Purpose: A 12-month experimental field trial was conducted to examine the survival of a three-strain cocktail of rifampicin-resistant generic Escherichia coli applied to soil amended with different animal manure types in California’s Central Valley.
Methods: High (107 CFU/ml) and low (104 CFU/ml) inoculum were separately applied by spraying the E. coli cocktail onto 4 untreated horse-, cattle-, goat-, and chicken litter- amended soil and control plots (2 m x 1 m) at the UC Davis vegetable crop field station. Soil samples were collected from November 2013 to October 2014 to determine the generic E.coli population by direct plating and MPN methods. The study was initiated during a time when the region was experiencing extreme drought conditions.
Results: We observed a 7.16 log reduction after 120 days from manure application. The generic E. coli populations survived longest in untreated chicken litter followed by horse, cattle and goat manure. E. coli populations increased after heavy rains by 5.87 and 5.61 log CFU in high and low inoculums plots, respectively. Time and manure type were statistically significant (P < 0.0001) and predicted the concentration of indicator E. coli in a linear regression model.
Significance: Although die-off was observed in soil by day 120 during a fall-winter period, resuscitation was observed for all manure types following heavy spring rains. The findings suggest that generic E. coli experiences multiple log reductions over 120 days, but exposures to rain fall can contemporarily reverse these reductions.