Yep, lovers of Taco Bell's peculiar brand of meat can rest easy, it was the lettuce. From the Seattle Times:
...
One likely problem is the proximity of ranching and farming operations in parts of California. Cattle and other animals harbor the bacteria, which is shed in their feces.
Supporting that theory, Acheson said officials found the same substrain of E. coli involved in the Taco John's outbreak in two unidentified "environmental samples" from a dairy adjacent to one of the chain's lettuce growers.
California lettuce linked to 2 E. coli outbreaks [Seattle Times]








