| Lactic acid bacteria（LAB）is not only the gut microorganism which has important physiological functions,but also one of the most important microorganisms in food industry.Lux S/AI-2 quorum sensing（QS）system is widely found in LAB,which can produce signal molecule AI-2 and participate in regulating the physiological functions of bacteria.The purpose of this study was to investigate the regulation of LAB physiological characteristics by signal molecule AI-2,and the Lux S/AI-2 QS system was deeply explored when LAB strains under environmental stresses and co-cultured with yeast or its cell-free fermentation supernatant（CFS）.（1）The Lux S/AI-2 QS system of Enterococcus faecium 8-3 and Lactobacillus fermentum 2-1 under environmental stresses were investigated.The results showed that acid stress induced the secretion of AI-2 and improved S-ribosylhomocysteine lyase（LuxS）and S-adenosylhomocysteine nucleosidase（Pfs）genes transcription level.Alkaline stress promoted/inhibitted the secretion of AI-2.Low temperature stress inhibited the AI-2 secretion of tested strains and high temperature promoted the AI-2secretion of L.fermentum 2-1,but the transcription of LuxS/AI-2 QS-related genes were promoted.The activity of AI-2 was decreased at the initial stage under mild osmotic stress and then increased with the extension of culture time,while under severe osmotic stress,the activity of AI-2 decreased significantly without further increase with the prolonging of culture time.While LuxS and Pfs genes transcription level were positively correlated with osmotic pressure.The deficiency of nutrition induced AI-2 secretion and the transcription of the LuxS gene,the effect was positively correlated with the level of nutritional stress.All of these suggested that the LuxS/AI-2 QS system of E.faecium 8-3and L.fermentium 2-1 played a regulatory role against the environmental stresses.（2）The Lux S/AI-2 QS system of lactic acid bacteria（LAB）cocultured with yeast and its CFS were examined.The results showed that the CFS of Saccharomyces cerevisiae YE4 inhibited the growth,acid production and biofilm formation of E.faecium8-3 and L.fermentium 2-1 by different degrees.However,the secretion of AI-2 was promoted in varying degrees and the transcription of LuxS and Pfs genes were significantly affected.Compared with the CFS,viable count,acid production and biofilm formation of the tested strains were less effected when co-cultured with the yeast,also showed changes in AI-2 activity and gene transcription levels.Therefore,the Lux S/AI-2QS system of LAB was affected by co-culturing with S.cerevisiae YE4 or its CFS.（3）Signal molecule AI-2 was synthesisd in vitro and its effects on the physiological characteristics of LAB were studied.The recombinant plasmids were successfully constructed by using the LuxS and Pfs genes from E.faecium 8-3.Then inducing expression of recombinant protein was optimized.Optimized conditions included the OD595nm reached from 0.3 to 0.7 by using SOC medium,then cultivated at 37°C last for12h by adding 0.1mmol/L（Isopropylβ-D-thiogalactopyranoside,IPTG）.The target protein could be able to detect in the supernatant.After the proteins were purified and concentrated,the AI-2 molecule with high biological activity was synthesized by reaction with S-adenosylmethionine.The activity of synthesized AI-2 decreased with the extension of incubation time and kept stable after 10h.At the same time,its activity in the alkaline condition declined faster,conversely,its activity decreased slowly under acidic conditions and retention time was relatively long.The synthesized AI-2 in vitro was added to the culture medium of E.faecium 8-3and L.fermentum 2-1.The promoting effect of growth was positively correlated with the concentration of added AI-2 for both strains.The promotion effect mainly appeared in the logarithmic phase and stable phase of E.faecium 8-3,and occured in the logarithmic phase of L.fermentium 2-1.When the concentration of exogenous AI-2 was 60μmol/L,there was no significant effect on the biofilm formation of the tested strains.But it promoted the acid and mild alkaline resistance of E.faecium 8-3,while there was no effect on the resistance to severe alkaline.Exogenous AI-2 has no effect on L.fermentium2-1 acid resistance,but improved its resistance to mild alkaline and reduced its resistance to severe alkali stress.The above results indicated that AI-2 was involved in regulating the physiological functions of the strains and played a part in resistance to multiple environmental stresses.（4）The regulatory effect of AI-2 on LAB and the co-culture process of LAB and yeast were explored from the perspective of proteomics.The results showed that 15proteins were differentially expressed under the action of exogenous AI-2.The metabolic pathways contained cysteine and methionine metabolism,amino acid synthesis,RNA transport and so on.With the addition of exogenous signal molecule AI-2,the differential protein expression in the metabolic pathway of cysteine and methionine increased significantly,which was mainly Pfs protein involved in AI-2 synthesis.There were 82proteins were differentially expressed with the addition of S.cerevisiae YE4 CFS.The metabolic pathways contained QS system,ABC transporter,biotin metabolism,peptidoglycan synthesis,protein transport,cysteine and methionine metabolism and so on.The expression of differential protein in QS metabolic pathway was significantly increased among others.There were 50 differentially expressed proteins when co-culturing with S.cerevisiae YE4.The metabolic pathways contained QS system,protein transport,amino acid biosynthetic pathway and so on.Differentially expressed proteins were down-regulated in the QS metabolic pathway.The expression of differential proteins in QS metabolic pathway were differently among co-culturing with S.cerevisiae YE4 or its CFS.The above results indicated that the effects of yeast and its CFS on the QS system of LAB were 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