Say It Ain’t So! Scientists Weigh In on the Five-Second Germ Rule

The five-second rule is a myth, says scientists. Bacteria contaminates dropped food upon contact.

[DIGEST: New York Times, Today, Nature World News]

You reach for a handful of almonds and one drops to the floor. You have five seconds to pick it up, right? Not so fast.

According to a new study by microbiologist Donald W. Schaffner at Rutgers University, no matter how soon you retrieve that food, bacteria will come along for the ride. The study appeared last month in the American Society for Microbiology’s journal Applied and Environmental Microbiology.

To test the five-second rule, Schaffner and master’s thesis student Robyn C. Miranda tested four different foods (cut watermelon, bread, buttered bread and gummy candy) against four different surfaces (steel, tile, wood and carpet). Each surfaced was treated with Enterobacter aerogenes, a non-pathogenic cousin of salmonella.

The researchers allowed the food to remain on the surface for varying amounts of time – 1 second, 5 seconds, 30 seconds or 300 seconds, yielding a total of 2,560 measurements. While they found that additional time on the floor did lead to more bacteria on the food, significant amounts of bacteria were transmitted within the first second of contact. “Bacteria can contaminate instantaneously,” said Schaffner. “At the shortest amount of time we studied. . . no matter what food and what surface, we always found some bacteria transfer in at least one of our experimental trials,” he continued.  

“We sort of joke about the five-second rule, but people act as if germs take some period of time to race to the item that fell on the floor,” joked William K. Hallman, an experimental psychologist and professor at Rutgers University.

Five-Second
Credit: Source.

While carpet may be the hardest to get clean, it also had the lowest rate of bacteria transmission among the four surfaces. That’s because the bacteria sinks down away from the surface, into the carpet fibers, said Schaffner.

Among the foods, watermelon had the most contamination, with the gummy candy having the least. “Watermelon was the most moist food we studied, and we saw almost all the bacteria transfer in the fraction of a second,” said Schaffner. “Transfer of bacteria from surfaces to food appears to be affected most by moisture,” he continued. The wetter the food, the higher the risk of transfer.

Transfer of bacteria to food raises an important public health issue. Schaffner pointed to research by the Centers for Disease Control that found that surface cross-contamination of food is the sixth most common contributing factor out of 32 in outbreaks of foodborne illness. About one in six Americans get sick every year from consuming contaminated foods.

Scientists have found that exposure to some bacteria, particularly in early life, can boost the immune system. However, when it comes to salmonella, you don’t want to play the odds. Said Schaffner, “I will tell you on the record that I’ve eaten food off the floor.” But, he added, “If I were to drop a piece of watermelon on my relatively clean kitchen floor, I’m telling you, man, it’s going in the compost.” 

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