Cooked rice turns out to be a potential source of food poisoning. Raw rice almost always carries dormant spores of the bacterium Bacillus cereus, which produces powerful gastrointestinal toxins. The spores can tolerate high temperatures, and some survive cooking. If cooked rice is left for a few hours at room temperature, the spores germinate, bacteria multiply, and toxins accumulate. Ordinary cooked rice should therefore be served promptly, and leftovers refrigerated or frozen to prevent bacterial growth. The rice in Japanese sushi is served at room temperature, but the surface of its cooked grains are coated with a flavorful and antimicrobial mixture of rice vinegar and sugar. Rice salads should be similarly acidified with vinegar, lemon or lime juice.