The New York Times reports that, despite its reputation, mayonnaise can reduce food spoilage.

Most commercial brands of mayonnaise contain vinegar and other ingredients that make them acidic — and therefore very likely to protect against spoilage.
 
When problems occur, they usually result from other contaminated or low-acid ingredients (like chicken and seafood), improper storage and handling, or homemade versions that contain unpasteurized eggs.

One prominent study published in The Journal of Food Protection found, for example, that in the presence of commercial mayonnaise, the growth of salmonella and staphylococcus bacteria in contaminated chicken and ham salad either slowed or stopped altogether. As the amount of mayonnaise increased, the rate of growth decreased. When temperatures rose to those of a hot summer day, the growth increased, but not as much as in samples that did not contain mayonnaise.

Or, as Bill Marler quipped, for his summer picnic, “I’ll just have the bun please.”