Analysis Of Biological Characteristics Of Salmonella Enteritidis In Different Food Matrixs With Low Water Activity

Low-water activity foods are generally considered unsuitable for the growth of pathogenic bacteria.However,in recent years,Salmonella infections related to low-water activity foods have occurred frequently,and Salmonella has been detected in a variety of low water activity foods,including almonds,peanut butter,milk powder,chocolate and spices.Low-water activity environment can induce cross-resistance of pathogenic bacteria,and even cause changes in virulence of pathogenic bacteria.Previous studies have shown that Salmonella exposed to low water activity can produce cross-tolerance to other environmental stresses including low p H value,bile salt,heat treatment and disinfectants and ultraviolet radiation.In addition,the stress response of Salmonella is also different in different ingredients of low water activity foods.At present,there are few studies on the effects of different low-water activity food substrates on the physiological characteristics of Salmonella,and the specific adaptation mechanism of Salmonella is not clear.In this study,Salmonella Enteritidis was taken as the research object to study the survival rate of Salmonella Enteritidis in the drying storage of 24 h and the survival curve in different low water activity food substrates.And the changes of heat tolerance,gastrointestinal environment tolerance and adhesion and invasion ability of Salmonella in different low-water activity food matrixes of storage.The effects of different foods with low-water activity on the physiological characteristics of Salmonella were studied,and the expression of related genes was detected.The results are as follows:(1)During the drying storage of 24 h,the reduction of Salmonella Enteritidis in chocolate powder,nonfat milk powder and ginger powder was 0.72 ±0.19,2.2 ±0.22 and0.84 ±0.20 Log CFU/g,respectively.In the subsequent long-term storage process,Salmonella Enteritidis in different samples had a similar survival curve and maintained a high survival rate.It shows that the influence of food ingredients on the survival of Salmonella may be more in the initial drying process,and then Salmonella adapts to low water activity and maintains similar survival ability in different samples.(2)In the three food matrixs,the cross-resistance of desiccation-adapted Salmonella Enteritidis increased with the extension of storage time(P < 0.05).The desiccation-adapted Salmonella Enteritidis in ginger powder has the strongest heat resistance.Compared with desiccation-adapted Salmonella in nonfat milk powder,desiccation-adapted Salmonella Enteritidis in chocolate has relatively strong thermal cross resistance.(3)In the in vitro digestion experiment,the food matrix had an important effect on the gastrointestinal simulated environment of desiccation-adapted Salmonella Enteritidis.The tolerance of desiccation-adapted Salmonella Enteritidis in three food substrates was in the following order: chocolate powder > skimmed milk powder > ginger powder.Food ingredients have an important influence on the survival of low water activity adaptive Salmonella Enteritidis in the in vitro digestion process.(4)Compared with desiccation-adapted Salmonella Enteritidis in chocolate,desiccation-adapted Salmonella Enteritidis in nonfat milk powder had stronger adhesion and invasion ability to Caco-2 cells,while desiccation-adapted Salmonella Enteritidis in chocolate powder had no invasion ability to Caco-2 cells.In the first month of dry preservation,extension of the storage time could increase the adhesion rate to Coca-2 cells for desiccation-adapted Salmonella Enteritidis in chocolate powder and nonfat milk powder,and reduce the invasion rate to Coca-2 cells for desiccation-adapted Salmonella Enteritidis in nonfat milk powder.However,there was no significant difference in adhesion rate and invasion rate of desiccation-adapted Salmonella Enteritidis to Caco-2 cells after 4 weeks and8 weeks of storage.(5)The expression of fad A of desiccation-adapted Salmonella Enteritidis stored in ginger powder for 4 weeks was significantly higher than that in chocolate powder and nonfat milk powder.However,for the seven genes ots B,ots A,rpo S,dna K,rpo H,grp E and pro V,the effect of food composition on the gene expression of desiccation-adapted Salmonella Enteritidis was not significant.However,the expression of related genes in desiccation-adapted Salmonella Enteritidis was different in different storage time.The up-regulation of otsB,otsA,dnaK,rpoH,grpE and proV genes in desiccation-adapted Salmonella enteritidis stored for 4 weeks was significantly lower than that in the initial storage samples.(6)Compared with non desiccation-adapted Salmonella Enteritidis,the expression of osmotic protective substance related genes ots B,ots A,proV and fadA and stress response regulatory gene rpo S in desiccation-adapted Salmonella Enteritidis were significantly up-regulated.This suggested that that the production of osmotic protective substances and stress response regulatory factor Rpo S are important for the survival of Salmonella Enteritidis in low water activity environment.At the same time,the expression of heat tolerance-related genes dnaK,rpoH and grpE were also significantly up-regulated,which may be the reason why Salmonella Enteritidis has stronger heat tolerance in dry environment.