The following is from Science Sept 11, 1998;281:1666-1668, 1578-1579:
and
shows the contamination levels of grain-fed cattle.

Best, Peter

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Bovine digestive tracts and manure can harbor a variety of E.coli
strains,
including E. coli 0157:H7, which has been associated with potentially
deadly infections in humans.  When beef is slaughtered, it is often
contaminated with cattle manure.  Produce can also be contaminated
with the
bacteria if fertilized with manure.  Despite more stringent food
safety
standards, E. coli, 0157:H7 outbreaks have become increasingly common
since
World War II.  Since then, most farmers have been feeding their cattle
grain, rather than less costly, less fattening hay.  Cattle fed grain
had
300 times as much E. coli in their feces as did cows fed hay, the
researchers found.  Moreover, the bacteria in the grain-fed cattle's
feces
were much more likely to survive after being soaked for an hour in
acid.
This is significant because human stomach acid usually kills pathogens
that
could otherwise cause infection.  Why were the grain-fed cattle more
likely
to have acid-resistant bacteria in their feces?  Cattle produce only
small
quantities of an enzyme that breaks down starch, the researchers
explain.
As a result, large quantities of starchy grain pass into cows'
intestines
undigested, and the starch ferments there, producing fatty acids.
Bacteria
"raised" in this acidic environment may become acid resistant, and can
survive and multiply, if they later find their way into human
stomachs.
Switching cattle from a grain diet to a hay diet, however, quickly
changes
the intestinal environment and eliminates acid-resistant ED. colic in
the
digestive tract within 5 days, the researchers report.