Mortality of Escherichia coli O157:H7 in Two Soils with Different Physical and Chemical Properties

Wild and domesticated animals can harbor a pathogenic Escherichia colt strain designated as O157:H7. Potential health problems could occur if strain O157:H7 is a more robust survivor in defecated waste than commonly used indicator bacteria. A laboratory study was conducted to assess E. colt O157:H7 survival relative to a nonpathogenie E. colt strain in two soils with different physical and chemical characteristics. Bacteria in the inoculated soils were enumerated on a weekly basis for 8 wk using a most probable number (MPN) technique. First-order decay models were used to describe bacteria mortality in the soils. Decay series were described slightly better by a two-stage function than by a single-stage function. Strain O157:H7 exhibited similar mortality patterns to the nonpathogenic E. colt in the same soil environment. Both E. ¢oli strains bad greater mortality rates in Pope silt loam (coarse-loamy, mixed, active, mesic Fluventic Dystrudept) than Zanesville silt loam (fine-silty, mixed, active, mesic Oxyaquic Fragiudalf). Differences in available soil water probably were the overriding factor in E. colt survival. Escherichia colt O157:H7 survival could be modeled in the same way as nonpathogenic E. colt and appears to have a slightly higher mortality rate. O of the main concerns with pathogenic enteric microorganisms is their survival in soil after manure or biosolids deposition. Any soil condition that favors extended growth and survival of enteric pathogens presents a health hazard because it increases the likelihood of disease transmission. Consequently, it is important that soil conditions influencing the survival of these microorganisms be determined. Several studies on the survival of enteric microorganisms outside of the human or animal gastrointestinal tract indicate that the mortality rate is initially very high (Crane et al., 1980). Two to three months is sufficient in most cases to reduce pathogens to negligible numbers once they have been applied to soil (Zhai et al., 1995). However, survival for as long as 5 yr has been documented (Gerba and Bitton, 1984; Rudolfs et al., 1950). Survival times among different bacteria and even different bacterial strains vary greatly and it is generally expensive and time consuming to test for enteric pathogens individually. Nonpathogenic indicator bacteria of enteric origin, which are easier and less expensive to monitor, are often used to model the survival of pathogens in soil and water. Current methods to rapidly detect fecal indicator bacteria use defined substrate technology Department of Agronomy, Univ. of Kentucky, Lexington, KY 405460091. Contribution (Paper no. 00-06-33) of the Kentucky Agric. Exp. Stn., published with the approval of the director. Received 17 Feb. 2000. *Corresponding author ([email protected]). Published in J. Environ. Qual. 29:1821-1825 (2000). to identify specific organisms (Covert et al., 1992). particular, the capacity of fecal coliforms (i.e., Escherichia coli) to hydrolyze the fluorescent indicator compound MUG (4omethylumbelliferyl [3-D-glucuronide) has been exploited to provide presumptive evidence for fecal contamination of water (Rice et al., 1990, 1991). Some fecal coliforms are unable to hydrolyze MUG, which means they give false negative responses to this quick test (Coyne and Shuler, 1994). Among the E. coli strains demonstrating a MUGnegative response is the virulent enterohemorrhagic (EHEC) pathogen designated as O157:H7, which has been found in asymptomatic cattle, sheep, swine, deer, dogs, horses, and fowl (USDA, 1997). This is the predominant EHEC strain in the USA and it has caused several notable disease outbreaks due to contamination of drinking water, ground beef, and swimming pools. The consequences of long-term pathogenic fecal coliform survival in soil are serious, particularly if those strains are more robust than nonpathogenic indicator bacteria and constitute an increasing fraction of the soil fecal coliform population with time. The detection and survival of O157:H7 has been examined in waste, food, and water (Hovde et al., 1999; Pyle et al., 1999), but comparative studies relating the survival of E. coli strain O157:H7 in soil to typical nonpathogenic indicator fecal coliforms are lacking. This study was therefore conducted to compare the mortality rate of E. coli O157:H7 with that of a nonpathogenic E. coli strain in two typical Kentucky soils with different physical and chemical characteristics. MATERIALS AND METHODS Soil Analysis and Experiment Design Two soils were selected for the study, Pope silt loam and Zanesville silt loam, that varied appreciably in physical and chemical properties (Table 1). The soils were analyzed for particle size and other common soil properties (e.g., exchangeable bases, cation exchange capacity [CEC], extractable N, and soil organic matter) using standard methods of soil analysis (Soil Survey Staff, 1992). Soil pH was determined in a 1:1 soil to deionized water slurry. Soil water potential was determined by the psychrometer thermocouple method using a pressure plate (Rawlins and Campbell, 1986). The experiment design was a two by two factorial with two types of soil and two strains of bacteria. Experimental units consisted of 96 polyethylene bags. Abbreviations: EHEC, enterohemorrhagic Escherichia coli; MUG, 4methylumbelliferyl ~3-D-glucuronide.