Purpose: This study quantifies cross-contamination rates of Enterobacter aerogenes from four common household surfaces to four representative foods considering contact time and the matrix containing the organism.
Methods: Two inoculum matrices were used: tryptic soy broth (TSB) containing an overnight culture and peptone buffer containing cells from overnight culture separated by centrifugation. Household surfaces (tile, carpet, wood, stainless steel) were inoculated with a nalidixic acid resistant strain of E. aerogenes and dried for 5 h. Surfaces contained ~7.0 log CFU/surface after drying. Squares (4x4 inches) of bread, bread with butter, watermelon and gummy squares were dropped on the respective surfaces from 5 inches and for four different times (0 to 300 sec), and the transferred cells enumerated. Each unique condition was replicated 20 times for 2,560 total measurements.
Results: The interactions of variables, as well as individual variables were analyzed for statistical significance by multiple linear regression analysis. The time, food, surface and the food*time interaction were all shown to have a significant effect (P < 0.0001) on the log % transfer of E. aerogenes. Time*surface, food*surface and matrix*surface interaction had a significant effect on log % transfer (P ≤ 0.0001). The matrix (P = 0.0129) and food*matrix interaction (P = 0.0445) were also significant. The time*matrix interaction was not statistically significant regarding log % transfer rate (P = 0.4949).
Significance: All variables and their interactions play a role in the transfer rate of E. aerogenes from surface to food. Overall, longer contact times promote greater transfer. Although time*matrix interaction was not statistically significant, the individual variables all have a significant effect on transfer.