4. DiscussionWe assessed the effect of organic fertilizer on the survival of akey bacterial pathogen,E. coliO157:H7 in agricultural soils withvarious physicochemical properties. With the same incubationtemperature and moisture, the survival ofE. coliO157:H7 variedin soils as a result of organic fertilizer amendment. The Weibullmodel proved to be a suitable model for describing the decline ofE. coliO157:H7 in the test soils. Even though the Weibull modelis an empirical model, it can be linked to different survival patterns of pathogen (linear whenp= 1, concave whenp> 1 and convexwhenp<1)[25].Ifp> 1, it means that the remaining cells becomeincreasingly susceptible to stress and their destruction rate willincrease with time[6,25]. Correspondingly, the destruction rate ofpathogens will decrease with time ifp<1.p= 1 is characteristic ofsimple, constant, first order die-off curves, indicating that the dyingprobability of the pathogen does not depend on time, i.e., thereis no accumulated damage and/or there is no biological variationin the pathogen population, antagonistic microbial community orpredatory activity[6]. With the present study, thepvalues of mosttest soils were more than 1, which indicates a small possibility offirst-order decline in the survival ofE. coliO157:H7 in manureamended soil. The previous study reported that the antagonism orpredation could change over time, resulting in fluctuating stress forE. coliO157:H7 survival in soil[6]. Hence, thepvalues in this studyrevealed that at the beginning of inoculation, the number and thevital force ofE. coliO157:H7 were high, and capable of competingfor ecological niches with soil indigenous microbial communities.However, along with incubation time, the metabolites of indigenous bacteria may result in the decay ofE. coliO157:H7 cells. Theıvalues that represent the time needed for first decimal reduction also varied from each other
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