It is likely that most of us have faced this dilemma: a tasty morsel of food has fallen on the floor, 
but rather than discard it, the thought of picking it up quickly and eating it crosses the mind. The so-called 5-second rule suggests that eating food from the floor is safe, so long as it is picked up within 5 seconds. A new study, however, says otherwise.
Researchers led by Prof. Donald Schaffner, of Rutgers University in New Brunswick, NJ, report their findings in the American Society for Microbiology's journal, Applied and Environmental Microbiology.
According to the Centers for Disease Control and Prevention (CDC), around
1 in 6 people
in the United States get sick each year due to foodborne illness, which is around 48 million people. Of these, around 128,000 people are hospitalized and 3,000 die of foodborne diseases.
The authors note that bacterial cross-contamination from surfaces to food can contribute to foodborne disease, which is why they wanted to investigate the topic further.
"We decided to look into this because the [5-second rule] practice is so widespread," says Prof. Schaffner. "The topic might appear 'light' but we wanted our results backed by solid science."
Researchers used varying surfaces and contact times
Previous studies have suggested that the 5-second rule holds scientific fact, but such research in peer-reviewed journals is limited.
Prof. Schaffner says the 5-second rule is rooted in the fact that "bacteria need time to transfer." However, is picking up food within 5 seconds fast enough?
To further investigate, the team used four surfaces: stainless steel, ceramic tile, wood, and carpet. They also used four different foods: watermelon, bread, bread with butter, and gummy candy.
In addition, they employed four different contact times: less than 1 second, 5 seconds, 30 seconds, and 300 seconds.
The type of bacteria they used is called Enterobacter aerogenes, which is a "cousin" of Salmonella that naturally occurs in the human digestive system, and they used either tryptic soy broth or peptone buffer to grow the bacteria.
Once the bacteria were cultivated, the researchers spread them on the varying surfaces and allowed the preparation to dry before dropping the food samples.
'Bacteria can contaminate instantaneously'
The team evaluated transfer sequences for each surface, food, contact time, and bacterial preparation. In total, there were 128 scenarios that the researchers replicated 20 times each, which resulted in 2,560 measurements.
Results showed that watermelon had the most contamination, whereas gummy candy had the least. The researchers also found that bacterial transfer from surfaces to food is affected by moisture.
"Bacteria don't have legs," says Prof. Schaffner, "they move with the moisture, and the wetter the food, the higher the risk of transfer. Also, longer food contact times usually result in the transfer of more bacteria from each surface to food."
Interestingly, compared with tile and stainless steel, carpet had a lower transfer rate. The transfer rate from wood was variable.
The researchers conclude their study by noting that, although longer contact times result in more bacterial transfer, "other factors including the nature of the food and the surface are of equal or greater importance." They say their findings disprove the 5-second rule.