A pair of studies from Purdue University researchers - one in the Journal of Food Protection and another in Food Research International (registration required for both journals) - have found that Salmonella and E. coli bacteria may be able to live inside plant tissue. A Purdue press release says:
E. coli 0157:H7 was present in tissues of mung bean sprouts and Salmonella in peanut seedlings after the plants' seeds were contaminated with the pathogens prior to planting. Amanda Deering, a postdoctoral researcher in food science, said seeds could be contaminated in such a manner before or after planting through tainted soil or water.
"The pathogens were in every major tissue, including the tissue that transports nutrients in plants," said Deering. . .
The issue both studies raise is that washing such produce would only remove the surface bacteria, but not the inner tissue. Cooking the produce to temperatures known to kill both pathogens would, however, remove them from the inner tissue.