotential probiotic characteristics.The strains of RJ2-1-4 and TG1-1-10 with better probiotic characteristics were screened for the study of EPS.The optimal culture conditions were obtained by optimizing the secretion conditions of EPS.The main results were as follows:The 41 strains of LAB isolated from fermented dairy products were used as research objects.The 10 strains of LAB with high biofilm production were screened by the method of crystal violet staining.According to the sequence analysis of 16 S rRNA gene fragment,RM1-1-4,TG1-1-10,RJ2-1-4 and TG1-1-11 were Pediococcus lactici.RJ1-1-4 was Lactobacillus plantarum.TG7-1-1 was Lactococcus lactis.TG7-1-6 was Lactococcus lactis subsp.cremoris.RM3-1-4 and RM2-1-5 were Entrococcus durans.TG5-1-5 was Leuconostoc mesenteroides.The results of acid and alkali tolerance test showed that Pediococcus lactici TG1-1-11,TG1-1-10,RJ2-1-4 and Enterococcus durans RM2-1-5 with high tolence of acid resistance.The Lactobacillus plantarum RJ 1-1-4,Lactococcus lactis TG7-1-1,Lactococcus lactis subsp.cremoris TG7-1-6,Leuconostoc mesenteroides TG5-1-5,Pediococcus lactici RM 1-1-4 and Eterococcusdurans RM3-1-4 showed better alkali resistance.Bile salt tolerance test results showed that the strains of Pediococcus lactici TG1-1-11,TG1-1-10,RJ2-1-4,Enterococcus durans RM2-1-5,RM3-1-4 and Lactobacillus plantarum RJ1-1-4 had good bile salt tolerance.The results of artificial stomach and intestinal fluid tests showed that the Pediococcus acidilactici of RJ2-1-4 and TG1-1-10 had the better tolerance to extreme environment.The Pediococcus acidilactici of RJ2-1-4 and TG1-1-10 with good probiotic characteristics obtained in previous experiments were used to further experiment.The change rule of EPS production was analyzed by changing the inoculum size,culture time,pH value of the medium and NaCl concentration.The results showed that the EPS production reached the maximum upon the condition of RJ2-1-4 and TG1-1-10 inoculum size was 3% and 1%,respectively.As the culture time prolonged,the production of EPSincreased first and then decreased.As the pH of the medium increased,the production of EPS increased first and then decreased.With the increase of NaCl concentration,the production of EPS decreased first,then increased and then decreased.Finally,the LAB medium composition was optimized.The optimal medium composition of RJ2-1-4 to produce eEPS was obtained by orthogonal test.The optimal medium combination was 20 g/L glucose,20 g/L soy peptone,and 8 g/L dipotassium hydrogen phosphate.The optimal medium combination for the strain of TG1-1-10 was 40g/L glucose,20 g/L soy peptone,and 2 g/L dipotassium hydrogen phosphate.