Question: I made some lasagna and mistakenly forgot it in the oven, where it sat at room temperature overnight. I know that can cause bacteria to form, but if I reheat the lasagna thoroughly won’t that make it safe?
Answer: No, unfortunately that’s not always the case.
According to the U.S. Food and Drug Administration, reheating your lasagna might indeed kill the bacteria that were likely produced when it sat out overnight.
But your problem doesn’t stop there, says the FDA.
Some types of bacteria also produce heat-resistant spores or toxins that can cause food poisoning. And these spores and toxins are often not destroyed by normal cooking or reheating.
Two such examples cited by the FDA: Staphylococcus aureus and Clostridium perfringens. Both can be caused by leaving cooked food at room temperature for more than two hours and reheating the food is not at all guaranteed to get rid of them.
Bottom line: You should not eat the lasagna. You're not certain to get sick if you do, but you’ll nonetheless be giving yourself a very good shot at contracting a serious foodborne illness.