> Marler also blames Americans’ love of convenience for the problem. “Mass-produced chopped, bagged lettuce that gets shipped around the US amplifies the risk of poisoning,” he said.

> Instead of shipping heads of lettuce or large carrot sticks that people wash, we chop them and mix them up in processing, then package them in plastic bags. In that process, Marler said, “The bacteria has a chance to grow. And a lot of people get sick.”

So it's not just the lettuce heads that get contaminated, it's how they are processed. This is reminiscent of the issues with large meat packing plants.

I've often wondered what effect that processing has on lettuce. Personally I prefer to buy a head and clean it myself because it tends to last a lot longer than a bag that's usually full of consendate and mashed onto the shelves by store employees, bruising half the contents...