How to Test a Folk Theory

For all aficionados of the five-second rule, this New York Times piece is fun must-reading:

Prof. Paul L. Dawson and his colleagues at Clemson have now put some numbers on floor-to-food contamination.

Their bacterium of choice was salmonella; the test surfaces were tile, wood flooring and nylon carpet; and the test foods were slices of bread and bologna.

First the researchers measured how long bacteria could survive on the surfaces. They applied salmonella broth in doses of several million bacteria per square centimeter, a number typical of badly contaminated food.

I had thought that most bacteria were sensitive to drying out, but after 24 hours of exposure to the air, thousands of bacteria per square centimeter had survived on the tile and wood, and tens of thousands on the carpet. Hundreds of salmonella were still alive after 28 days.

Professor Dawson and colleagues then placed test food slices onto salmonella-painted surfaces for varying lengths of time, and counted how many live bacteria were transferred to the food.

On surfaces that had been contaminated eight hours earlier, slices of bologna and bread left for five seconds took up from 150 to 8,000 bacteria. Left for a full minute, slices collected about 10 times more than that from the tile and carpet, though a lower number from the wood.

In my opinion, the scariest bit in there is the bit about the longevity of salmonella in your carpet. Scary, but perhaps not surprising. So there you have it: another cherished folk theory bites the dust, courtesy of science.