Persistence of Eschericha coli on Romaine Lettuce in Protected Agricultural Environments

Introduction: The ability of Escherichia coli populations to survive in non-host habitats is greatly influenced by environmental conditions; including thermal variability, temperature fluctuations and nutrient availability. The survival of pathogens on lettuce grown in controlled atmospheric conditions may differ than survival on field grown lettuce.

Purpose: This study investigated the persistence of E. coli on lettuce plants grown in hoop houses.

Methods: Over two growing seasons, 385 Romain lettuce (var. Melody) plants grown in a hoop structure were inoculated with irrigation water contaminated with bovine manure containing E. coli between 0 and 10,000  CFU/ml. Coliform and E. coli present on lettuce leaves on days 0 (pre- and post-inoculation), 1, 3, 5, 7,  10 and 13 post-inoculation were determined.

Results: Immediately post-inoculation, concentration of E. coli on plants in all treatment groups increased to approximately 100 CFU/g.  Concentrations of E. coli on days 1, 3, and 5 were inoculum-dose dependent and declined gradually to pre-inoculation concentrations (≤ 1 CFU/10 g) by one week post-inoculation; however, E. coli numbers subsequently increased to 3 CFU/g on plants inoculated with the highest concentrations of E. coli.

Significance: Low microbial quality irrigation water may introduce microorganisms to the surface of lettuce.  Although counts tended to decline over time, even under the controlled conditions of hoop house productions, E. coli populations may fluctuate dramatically as a result of factors that are yet to be determined.