Use of fluorescent microspheres as a tool to investigate bacterial interactions with growing plants.

Foodborne pathogens may exist as endophytes of growing plants. The internalization of Escherichia coli O157:H7 or other foodborne pathogens in growing lettuce plants may be independent of microbial factors. Mature lettuce plants were surface irrigated with E. coli O157:H7 or with FluoSpheres (fluorescent microspheres) and harvested 1, 3, and 5 days post-exposure. FluoSpheres were utilized as a bacterial surrogate. Microscopic examination of root, stem, and leaf tissue sections revealed that FluoSpheres were internalized into growing plants. Laser scanning confocal microscopy revealed that FluoSpheres were present within the root tissue and leaf stem tissue. The presence of FluoSpheres in internal portions of stem and leaf tissue suggests transport of the spheres from the root upward into the edible tissue. The level of uptake of FluoSpheres and E. coli O157:H7 was quantified using filtration. Numbers of FluoSpheres and E. coli O157:H7 cells in plant tissue were similar. The entry of E. coli O157:H7 into lettuce plants may be a passive event because the concentration of FluoSpheres was similar to that of the pathogen.