Differential Persistence of E. coli O157:H7 Strains on Romaine Lettuce Leaves

Authors: Patel, Jitu; Millner, Patricia

Submitted to: Annual Meeting of the Institute of Food Technologists
Publication Type: Abstract Only
Publication Acceptance Date: 2/17/2010
Publication Date: 7/19/2010
Citation: Patel, J.R., Millner, P.D. 2010. Differential Persistence of E. coli O157:H7 Strains on Romaine Lettuce Leaves. Annual Meeting of the Institute of Food Technologists. T168-03.

Interpretive Summary:

Technical Abstract: Outbreaks associated with leafy greens have focused attention on the persistence of Escherichia coli O157:H7 on produce. Limited information is available on ecological interactions influencing persistence of different E. coli O157:H7 strains on lettuce. Survival of six strains of E. coli O157:H7 isolated equally from produce and environmental samples was evaluated on lettuce grown in a BL-2 growth chamber. Each strain was grown independently in sterile water extracts of dairy manure solids. Romaine lettuce cvr. ‘Parris’ was grown in steam-pasteurized organic soil. Each strain (100 µL) was inoculated as 5µL droplets onto each of five leaves per plant of 3-week old plants and incubated at 22'C 16 h day and 18'C night, 50% RH. Initial E. coli O157:H7 populations on lettuce plants were 6 and 7 log CFU/leaf for produce and environmental isolates, respectively. Three replicates of each leaf sample per strain were analyzed on day 0, 1 and weekly for 28 days for E. coli O157:H7 by direct plating on SMAC medium, and by MPN. Three of the six E. coli O157:H7 strains were non-detectable on lettuce leaves after 7 days. Strain RM1918 (produce isolate) persisted on lettuce leaves for up to 21 days when determined by direct plating or MPN. Five E. coli O157:H7 strains persisted on lettuce leaves after 14 days as determined by MPN, whereas only two isolates were recovered after 14 days by direct plating. E. coli O157:H7 populations on lettuce leaves at day 1 were significantly lower from those on day 0 for all six strains. Die off of E. coli O157:H7 in water extract of manure progresses very quickly after inoculation onto lettuce. Persistence of E. coli O157:H7 on lettuce leaves varies substantially by strain. Enrichment MPN assay is required to detect persistence of low numbers of E. coli O157:H7 on lettuce.