If you leave stuffed mushrooms out all night at room temperature, they will not be safe to eat. Bacteria can grow at an incredible rate, so even if they are exposed to the air for just a few hours, stuffed mushrooms can present a health hazard. If you properly store and reheat stuffed mushrooms, however, they should be safe to eat the next day.
Leftovers, such as stuffed mushrooms, should be refrigerated no later than two hours after cooking. This includes the time that the food is sitting on the table or on your plate. If your mushrooms were left on the dinner table overnight, throw them in the trash. Store leftover stuffed mushrooms in a shallow dish, less than 3 inches deep, leaving air spaces around the mushrooms to let cold air get to the food. Set the refrigerator at 36 to 38 degrees Fahrenheit to keep the temperature inside below the safe limit of 40 degrees Fahrenheit.
Microwaves offer a convenient way to reheat stuffed mushrooms. Microwave cooking may dry out the mushrooms further, but it's important to heat leftover food to at least 165 degrees Fahrenheit. Nonetheless, microwaves won't kill off all bacteria. For example, the bacteria Staphylococcus aureus can cause nasty bouts of food poisoning and is resistant to heat. Therefore, if stuffed mushrooms are left out overnight, they could present a health hazard even if you microwave them. Turn the mushrooms halfway through cooking to to make sure they are heated throughout.
Some ingredients present more problems than others. For example, poultry, cream cheese, fish and seafood are all perfect breeding grounds for bacteria. If your stuffed mushrooms contain fillings such as minced chicken, crab or tuna, then it's a good idea to store them in the refrigerator as soon as possible after cooking. If you leave them out overnight, there is a much greater chance that the mushrooms are not safe to eat.
In hot climates with temperatures of 90 degrees Fahrenheit or more, food such as stuffed mushrooms should be refrigerated a maximum of one hour after cooking. Heat allows bacteria to multiply faster and the food to spoil more quickly. In very hot environments, refrigerator door temperatures can rise and fall. It's safer to store leftover food in the coldest part of the refrigerator, usually the middle shelf.