In general, if there are bacteria on the floor, they will cling to the food nearly immediately on contact, Dr. Gulick said. Factors that influence the risk and the rate of bacterial transfer include the type of floor; the type of food; the type of bacteria; and how long the bacteria have been on the floor.

In a study published in 2007 in The Journal of Applied Microbiology, Clemson University researchers tested salmonella placed on wood, tile or carpet, and dropped bologna on the surfaces for 5, 30 or 60 seconds. With both wood and tile, more than 99 percent of the bacteria were transferred nearly immediately, and there was no difference by the time of contact. Carpet transferred a smaller number of bacteria, again with no difference by contact time. The amount transferred decreased over hours, but there were still thousands of the bacteria per square centimeter on the surfaces after 24 hours, and hundreds survived on the surfaces for as long as four weeks. As few as 10 salmonella bacteria can cause gastroenteritis.C. CLAIBORNE RAY