It is widely accepted that the 5-second rule allows you to eat any food you pick up on the floor as long as it is within five seconds. However, a new study published in the journal Applied and Environmental Microbiology proves that bacteria transfer to any food regardless the time you pick it up.

Moisture, type of surface and contact time are factors that play a role in cross-contamination, the researchers from the Rutgers University add. Sometimes, transfer of bacteria can even start in less than one second.

“The popular notion of the ‘five-second rule’ is that food dropped on the floor, but picked up quickly, is safe to eat because bacteria need time to transfer,” says Donald Schaffner, professor and extension specialist in food science at the university. “We decided to look into this because the practice is so widespread. The topic might appear ‘light’ but we wanted our results backed by solid science.”

The research team used four surfaces: stainless steel, ceramic tile, wood and carpet, as well as four food items: watermelon, bread, bread and butter, and gummy candy.

The contact times were also taken into account, which included less than one second, five, 30 and 300 seconds.  The media they used were tryptic soy broth or peptone buffer, where they grew Enterobacter aerogenes, a naturally occurring non-pathogenic “cousin” of Salmonella in our digestive system.

They found that watermelon underwent the most contamination. The food item that got the least contamination was the gummy candy.

Overall, they concluded that transfer of bacteria from surfaces to food depends on the amount of moisture. Moreover, the longer the food contact, the more bacteria moved to the food.

They also found that carpet has very low transfer rates, unlike stainless steel or tile. The transfer from wood, on the other hand, was deemed more variable. It is most likely that the topography of the surface and the food also contributed to contamination.

The researchers prove that the 5-second rule is an oversimplification of what actually happens. Schaffner adds that people should always remember that bacteria can contaminate instantaneously.”