Few things match the delight of my dogs and myself at the sight of Floor Food. When it happens we're like "Ooo! Floor Food!" and compete to dive on it and eat it first. Sadly, The New York Times reports that the Five Second Rule—the cherished belief among some humans that it is 'safe' to eat Floor Food so long as it has been in contact with the floor for less than five seconds—has been debunked.
Professor Donald W. Schaffner, a food microbiologist at Rutgers University in New Jersey, said a two-year study he led concluded that no matter how fast you pick up food that falls on the floor, you will pick up bacteria with it.
They tested stainless steel, ceramic tile, wood, and carpet, with four different traditional floor foods: bread, buttered bread, watermelon and gummi bears. All resulted in the transfer of a salmonella-like bacterium.
HOWEVER. They also noted that while "bacteria can contaminate instantaneously," it was also the case that "longer contact times resulted in transfer of more bacteria," so I figure we're still good.