According to the U.S. Department of Agriculture, u003ca href=u0022https://ask.usda.gov/s/article/Does-mayonnaise-cause-foodborne-illnessu0022 target=u0022_blanku0022 rel=u0022noreferrer noopeneru0022u003emayonnaise does not cause food borne illnessesu003c/au003e due to the acid; lemon or vinegar in the mayonnaise, which prevents the growth of bacteria. After two hours, proteins in mayonnaise based foods can cause bacteria to grow. If you know this dish will be unrefrigerated for more than two hours, then leave out the bacon to be safe.