| Food safety has become a worldwide public health problem of common concern.Listeria monocytogenes which is one of the three major foodborne pathogens mainly spread through the food contamination pathway.The mortality rate can be up to 30%,once the body showed esepticemia,meningitis,corneal etc.The top three foodborne pathogenic bacteria contaminated in animal food in our country lead by Listeria monocytogenes.This pathogenic bacteria has a very tenacious survival characteristic,with high adhesion ability,anti-drying,low temperature resistance,acid and heat resistance,and even the formation of biofilms.In the same study found that Listeria monocytogenes obtained acid tolerance resistance(ATR)lead to the change of protein synthesis patterns after direct exposure to weak acid environment.The improvement of the acid resistance could enhance the ability of the bacteria going through the stomach and improving its pathogenicity,which will eventually lead to the emergence of high-risk strains and further enhance the harm to human.The first step of this experiment for two tested strains Listeria monocytogenes(1/2a)and L.innocua(ATCC 33090)under TSB growth conditions at 37?were monitored,while the basic induced(pH 5.5,lactic)acid reaction were determined.After that the TSB broth(pH = 7.2)was set as the control group,two different extreme pH(5.5 and 5.8)beef during rigor mortis process were set as growth substrates,and cells were collected at 3?5?7?24 and 72 h after cattle slaughtering,bacterial suvivals were measured after being challenged 1h inmedium with pH 3.0 for 1h.of the concentration of lactic acid were tested accordingly.The main results and conclusions are as follows:1.Growth curves of two tested strains of Listeria and basic acid resistance determination: Both strains grew well at 37?TSB and entered the logarithmic growth phase at 1.9h and 2h,respectively,while went into stable phase the maximum number of the bacteria were 9.67 and 8.95 lg CFU/mL,respectively.The growth states were different between two strains,with Listeria innocua faster than Listeria monocytogenes.After exposure in weak acidic medium(pH 5.5)for 4hours,then be challenged in TSB with pH at 3.0 for one hour,the survivals of two strains of bacteria was significantly increased compared with the control group,while Listeria innocua showed a relatively stronger acid resistance than Listeria monocytogenes.2.The acid resistance of two tested strains after being inoculated on cattle carcass during rigor mortis process.The cattle carcass were divided into two groups based on the ultimate pH values(5.5 and 5.8),and the cultures were collected at 3?5?7?24 and 72 h post-slaughter.Results showed that both strains had higher survival percentage than the control group(pH = 7.2)when challenged in pH 3.0.It indicated that both strains of bacteria growth in the beef during the process of rigor mortis can significantly improve its resistance to acid.Through determinanting the lactic acid content in beef during the process of rigor mortis,we find that beef with different ultimate pH value as the increase of rigor mortis and storage time,the lactic acid content also increased.But the increase of two strains bacteria's induced acid strength did not increase the content of lactic acid.There has some relationship between the Listeria monocytogenes enhanced resistance to acid in beef cattle and lactic acid content,but the specific mechanism remains to be further discussed. |
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