| Due to the characteristics of high pathogenicity and low infection threshold dose,E.coli O157:H7 has become a pathogen of concern worldwide,seriously endangering human health,and even life safety.Seasonal frozen soil is widely distributed in China,and soil freezing is a common natural phenomenon.E.coli O157:H7 is mainly derived from livestock and poultry manure.If its harmless treatment is not thorough,it will greatly increase the risk of infection caused by E.coli O157:H7,if it is applied as organic fertilizer to farmland or discharged into the environment.At low temperatures,the pathogenic bacteria in the soil will enter a dormant state as the soil freezes,and their activity will be greatly reduced.As the temperature in spring rises,they will enter an active state,and even multiply or spread in large areas under certain conditions,increasing the risk of infecting people.Therefore,studying the survival dynamics of E.coli O157:H7 under freezing conditions can better assess the environmental health risks of E.coli O157:H7 in seasonally frozen soils and provide scientific basis for the prevention and control of infection risks of bacterial pathogens.Experiments were conducted to simulates the persistence of E.coli O157:H7 in soils under four freezing conditions(-5? and 30% WHC,-5? and 60% WHC,-15? and 30% WHC,-15? and 60%WHC),and the survival behaviors during the 5-month freezing process were investigated.The scale factor(? days),shape factor(p)and monthly average log reduction of flora(MAR log/m)were obtained by fitting the survival data to the Weibull model.The physical and chemical properties of soil and bacterial communities were determined,the effects of freezing temperature and soil water content on survival were analyzed,and the key factors affecting the survival of E.coli O157:H7 under four experimental conditions were identified.1)The MAR value at-5? is between 0.59 and 0.94(log/m),which is less than 1log/m;at-15?,the MAR value is between 0.15 and 0.46,which is less than 0.5log/m.The shape coefficient p of all survival curves is less than 1,showing a concave curve.When the freezing temperature was-5?,the concentrations of E.coli O157:H7 went down continuously;when the freezing temperature was-15?,the concentrations of E.coli O157:H7 showed a decreasing trend with obvious fluctuation.2)The results of ANOVA showed that freezing temperature was a key factor affecting MAR value,and soil moisture had no significant effect on MAR value.The interaction between freezing temperature and water content on MAR was statistically significant,i.e.,the effect of soil moisture on MAR was different when the freezing temperature varied.While for the scale factor ? and shape parameter p,freezing temperature and soil moisture had a significant effect on ?,and the effect of soil moisture on ? was the same under different freezing temperatures.Freezing temperature and soil moisture had significant impact on p,and soil moisture had different effects on p under different freezing temperatures.3)Mantel and Partial Mantel analysis of MAR,soil physical and chemical properties,and bacterial communities,the results showed that soil physical and chemical properties(such as p H,EC,soil particle size,WSOC,etc.)were the key influencing factors that affected the MAR value,and bacterial communities had no significant impact on survival.Under the condition of-15? and 30% WHC,there was no significant correlation between MAR value and physical and chemical properties,soil bacterial community structure.It might due to the fact that most of the water in the soil exists in frozen state,and the metabolism of microorganisms in the soil was restricted.Combining the coexistence network analysis and the Pearson correlation analysis between MAR and the bacterial phyla with relatively higher abundances,it could be seen that although the soil bacterial community had no significant effect on survival as a whole,there were some bacterial phyla including Acidobacteria and Firmicutes that had a significant correlation with survival. |
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