Yes, says one of Britain’s leading microbiologists, Professor John Threlfall, of the government’s Health Protection Agency (HPA).

The Scotsman reports today that Threlfall said prepared salads and other "ready-to-eat" foods pose a salmonella threat and he urged consumers to disregard assurances on packaging and wash the contents again before eating.

The U.K. Food Standards Agency (FSA) also agreed that extra precautions may be necessary with pre-washed products, with a spokesman quoted as saying,

"Our advice is to wash all lettuce, including bagged lettuce, when you get it home. We will review this advice if we receive extra evidence and reassurances from the industry about their cleaning processes."

No, says the industry, some government agencies and some academics. The products are sufficiently washed at the processing facility, are ready-to-eat, and there is a potential for cross-contamination.

David Barney of the Fresh Prepared Salads Producer Group, which represents UK salad companies, was quoted as saying,

"I am very puzzled by this advice. I don’t understand why he is saying this and we would strongly argue against it. Our cleaning processes are robust and well-managed. The wash the salad gets is as good as any wash you would give in the home, and washing it again at home is not going to make a substantive difference to the safety of the product. … There is almost no food-borne illness directly associated with retail prepared salads, because the washing systems have been particularly good. … It’s widely known that kitchens – and particularly kitchen sinks – are the source of much cross-contamination."

What’s missing in all of this is data to support either recommendation. And the question is the wrong one, focusing on what consumers can do. Washing of fresh produce, particularly leafy greens like spinach and lettuce, is of limited use in removing dangerous microorganisms. The contamination, especially with E. coli O157 and Salmonella, must be prevented on the farm. A  2005 article from the Atlanta Journal-Constitution discussing some aspects of the issue is available here.

A table of known outbreaks of verotoxigenic E. coli — including but not limited to E. coli O157:H7 — associated with fresh spinach and lettuce is available at http://foodsafety.ksu.edu/en/article-details.php?a=3&c=32&sc=419&id=903.