According to Consumer Reports, some "pre-washed" and "triple-washed" salads are not what they represent themselves to be. Almost 40 percent of 208 samples from sixteen brands were contaminated with bacteria.
"The tainted salads were not contaminated with more serious bacteria like salmonella or E. coli, but 39 percent of them did contain coliform levels that exceeded 10,000 colony forming units per gram (CFU/g) and 23 percent of them contained enterococcus levels exceeding 10,000 CFU/g. Industry experts generally agree that acceptable levels of these types of bacteria for leafy greens should be below 10,000 CFU/g."
It is likely that the contamination is due to poor sanitation. The U.S. Department of Agriculture concedes that approximately two in 4,000 bags of salad have traces of salmonella in them. This is usually attributable to cattle feedlot runoff, which can occur when rain and other factors cause animal waste to spread to nearby fields, and can also contaminate water supplies.
http://www.naturalnews.com/028628_salad_feces.html