Taking the lead from MrWizard, I did a little look for what the
boys and girls in white lab-coats say. In addition to the
evidence presented at Snopes.com and the experts at the Ketchup
Advisory Board (see MrWizard's answer) a few interesting
articles (see below) all seem to point to this conclusion:
Commercially produced mayonnaise made with acetic acid is safe
and due to its low pH, as well as the salt, and other additives
it contains. That, combined with the fact that none of the
outdoor experts here have ever had mayo go bad on them... well,
draw your own conclusions. References (added emphasis mine):
Smittle, R.B. “Microbiological Safety of Mayonnaise, Salad
Dressings, and Sauces Produced in the United States: A Review.”
Journal of Food Protection 63, no. 8 (2000): 1144–1153. The
absence of reports of foodborne illness associated directly with
the consumption of commercially prepared acidic dressings and
sauces is evidence of their safety. Salmonella, E. coli O157:
H7, E. coli, L. monocytogenes, Staphylo