Precaution about left over onions

I have taken the idea of this post through an email which I received recently. As we try to save our time and money by reusing or recycling our food, we might use the left over onions for making food.
Are left over onions safe for reuse? The simple answer is no, left over onions are very harmful. 

LEFT OVER ONIONS ARE POISONOUS

When someone gets sick, we blame oily foods while nobody cares about the bowl of potato salad sitting on the table. Mostly when we suffer from food poisoning, we assume that it might be due to the use of mayonnaise. Usually commercially made mayonnaise is completely safe. It doesn't even have to be refrigerated. No harm in refrigerating it, but it's not really necessary."
The pH in mayonnaise is set at a point that bacteria could not survive in that environment.