Warren Taylor posted this on another ng.

It will probably always be impossible to get 100% protection
against e coli contamination in spinach / fresh salad greens,
according to the weblink below.

This is for multiple reasons, not the least of which is
that it is impossible to wet and wash the complex cracked
and rolled up natural design of contorted leaf surfaces
and stems fully, so as to exterminate the contamination.

This is true, despite using commercial washing equipment with
surfactant-aided, [http://en.wikipedia.org/wiki/Surfactant]
detergent and emulsifying compounds in the best
chlorine/peroxide disinfectant washes available today.

If you want fresh and chewy salad, it will always be risky
to some degree.  Only undercooked hamburger is more risky
on a population-wide basis (per URL weblink below).

It is also probably overly optimistic to believe the we
ourselves can design our own personal kitchen washing
techniques with our own chemicals and disinfectants to
be superior to those in the best commercial processing
facilities.

Quotes from weblink below:

1) "The safeguards are not in place to protect fruits and
   vegetables in the same way that they are for beef and
   poultry."

2) Leafy vegetables are the second leading source of E. coli
  infections in the United States, behind ground beef,
  but the government relies primarily on voluntary safety
  steps by farmers and packagers to prevent outbreaks.

http://www.cbsnews.com/stories/2006/09/19/health/main2020591.shtml