Purpose: To investigate 1) survival of non-pathogenic Escherichia coli (Ec) and attenuated E. coli O157:H7 (attO157) in soils amended with poultry litter (PL), dairy manure (DM), dairy manure liquid (DL), or horse manure (HM) and 2) to compare the survival of Ec and attO157 in manure-amended soil.
Methods: Field plots (Othello soil) were amended with PL, DM, DL, or HM. Inocula containing equal numbers of three rifampicin-resistant (RifR) strains of Ec and two attO157-RifR strains grown in poultry-litter extract, were spray-inoculated (1-l per 2 m2) at low, 4. 2 x 103 CFU/ml, or high, 1.29 x 106 CFU/ml, cell densities. Soil samples collected periodically 0-154 days post-inoculation (dpi) were analyzed for viable Ecand att0157 by direct plating and/or mini-MPN.
Results: Persistence of all strains was influenced by manure type, inocula concentration, and environmental factors. Populations declined more rapidly in DM compared to other treatments, except non-manured soils. By 7-dpi, all strains from both inoculum-level treatments declined to ≤1 CFU (MPN) gdry soil-1 (gds-1), except DL. E. coli populations in all manure treatments increased after rainfall except in DL. Populations at 28-dpi and thereafter, continued at ca. ≤1 CFU (MPN) gds-1. No Ecor attO157 were found in uninoculated control soil.
Significance: Manure type and environmental factors influenced survival of E. coli in soil, with attO157 declining more rapidly than Ec. Both Ec and attO157 survived at higher populations in manure-amended soils compared to non-amended soils, indicating the effect manure has in extending survival of bacteria in soils. These findings should be taken into consideration when setting guidelines for fresh produce safety relative to manure application to soil as stated in proposed FDA standards.