Survival and Persistence of Nonpathogenic Escherichia coli and Attenuated Escherichia coli O157:H7 in Soils Amended with Animal Manure in a Greenhouse Environment

Abstract

Animal manure provides benefits to agriculture but may contain pathogens that contaminate ready-to-eat produce. U.S. Department of Agriculture National Organic Program standards include 90- or 120-day intervals between application of manure and harvest of crop to minimize risks of pathogen contamination of fresh produce. Data on factors affecting survival of Escherichia coli in soils under greenhouse conditions are needed. Three separate studies were conducted to evaluate survival of nonpathogenic E. coli (gEc) and attenuated E. coli O157:H7 (attO157) inoculated at either low (4 log CFU/ml) or high (6 log CFU/ml) populations over 56 days. Studies involved two pot sizes (small, 398 cm(3); large, 89 liters), three soil types (sandy loam, SL; clay loam, CL; silt loam, SIL), and four amendments (poultry litter, PL; dairy manure liquids, DML; horse manure, HM; unamended). Amendments were applied to the surface of the soil in either small or large containers. Study 1, conducted in regularly irrigated small containers, showed that populations of gEc and attO157 (2.84 to 2.88 log CFU/g) in PL-amended soils were significantly (P < 0.05) greater than those in DML-amended (0.29 to 0.32 log CFU/g [dry weight] [gdw]) or unamended (0.25 to 0.28 log CFU/gdw) soils; soil type did not affect E. coli survival. Results from study 2, in large pots with CL and SIL, showed that PL-amended soils supported significantly higher attO157 and gEc populations compared with HM-amended or unamended soils. Study 3 compared results from small and large containers that received high inoculum simultaneously. Overall, in both small and large containers, PLamended soils supported higher gEc and attO157 populations compared with HM-amended and unamended soils. Populations of attO157 were significantly greater in small containers (1.83 log CFU/gdw) than in large containers (0.65 log CFU/gdw) at week 8, perhaps because small containers received more regular irrigation than large pots. Regular irrigation of small pots may have affected E. coli persistence in manure-amended soils. Overall, PL-amended soils in both small and large containers supported E. coli survival at higher populations compared with DML-, HM-, or unamended soils.