Living the Scientific Life (Scientist, Interrupted)

tags: five-second rule, food, bacteria, microbiology

Have you ever heard of the five-second rule, where you can pick up food that has fallen on the floor within five seconds and eat it without risk of illness? Do you follow it? In 2003, a then-high school science intern at the University of Illinois, Jillian Clarke, conducted a survey and found that slightly more than half of adult men and 70 percent of adult women knew about the five-second rule and many said they followed it. Clarke then conducted an experiment to find out if various food became contaminated with bacteria after just five seconds on the floor.

For performing this first test of the five-second rule, Clarke was awarded the 2004 Ig Nobel Prize in public health by the Annals of Improbable Research.

Clarke's study inspired another research group at Clemson University to investigate several questions regarding the five-second rule: Does the type of contaminated surface affect the numbers of bacteria collected? How many bacteria does a food item collect in just five seconds? Does it collect more if it sits on the contaminated surface longer? Does it collect enough to make you sick?

To answer these questions, Paul Dawson and his colleagues conducted several experiments of floor-to-food contamination. They chose Salmonella as their pathogenic bacteria; they examined several surfaces; tile, wood flooring and nylon carpet; and the test foods were slices of bread and bologna.

First, the researchers determined how long the bacteria could live on these test surfaces. To do this, they applied a broth containing several million live bacteria per square centimeter to their test surfaces -- typical numbers for badly contaminated food -- and found that, after 24 hours, thousands of Salmonella survived per square centimeter on both the tile and wood surfaces, while tens of thousands survived on the carpet. Surprisingly, hundreds of Salmonella were still alive after 28 days.

Next, Dawson and his colleagues placed test food slices onto Salmonella-covered surfaces for varying lengths of time, and counted how many live bacteria were transferred to the food. Slices of bologna and bread left for five seconds took up between 150 to 8,000 bacteria from surfaces that had been contaminated eight hours earlier. However, left for a full minute, the food slices collected about 10 times more bacteria from the tile and carpet, and a lower number from the wood surface.

Dawson and colleagues found that the type of contaminated surface did affect the number of bacteria that the food slices took up, and that the length of time that the food remained on the contaminated surface did affect the numbers of bacteria they absorbed. Apparently, this amount of bacteria is potentially enough to cause illness in people; the infectious dose -- the smallest number of bacteria that can actually cause illness -- is as few as 10 for some Salmonellas.

But consider that Clarke, the original investigator, found that bacterial contamination was so low on the floor at the University of Illinois that it couldn't be measured, unlike the levels of contamination that the Clemson group were using for their studies. So the likelihood that a cookie, quickly picked off the floor and consumed, can make you ill, is somewhat remote, but it is a factor worth considering if you are in an area where there could be significant levels of bacteria present.

Source:

Dawson, P., Han, I., Cox, M., Black, C., Simmons, L. (2007). Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five-second rule. Journal of Applied Microbiology , 102(4), 945. DOI: 10.1111/j.1365-2672.2006.03171.x [Original paper]