1) If you drop food onto a surface that you've intentionally contaminated with bacteria, it's going to pick up bacteria immediately.
2) But most surfaces have surprisingly little harmful bacteria.
As part of the only peer-reviewed study on this topic, scientists at Clemson dropped pieces of bologna and bread onto wood, tile, and carpet floors that had been heavily contaminated with salmonella.
There was some variation between the foods and surfaces, but in general, they found that 150 to 8,000 bacteria were picked up by the food within five seconds; after a full minute, these numbers were about ten times higher. Given that as few as ten individual Salmonella bacteria can cause an illness, this sounds like a pretty convincing case against eating off the floor.
"If you drop food onto a contaminated surface, it is going to pick up bacteria"
Except that's not exactly what Paul Dawson, the food scientist who led the study, took away from it. Although he certainly doesn't advocate eating off the floor, he told me that "the risk of getting sick from eating food dropped on the floor is very low, since most surfaces do not harbor pathogenic bacteria — unless you're in an environment likely to have harmful bacteria, like a hospital."
This point is underscored by an earlier (unpublished) study, conducted at the University of Illinois. In it, researchers (led by high school intern Jillian Clarke) started by testing how many bacteria of few particular species were on various floors across the campus — and found none.
"We were shocked," Meredith Agle, a then-Ph.D. candidate (a now food scientist) who worked on the study, told whoever wrote the university's press release. "We didn't even find a countable number of bacteria on the floor. We thought we might have made a mistake, so we tried again with the same result."
