From:
http://fycs.ifas.ufl.edu/linda11.htm

Recent research by UGA researchers revealed that after
lettuce is washed, E. coli O157:H7 can still be found
lurking on leaves. Although supermarket lettuce is
rinsed in a mild chlorinated water to kill bacteria,
often pathogens still survive, especially around
breathing holes and brown spots. According to the UGA
researcher, Dr. Joe Frank, the E. coli and other
pathogens can penetrate into the surface of the
lettuce tissue through its stomata (the holes in which
leaves breathe), a bruise, or a damage area on the
leaf. There the E. coli is protected from the
chlorinated rinse water. People should realize that
any time they eat raw agricultural produce, there is a
risk of pathogenic infection, even if the risk is low.
However, it is possible to further reduce the risk by
washing produce and removing bruised or brown spots.

For more information, contact Dr. Joe Frank at (706)
542-2286.