A common superstition, the five-second rule states that food dropped on the ground will not be contaminated with bacteria if it is picked up within 5 seconds of being dropped. There are numerous variations on the rule including “three-second rule”, “seven-second rule”, “thirty-second rule”, and “ten-second rule”, and others.
In 2006 using salmonella on wood, tiles, and nylon carpet, researchers found that the bacteria were still thriving after twenty-eight days of exposure under dry conditions. Tested after eight hours’ exposure, the bacteria could still contaminate bread and bologna in under five seconds, but a minute-long contact increased contamination about tenfold (with tile and carpet surfaces only). The five-second rule was also featured in an episode of the Discovery Channel series MythBusters. There was no significant difference in the number of bacteria collected from 2 seconds exposure as there was from 6 seconds exposure. The moisture, surface geometry and the location the food item was dropped on did, however, affect the number of bacteria.
Source: Wikipedia