| Nowadays,foodborne diseases caused by foodborne pathogenic microorganisms have become one of the most serious food safety issues in the world.Salmonella,as a common foodborne pathogen that caused 40%～60%of bacterial food poisoning of the whole country,is one of the key monitoring objects in food safety surveillance.Fresh vegetables are popular with people due to their high nutrition.However,they have been considered sources of many foodborne diseases due to pathogenic bacteria pollution.The bacterial absorption on the surface of fresh vegetables is the important stage of the whole process of contamination.So to scavenge the adsorption of bacteria on the surface of fresh vegetables is of great significance to ensure public health.In this study,lettuce was taken as the object used as the research,Salmonella Enteritidis（ATCC13076）and Salmonella Typhimurium（ATCC14028）were used as tested strains.Through the way of artificial pollution,the adsorption of Salmonella on the lettuce surface under different conditions and the scavenging effects of bacteriophage（LPSE1 and LPST10）to Salmonella were studied.The main results are as follows:1.The study of the adsorption mechanism of Salmonella on the surface of lettuce under different conditions.In this chapter,the adsorption of Salmonella on lettuce surface under different temperatures（4,16,24 and 32℃）,different p H（4.0～9.0）and different ionic strength（0～100 m M）was studied and DLVO theory was used to analyze adsorption mechanism of Salmonella on lettuce surface.（A）With the increase of temperature（4～32℃）and time（5～60 min）prolongation,the adsorption capacity of Salmonella Enteritidis improved on lettuce surface（3.59～5.20log CFU/cm2）,and the adsorption isotherm of Salmonella Enteritidis on lettuce surface can be well matched with the isothermal adsorption equation--Freundlich equation Y=kf×X1/n（R2>0.95）.With the increase concentration of Na+（1～100 m M）,the adsorption of Salmonella Enteritidis increased which conformed to the DLVO theory.However,with the increase concentration of Ca2+（1～100 m M）,the adsorption capacity showed a declining trend,which deviated from the DLVO theory.It is possible that the double electric layer was extremely compressed in the high concentration of Ca2+.The adsorption capacity decreased with the increase of p H values（4.0～9.0）.Higher p H values led to OH-migrate to the double electric layer and the accumulation of negative charge,resulting in the increase of the electrostatic repulsion.Meanwhile,the p H value can affect the adsorption capacity by changing the surface charge density,which influenced by the chemical groups of ionization degree on the surface of microorganism.（B）With the increase of temperature（4～32℃）and time（5～60 min）prolongation,the adsorption capacity of Salmonella Typhimurium improved on lettuce surface（3.59～5.20 log CFU/cm2）,and the adsorption isotherm of Salmonella Typhimurium on lettuce surface can also be well matched with the isothermal adsorption equation--Freundlich equation Y=kf×X1/n（R2>0.95）.With the increase concentration of Na+（1～100 m M）,the adsorption of Salmonella Typhimurium increased which conformed to the DLVO theory.However,with the increase concentration of Ca2+（1～100 m M）,the adsorption capacity showed a declining trend,which deviated from the DLVO theory.It is possible that the double electric layer was extremely compressed in the high concentration of Ca2+.The adsorption capacity decreased with the increase of p H values（4.0～9.0）.Higher p H values led to OH-migrate to the double electric layer and the accumulation of negative charge,resulting in the increase of the electrostatic repulsion.Meanwhile,the p H value can affect the adsorption capacity by changing of the surface charge density,which influenced by the chemical groups of ionization degree on the surface of microorganism.2.The comparison of sodium hypochlorite and bacteriophage on the scavenging effect of Salmonella on the surface of lettuce.（A）The results of the scavenging experiments of Salmonella Enteritidis（or Salmonella Typhimurium）on lettuce surface by using 10 mg/L sodium hypochlorite were shown below.Under the condition of 4℃,with the prolongation of interaction time（0～48 h）between Salmonella Enteritidis（or Salmonella Typhimurium）and lettuce,the scavenging amount of Salmonella Enteritidis（3.84～0.55 log CFU/cm2）（or Salmonella Typhimurium（3.83～0 log CFU/cm2））adhere to lettuce surface showed a decreasing trend after measured by sodium hypochlorite.However,the scavenging rate reached 90%in the first two hours of Salmonella Enteritidis（or Salmonella Typhimurium）interacting with lettuce under both 4℃and 24℃.（B）The scavenging of Salmonella Enteritidis（or Salmonella Typhimurium）on lettuce surface by phage（LPST1 and LPST10）under different times,temperature and MOI value.a)Under the condition of 4℃,the scavenging amount increased with the prolongation of the interaction time（0～48 h）between Salmonella Enteritidis and lettuce and the increase of phage MOI（0.1,1,10）,and the scavenging amount of MOI=10（0～1.75 log CFU/cm2）is much higher than that of MOI=1（0～1.35 log CFU/cm2）and MOI=0.1（0～1.08 log CFU/cm2）.Under the condition of 24℃,with the prolongation of reaction time（0～48 h）between Salmonella Enteritidis and lettuce and the increase of phage MOI（0.1,1,10）,the scavenging amount increased at first and then decreased.At12 h,the scavenge amount reached the maximum（1.80 log CFU/cm2）,and the scavenge amount of MOI=10（0～1.80 log CFU/cm2）was much higher than that of MOI=1（0～1.35 log CFU/cm2）and MOI=0.1（0～1.08 log CFU/cm2）,the maximum scavenging rate reached 60%.b)Under the condition of 4℃,the scavenge amount increased with the prolongation of the interaction time（0～48 h）between Salmonella Typhimurium and lettuceand the increase of phage MOI（0.1,1,10）,and the scavenge amount of MOI=10（0～2.36 log CFU/cm2）is much higher than that of MOI=1（0～0.67 log CFU/cm2）and MOI=0.1（0～0.21 log CFU/cm2）.Under the condition of 24℃,with the prolongation of reaction time（0～48 h）between Salmonella Typhimurium and lettuce and the increase of phage MOI（0.1,1,10）,the scavenging amount increased first and then decreased.At 12 h,the scavenging amount reached the maximum（2.65 log CFU/cm2）,and the scavenging amount of MOI=10（0～2.65 log CFU/cm2）was much higher than that of MOI=1（0～2.16 log CFU/cm2）and MOI=0.1（0～1.47 log CFU/cm2）,the maximum scavenging rate reached 70%.（C）Scavenging effects of sodium hypochlorite and mixed phage on the mixed strains of Salmonella Enteritidis and Salmonella Typhimurium at 24℃a)With the prolongation of reaction time（0～48 h）between the mixed strains and lettuce,the scavenging amount of the stains adhered to the lettuce surface caused by 10mg/L sodium hypochlorite decreased from 3.84 to 0.29 log CFU/cm2.At the same time,the scavenging rate in the first two hours of sodium hypochlorite was better than that of the other time,which reached to 90%.b)The scavenging effects of mixed phage（LPSE1:LPST10=1:1）on the mixed strains of Salmonella Enteritidis and Salmonella Typhimurium at different time and MOI were studied.The results was shown that:with the prolongation of the interaction time（0～48 h）between the mixed strains and lettuce and the increase of phage MOI（0.1,1,10）,the scavenging amount increased at first and then decreased.The scavenge amount（1.44 log CFU/cm2）of MOI=1 reached maximum after reacting 8 h and the scavenging amount（1.35 log CFU/cm2）of MOI=10 reached maximum after reacting 12 h with the scavenging rate of 60%. |
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