A high school senior in the U.S. has dealt a blow to the gastronomic principle known as the five-second rule.

The rule states if food falls on the floor and remains there for five seconds or less then it's fine to pick it up and polish it off.

Jillian Clarke who attends the Chicago High School for Agricultural Sciences put the rule to the test.

Clarke says the rule dates back to Genghis Khan, founder of the Mongol Empire. He apparently considered food safe to eat so long as it had been on the floor for 20 hours or less.

As part of her seven-week internship at the University of Illinois at Urbana-Champaign, Clarke tested the five-second rule around campus.

The researchers tested how many microorganisms transferred onto food from rough and smooth tiles. Gummy bears and fudge-type cookies – the two most commonly dropped and eaten snacks – were tested.

The campus floors were actually quite clean. The researchers found fewer than 20 so-called colony-forming units of E. coli on the floors. The lower limit for detecting the microbes is 25 colony-forming units.

When the researchers purposely inoculated food with bacteria, they found it doesn't take long for the bugs to contaminate.

"People think if it made contact for only five seconds, then it is OK to eat, but it's false because if you do drop anything full of microorganisms such as E. coli, it will transfer and transfer immediately," Clarke told CBC Radio's As It Happens.

The texture of the food and floor tiles also made a difference, she found. Microbes transfer faster on smooth foods like gummy bears falling on smooth tiles compared to rough tiles or soft cookies.