But most mayonnaise contains vinegar and other ingredients that make it acidic, and therefore very likely to protect against spoilage. When problems occur, they usually result from low-acid ingredients like chicken and seafood.
One study published in The Journal of Food Protection found that in the presence of mayonnaise, the growth of salmonella and staphylococcus bacteria in contaminated chicken and ham salad slowed down or stopped completely. The more mayonnaise used, the more the rate of growth decreased.
Sources: New York Times July 1, 2008