Well that’s interesting. Mythbusters busted this one. http://www.discovery.com/tv-shows/mythbusters/mythbusters-database/5-second-rule.
They found results varied more with different foods and surfaces, versus time periods. Not all bacteria is harmful though, and I don’t recall if the episode went that in-depth as far as discussing what types of bacteria they found in the results. Either way, now that we have a dog, we don’t have to worry about it much anymore.