I think we have all been somewhere in this situation before: a delicious morsel of food makes its way onto the floor, someone yells, “5-second rule!” and snatches it up and pops it into their mouth. Usually this results in a mixed response from onlookers. Some cringe, others laugh, and some nod on with encouragement and agreement. Occasionally, a discussion will follow after someone invokes the 5-second rule. Is it real? If you pick up food off the ground within 5 seconds, does that make it a safe alternative to the food that was on the ground for more than 5-seconds?
So, what is this rule, precisely? Well, it is often cited in discussion, the idea that harmful bacteria will take around 5-seconds to attach themselves or otherwise move onto a piece of food after it has fallen onto the floor. But, how true is this, really? To make a long story short, it’s not all that true. The truth of the matter is a mixed bag of creepiness, biology, and harsh reality.
Unfortunately, there is not any real science or evidence to support the legitimacy of the 5-second rule, and actually, there have been some studies that actually debunk the myth entirely. The most significant of these is a set of experiments which were completed by Rutgers University and published in September of 2016. The team at Rutgers completed 2158 measurements with a number of different food items which were dropped onto various surfaces. These included moisture-rich items like watermelon and relatively dry items like gummy candy. They tested dropping the food onto four different surfaces including smooth ceramic and rough carpet.
After each food item was dropped for a measured amount of time (The time points studied were 1, 5, 30, and 300-seconds spent on the “floor”), the piece of food was cultured on an agar plate specifically designed to encourage and support the growth of Enterobacter aerogenes, a tiny, rod-shaped species of bacteria that colonizes the GI tract of most individuals. This particular strain is related and behaves similarly to salmonella, but is less pathogenic and less dangerous for scientists to work with. This species of Enterobacter is considered an opportunistic pathogen, which means that while a healthy person has no trouble defending against their internal population of bacteria, Enterobacter waits on the sidelines for poor health or injury to arise so that it may take advantage of the situation and cause an infection.
The results of the study will no doubt be disconcerting to those who have lived by the 5-second rule. Ultimately, bacteria can transfer from one surface to another in less than a second. Once the food hits the floor, consider it colonized! In the brief moments after impact, the food has already become a new home for a variety of bacterial species. There are a few variables in this process that can make the bacterial transfer more or less substantial. From the Rutgers study, some variables and factors discovered are that flat surfaces (the ceramic) have a greater transfer potential than the rougher surface (the carpet), which can be simply attributed to the greater surface area of contact provided by flatter materials. In addition, you could imagine how thick carpeting might attenuate some of the messiness of wet food in comparison to a slick surface. However, above all, it seems that moisture is the biggest factor. The more moisture in the food/surface, the more potential there is for a large transfer of bacteria.
Why moisture? Liquids provide a medium through which bacteria may flow and be pushed around in the tiny currents that move around upon the food impacting the floor. A lot of bacteria can move of their own accord too. These are classified as “motile” bacteria and often possess one or more flagella, whip-like appendages that stick out from the bacterial membrane and move in some fashion to coordinate movement. This micro-whips, almost a form of primitive tentacle, undulate and propel the bacterium closer or farther away from some particular stimulus, like moving toward food or moving away from a hostile environment. These motile bacteria will be both moved through the liquid and will also move of their own volition onto new surfaces. There are also “immotile” bacteria, or bacteria that do not move. These remain fixed in position and grow wherever it is they find themselves, if possible. Without the opportunity to travel of their own volition, these kinds of bacteria rely on other factors to transport them if it becomes necessary. Moisture is one of the greatest ways of moving bacteria around and immotile bacteria have used this factor to successfully move, spread, and thrive around the world! For this reason, dropping a piece of wet food on the floor often rolls out the red carpet for some bacteria, while also acting as the limousine that picks them up and brings them in style (and en mass) to the food party!
In conclusion, please don’t eat food off the floor. The freshly-mixed bacterial cocktail is not worth the risk. Each time you do, you roll the dice with whatever bacteria happen to be picked and are then inserted into your body. Sometimes you may get sick, on rare occasions you may be unlucky and pick up a dangerous infection, and in the best case scenario you may not get sick at all. However, in the latter, most fortunate, scenario: your immune system doesn’t need the stress! Our bodies are under constant assault from bacteria from all corners and walks of life and your body generally doesn’t appreciate you facilitating the next round of reinforcements! I’m sure we’d all like to live in a world where the 5-second rule is a reality, saving thousands of potential pieces of food from floor-touching oblivion, but unfortunately, it simply is not true.