Paul Dawson, a professor of Food Science at Clemson University, investigated the history of the "Five Second Rule" and ran experiments to see how much bacteria actually transfers from the floor onto dropped food.
From The New Republic:
To find out, we inoculated squares of tile, carpet or wood with Salmonella. Five minutes after that, we placed either bologna or bread on the surface for five, 30, or 60 seconds, and then measured the amount of bacteria transferred to the food. We repeated this exact protocol after the bacteria had been on the surface for two, four, eight, and 24 hours.
We found that the amount of bacteria transferred to either kind of food didn't depend much on how long the food was in contact with the contaminated surface—whether for a few seconds or for a whole minute. The overall amount of bacteria on the surface mattered more, and this decreased over time after the initial inoculation. It looks like what's at issue is less how long your food languishes on the floor and much more how infested with bacteria that patch of floor happens to be.
We also found that the kind of surface made a difference as well. Carpets, for instance, seem to be slightly better places to drop your food than wood or tile. When carpet was inoculated with Salmonella, less than 1 percent of the bacteria were transferred. But when the food was in contact with tile or wood, 48 percent to 70 percent of bacteria transferred….
So the next time you consider eat eating dropped food, the odds are in your favor that you can eat that morsel and not get sick. But in the rare chance that there is a microorganism that can make you sick on the exact spot where the food dropped, you can be fairly sure the bug is on the food you are about to put in your mouth.
"Is the "Five Second Rule" Real? A Food Scientist Explains." (The New Republic)
Comic: "Crop from Greg Williams' WikiWorld. Original version," (CC BY 2.5).