Lack of Internalization of Escherichia coli O157:H7 in Lettuce (Lactuca sativa L.) after Leaf Surface and Soil Inoculation

Survival and internalization characteristics of Escherichia coli O157:H7 in iceberg, romaine, and leaf lettuce after inoculation of leaf surfaces and soil were determined. A five-strain mixture of E. coli O157:H7 in water and cow manure extract was used as an inoculum for abaxial and adaxial sides of leaves at populations of 6 to 7 log and 4 log CFU per plant. The five strains were individually inoculated into soil at populations of 3 and 6 log CFU/g. Soil, leaves, and roots were analyzed for the presence and population of E. coli O157:H7. Ten (4.7%) of 212 samples of leaves inoculated on the adaxial side were positive for E. coli O157:H7, whereas 38 (17.9%) of 212 samples inoculated on the abaxial side were positive. E. coli O157:H7 survived for at least 25 days on leaf surfaces, with survival greater on the abaxial side of the leaves than on the adaxial side. All 212 rhizosphere samples and 424 surface-sanitized leaf and root samples from plants with inoculated leaves were negative for E. coli O157:H7, regardless of plant age at the time of inoculation or the location on the leaf receiving the inoculum. The pathogen survived in soil for at least 60 days. Five hundred ninety-eight (99.7%) of 600 surface-sanitized leaf and root samples from plants grown in inoculated soil were negative for E. coli O157:H7. Internalization of E. coli O157:H7in lettuce leaves and roots did not occur, regardless of the type of lettuce, age of plants, or strain of E. coli O157:H7.