Purpose: This study was performed (i) to calculate the die-off rate for E. coli in feces and on produce under field conditions, and (ii) to calculate transfer coefficients for E. coli from feces to lettuce.
Methods: Two studies were conducted. In the first study, lettuce was inoculated with E. coli, and harvested 0-10 days following inoculation. E. coli concentration on the lettuce was determined and die-off rates were calculated. Correlation tests were used to examine the relationship between weather and die-off rate. In the second study, feces inoculated with E. coli were placed in a lettuce field 0-3 days before irrigation. After irrigation E. coli concentration on the lettuce and in the feces was determined, and transfer rates were calculated.
Results: In the first study the average daily die-off for E. coli on the lettuce over 10 days was 0.52 log MPN (Standard Deviation [SD] = 0.56). The only weather factor that was significantly associated with die-off rate was leaf wetness (P < 0.002). In the second study, 89% of the lettuce heads tested positive for E. coli, and, on average, 0.027% (SD= 0.17) of the E. coli present in the feces transferred to a given lettuce head.
Significance: These findings provide die-off and transfer rates for E. coli in feces and on lettuce, respectively, that can be used in quantitative risk assessments to help determine the level of risk of human illness from such events.