Yes, we all want to believe that that chocolate-covered pretzel stick that we just dropped on the floor of the kitchen which hasn't been mopped in an unmentionable number of weeks is good to eat as long as you pick it up lickety-split. And I hate to be the bearer of bad news guys, but it turns out that the most recent studies by food microbiologists from Rutgers University show that every child's (and many adults') favorite Five-Second Rule doesn't actually hold up in most scenarios. According to the scientists, the legitimacy of how much bacteria will actually invade your food droppings depends more on what food it is and where you dropped it, not the amount of time you had left it.
In their study, the scientists put the bacteria Enterobacter aerogenes on four different surfaces: stainless steel, wood, tile, and carpet and dropped one of four different foods - bread, buttered bread, watermelon, and a gummy bear - on each material. They also varied the amounts of time that they let the food sit, from less than 1 second to five minutes. They found that the moistness of the food and the flatness of the surface it is dropped on matters way more than how long it has been on the ground. In other words, the watermelon didn't fare so well on the stainless steel because there was a lot of surface area for the bacteria to transfer themselves onto the food.
Now eating food off the ground doesn't necessarily mean you will get sick - that has to do more with what type of bacteria your food has collected! Nevertheless, usually when you drop something you don't have the tools or time to see if the bacteria that's stuck to your snack is harmful or not, so better to stay on the cautious side.