We were always taught to be wary of eating foods laced with mayonnaise on hot summer days for fear of eating something contaminated with excessive bacteria. But that fear really only applied to homemade mayonnaise (which, we highly recommend making, by the way — just not for a picnic); the preservatives in commercial mayo keep bacteria at bay. In fact, it seems to retard bacterial growth:
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.
So lather on the mayonnaise this summer; you may not fit into your swimsuit, but hey — no salmonella!
The Claim: Mayonnaise Can Increase Risk of Food Poisoning [New York Times]