Factors That Influence Survival of Escherichia Coli O157:H7 in Field-Inoculated Lettuce

Introduction: Although Escherichia coli O157:H7 is not a natural colonist of plants, it can persist at low levels in the lettuce phyllosphere for extended periods of time. High humidity combined with warm temperatures have been shown to support the growth of E. coli O157:H7 on lettuce plants grown in an environmental chamber; the impact of these factors in the production environment is unknown.

Purpose: Our purpose was to evaluate the impact of inoculation time and plant age on the survival of E. coli O157:H7 on field-inoculated lettuce.

Methods: Five field trials were conducted in the Salinas Valley (spring 2010, summers 2010, 2011, and 2012, and fall 2012). For each field trial, lettuce plants that were 4 or 6-weeks post seeding were inoculated with a rifampicin-resistant attenuated E. coli O157:H7 (ATCC 700728) at a target level of 5 or 7 log CFU/plant. Two sets of plants were inoculated: one between 9:00 to 11:30 pm (nighttime) and another the next morning between 9:00 and 10:00 am (morning). E. coli O157:H7 was enumerated on the plants by direct plating, filtration and plating, or by MPN. When counts were below the limit of detection, the entire above-ground portion of the plant was enriched for E. coli O157:H7.

Results: Inoculation at nighttime when relative humidity and leaf wetness were high enhanced survival of E. coli O157:H7 during the first several hours to 1 day after inoculation but not thereafter. Population size declined in the first 8 h by 0.5 to 1.5 log CFU/plant for nighttime inoculations and by 3.5 log CFU/plant after the morning inoculations regardless of plant age. A greater proportion of E. coli O157:H7 positive plants was observed 2 weeks after inoculation of 6-week old plants. 

Significance: The time of inoculation influences the short-term survival of E. coli O157:H7 on lettuce plants while plant age and size influence long-term prevalence rates.