We think of mayonnaise as a contaminate killer if not refrigerated or handled carefully. Wrong! We do not need to worry about mayonnaise; it is completely safe and does not need refrigeration. The pH in mayonnaise is set at a point that bacteria could not survive in its environment.
What we do need to worry about is food poisoning from ONIONS and where those onions came from (in the potato salad?). It’s probably the onions, and if not onions, it’s the POTATOES. Onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion. It is not safe even if you put it in a zip-lock bag and put it in your refrigerator. It’s already contaminated enough just being cut open and out for a minute. It can become hazardous to you. When you are eating out and putting onions on that hotdog, you are asking for food poisoning on a mega level.
Onions and moist potatoes in potato salad will attract and grow bacteria faster than any commercial mayonnaise will even begin to break down. Onions, when cut and left in the open air can cause adverse stomach infections because of excess bile secretions and even food poisoning.
Remember, onions were cut and left in rooms to fight off the Black Plague in the early 1900’s because people thought they had the ability to absorb airborne bacteria. The onion would be black the next morning from absorbing bacteria.
What’s in your onion?