If you’ve ever dropped a piece of food on the floor and quickly popped it into your mouth, you may have been relying on a long-standing health myth: the five-second rule. In fact, the notion that food dropped on the floor is safe if it’s picked up within a few seconds has no merit. In a recent study using a variety of foods and surfaces, researchers found that bacteria can accumulate on dropped food in less than one second. You’d have to have superpowers to grab food that fast! The amount of moisture in the food matters most, since bacteria moves with moisture. In other words, watermelon or an orange slice will accumulate bacteria more quickly than, say, a cracker. The floor surface matters too, with tile and stainless steel leading to quicker contamination than carpet.
Source: Longer Contact Times Increase Cross-Contamination of Enterobacter aerogenes from Surfaces to Food
When in doubt, throw it out.