Scientists Ran 2,560 Tests on the Five-Second Rule. Some Bacteria Didn’t Need One Second.

Everybody knows the rule. Food on the floor is fine if you grab it fast, because germs need a few seconds to climb aboard.

At Rutgers University, food scientist Donald Schaffner and graduate student Robyn Miranda decided to test it properly. Their study appeared in 2016 in Applied and Environmental Microbiology.

The setup was thorough. Four surfaces: stainless steel, ceramic tile, wood and carpet. Four foods: watermelon, bread, buttered bread and gummy candy. Four contact times: under one second, 5 seconds, 30 seconds and 5 minutes. The bacteria were also prepared two different ways, which made 128 scenarios, and each one was repeated 20 times, for 2,560 measurements. The surfaces were coated with a harmless, food-grade bacterium and allowed to dry before anything was dropped.

Here’s what came back.

Some bacteria transferred in under a second. That alone knocks out the rule. In Schaffner’s words, bacteria “can contaminate instantaneously.”

Wet food is the worst. Watermelon picked up the most bacteria. Gummy candy picked up the least. As Schaffner put it, “Bacteria don’t have legs, they move with the moisture.”

Longer is still worse. In general, more contact time meant more transfer. So the rule isn’t pure fiction. It’s just a big oversimplification.

Carpet was the surprise. It transferred far less than tile or stainless steel. Wood was all over the place. The researchers pointed to the texture of the surface and the food as the likely reason.

Keep in mind: this was a lab test with one bacterium on small squares of material. Your floor, and whatever lives on it, is a different story. Carpet transferring less in this test doesn’t mean carpet is clean.

National Geographic was among the outlets that picked up the study at the time. The verdict from the lab itself was blunt: Schaffner called the rule “a significant oversimplification.”