Vinegar can be used to kill salmonella, E. coli, and other "gram-negative" bacteria. It cannot kill gram-positive bacteria such as staphylococcus or streptococcus. Lettuce is susceptible to contamination by various bacteria and fungi. However, the presence of vinegar should not cause any problems for someone who is eating the lettuce. Lettuce has a natural resistance to many pathogens due to the presence of oxalic acid and acetylcholine, two substances that deter most organisms from attacking it. The amount of vinegar needed depends on the concentration of acetic acid. One tablespoon of vinegar contains about 5% acetic acid. A solution containing this percentage of acetic acid will be very effective in killing E. coli and other bacteria. Lower concentrations of vinegar may be effective at lower temperatures but higher concentrations at room temperature. Lettuce is an easy food to prepare because it does not require difficult techniques or special equipment. It can be eaten raw or cooked. If you choose to cook it, please follow safe cooking instructions so you do not end up with contaminated food. People vary in their sensitivity to acetic acid.