Longer Contact Times Increase Cross-Contamination of Enterobacter aerogenes from Surfaces to Food

Robyn C. Miranda, Donald W. Schaffner

Bacterial cross-contamination from surfaces to food can contribute to foodborne disease. The cross-contamination rate of Enterobacter aerogenes was evaluated on household surfaces using scenarios that differed by surface type, food type, contact time (<1, 5, 30 and 300 s), and inoculum matrix (tryptic soy broth or peptone buffer). The surfaces used were stainless steel, tile, wood and carpet. The food types were watermelon, bread, bread with butter and gummy candy. Surfaces (25 cm(2)) were spot inoculated with 1 ml of inoculum and allowed to dry for 5 h, yielding an approximate concentration of 10(7) CFU/surface. Foods (with 16 cm(2) contact area) were dropped on the surfaces from a height of 12.5 cm and left to rest as appropriate. Post transfer surfaces and foods were placed in sterile filter bags and homogenized or massaged, diluted and plated on tryptic soy agar. The transfer rate was quantified as the log % transfer from the surface to the food. Contact time, food and surface type all had a highly significant effect (P<0.000001) on log % transfer of bacteria. The inoculum matrix (TSB or peptone buffer) also had a significant effect on transfer (P = 0.013), and most interaction terms were significant. More bacteria transferred to watermelon (∼0.2-97%) relative to other foods, while fewer bacteria transferred to gummy candy (∼0.1-62%). Transfer of bacteria to bread (∼0.02-94%) and bread with butter (∼0.02-82%) were similar, and transfer rates under a given set of condition were more variable compared with watermelon and gummy candy. The popular notion of the "five second rule" states food dropped on the floor for less than five seconds is "safe", because bacteria need time to transfer. The rule has been explored by a single study in the published literature and on at least two television shows. Results from two academic laboratories have been shared through press release, but remain unpublished. We explore this topic using four different surfaces (stainless steel, ceramic tile, wood and carpet), four different foods (watermelon, bread, bread with butter and gummy candy), four different contact times (<1, 5, 30 and 300 s), and two bacterial preparation methods. Although we show that longer contact times result in more transfer, we also show that other factors including the nature of the food and the surface are of equal or greater importance. Some transfer takes place "instantaneously" at times <1 s, disproving the "five second rule".