Persistence of Escherichia coli O157:H7 on lettuce plants following spray irrigation with contaminated water.

Irrigation water collected at farms growing crops for human consumption was artificially contaminated with E. coli O157:H7 and used to irrigate lettuce plants. Plants in a growth chamber were spray irrigated either once or intermittently with water contaminated with 10(2) or 10(4) CFU of E. coli O157:H7 per ml and were then sampled over a 30-day period. Only plants exposed to 10(2) CFU/ml on day 1 did not harbor the pathogen at the end of the sampling period. All other treatments resulted in contaminated plants at harvest. Plants irrigated with 10(4) CFU/ml contained high levels (up to 5 log CFU/g) of the pathogen at harvest. The results obtained in this study underscore the assertion that spray irrigation (the application of water directly to plant leaves) is linked to the contamination of crops and suggest that repeated exposure increases the E. coli O157:H7 level on the plant.