Purpose: This study quantifies cross-contamination rates of Enterobacter aerogenes from four common household surfaces to two 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 (ceramic unglazed tile, indoor/outdoor 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 white bread and gummy squares were dropped on the respective surfaces from 5 inches and left to rest for four different times (0 to 300 s), and the transferred cells enumerated. Each unique condition was replicated 20 times for 1,280 total measurements.
Results: E. aerogenes showed increased transfer from tile, wood or stainless steel to white bread with increased contact time. Transfer ranged from below detection limit (0 sec, buffer inoculum on tile to white bread) to a high of 19.0% (TSB inoculum on tile to white bread at 300 s). The transfer from carpet to white bread was always below the detection limit (2 log CFU/sample) for each experiment. The transfer from all surfaces, inoculum suspension types, and contact times to gummy squares was below the detection limit except for tile and wood at 300 s in TSB inoculum, where ~0.20% transfer occurred.
Significance: Longer contact times promote greater transfer. Surface type (both contact surface and food) often has a more significant effect than contact time.