People often cite mayonnaise as a source for food poisoning, but studies have shown the condiment is not very conducive to bacterial growth. This is due to the ingredients used in commercially–prepared mayonnaise which typically include pasteurized eggs, salt and an acid like vinegar (acetic acid) or citric acid.
In fact, "mayonnaise manufactured in the United States is quite acidic, with a pH of 3.6 to 4.0. Not only that but the organic acids used in commercially prepared mayonnaise have other antimicrobial properties.
Studies have shown that the lethal effect of mayonnaise on foodborne pathogens has been well documented. According to Michael P. Doyle, Ph.D., Professor and Director of the University of Georgia’s Center for Food Safety and Quality Enhancement, most harmful bacteria, including E. coli O157:H7, Salmonella and Listeria monocytogenes, die off within hours in the presence of mayonnaise, due to its acidity."
Extra mayo, anyone?