Suvival of Six Shiga-toxigenic Escherichia coli Serotypes in Field Lettuce

Introduction: Most outbreaks of illness caused by Shiga-toxigenic Escherichia coli in leafy vegetables have been caused by serotype O157 but several incidents involving non-O157 serotypes have been reported in recent years. Little is known about the survival of non-O157 Shiga-toxigenic E. coli on field crops.

Purpose: The purpose of this work was to compare the survival of six Shiga-toxigenic E. coli serotypes on growing field lettuce.

Methods: E. coli isolates from serotypes O26:H11, O44:H18, O103:H2, O111:HNM, O145:HNM and O157:H7 lacking both stx1 and stx2 genes were applied (10⁶ CFU/g) to six separate plots of Romaine lettuce in an experimental site located at Summerland, British Columbia, Canada (dry climate, sandy soil) in 2011 and 2012. Residual populations on lettuce leaves were estimated on modified MacConkey agar (10mg/ml of vancomycin, 3mg/ml cefsulodin, 42°C incubation T) and/or enrichment in modified TSB.

Results: With the exception of serotype O103:H2, E. coli populations were reduced to ≤ 1 log CFU/g leaf tissue after 7 days in both trial years. E. coli O157:H7 was consistently detected by enrichment for up to 21 days, whereas recovery of other serotypes was sporadic. In contrast, 2 log CFU/g E. coliO103:H2 remained on the leaves 21 days post-inoculation. The latter was clearly able to survive longer on lettuce leaves than the other experimental strains examined in this work.

Significance: Non-O157 serotypes of E. coli are capable of similar or longer survival on growing lettuce plants. The present work hints at innate differences in the fitness of individual E. coli serotypes or strains in the production environment.