Conditions at the time of inoculation influence survival of attenuated Escherichia coli O157:H7 on field-inoculated lettuce

The impact of plant development, environmental conditions at the time of inoculation, and inoculum concentration on survival of attenuated BSL1 Escherichia coli O157:H7 strain ATCC 700728 on field-grown romaine lettuce was evaluated over 3 years. E. coli 700728 was inoculated onto 4- and 6-week-old romaine lettuce plants in the Salinas Valley, CA, at night or the next morning with either low (5 log) or high (7 log) cell numbers per plant to simulate a single aqueous contamination event. At night, when leaf wetness and humidity levels were high, E. coli cell numbers declined by 0.5 log CFU/plant over the first 8-10 h. When applied in the morning, E. coli populations declined up to 2 log CFU/plant within 2 h. However, similar numbers of E. coli were retrieved from lettuce plants at 2 and 7 days. E. coli cell numbers per plant were significantly lower (P < 0.05) 7 days after application onto 4-week-old compared to 6-week-old plants. E. coli 700728 could be recovered by plating or enrichment from a greater proportion of plants for longer times when inoculated at high compared with low initial concentrations and after inoculation of 6-week-old plants compared with 4-week-old plants, even at the low initial inoculum. A contamination event near harvest or when leaf wetness and humidity levels are high may enhance survivability, even when low numbers of E. coli are introduced.