Experts say that washing loose-leaf lettuce will not always do the trick. The most thorough washing will not necessarily kill all of the bacteria. Rather, consumers should remove the outer leaves and wash each leaf individually.
When it comes to bagged or boxed lettuce, make sure that the leaves are not bruised or damaged and that they are refrigerated.
If the package says "pre-washed," "triple washed" or "ready to eat," avoid the temptation to wash the leaves again. Rewashing them could risk cross-contamination from your own hands or from bacteria in the sink or other kitchen appliances.