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 study
revealed that at the beginning of inoculation, the number and the
vital force ofE. coliO157:H7 were high, and capable of competing
for 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. The
ıvalues that represent the time needed for first decimal reduction also varied from each other (Fig. 2). This confirmed that the
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