Escherichia coli Contamination on Ready-To-Eat (RTE), Lettuce

Escherichia coli outbreaks associated with Ready-to-Eat vegetables have brought attention to irrigation water contaminated with pathogens. A greenhouse study was conducted to determine the potential transfer and survival of E. coli from contaminated irrigation water to lettuce leaves and in soil. Contamination of lettuce was studied using two irrigation methods (drip and sprinkler irrigation) and two soil types (organic and mineral soil) during June–August 2014. There were pots inoculated with bacteria E. coli ATCC8739, and un-inoculated pots without bacteria. The four treatments were replicated five times each for the inoculated and un-inoculated pots. One-time application of E. coli-contaminated bacterial water (7.23 log cfu/ml) was used to irrigate lettuce, after two weeks of transplanting the seedlings into pots. The transfer and survival of bacteria were observed in the soil and lettuce leaves over a time period of 30 days after inoculation. It was observed that there was highest transfer and survival of bacteria on organic soil with drip irrigation, i.e., 7.03 log cfu/g (63.1% bacterial retention) and dropped to 4.71 log cfu/g (0.3% retention) on the 30th day. For lettuce leaves, maximum contamination was observed with plants growing in organic soil with sprinkler irrigation and it decreased from 5.32 log cfu/g (1.23% retention) to 2.88 log cfu/g (0.004% retention) by the 30th day. Soil type significantly influenced survival of the bacteria, as organic soil showed more contamination (i.e., 63.1 and 0.57% retention) than mineral soil (i.e., 0.47 and 0.04% retention) on the 2nd day. Also, an early bacterial decay or movement to lower depths was observed in mineral soil compared to the organic soil. Irrigation method has a significant influence in contaminating the lettuce leaves, as sprinkler irrigation resulted in higher transfer and retention of bacteria on the edible portion of the plant. Also, in the case of sprinkler irrigation, fate and transport of bacteria were independent of the soil type. Drain water from the pots was also collected to study bacteria transport, but none were positive for the bacterial presence.