Cooked food isn't entirely safe either. You should avoid eating cooked food that has been left in the open, especially overnight. "Bacteria that have contaminated cooked foods can multiply to dangerous levels in one or two hours at room temperature," said Dr Look. HOW SOON DO THE SYMPTOMS USUALLY APPEAR? “Food poisoning is usually the result of consuming contaminated food or drinks, with symptoms appearing rapidly – sometimes within one to six hours,” said Dr Lee. But when it comes to bacterial food poisoning that involves salmonella or E. coli, the microbes may take longer to multiply and cause symptoms, said Dr Reuben Wong, a gastroenterologist at Gleneagles Hospital’s gutCARE. For instance, said Dr Wong, salmonella starts to show signs 12 to 72 hours after infection, while E. coli can take one to eight days to hit. For listeria, which is particularly fatal to unborn babies and newborns, the first signs of infection may only show 30 days or more after infection. However, there are some bacteria, such as staphylococcus aureus, that can produce toxins that show symptoms within hours, said Dr Yip. “The food poisoning symptoms often appear quickly, sometimes within one to six hours.”