Cross-contamination of lettuce (Lactuca sativa L.) with Escherichia coli O157:H7 via contaminated ground beef.

A lettuce outbreak strain of E. coli O157:H7 was used to quantitate the pathogen's survival in ground beef and its transfer to hands, cutting board surfaces, and lettuce. Overnight storage of inoculated beef at 4 degrees C resulted in no pathogen growth, while room-temperature storage allowed multiplication. Hamburger patty formation allowed the transfer of bacteria to hands. Contaminated fingers subsequently transferred the pathogen to lettuce during handling. E. coli was transferred from hamburgers to cutting board surfaces; overnight storage of boards decreased the numbers of recoverable pathogens by approximately 1 log CFU. A 15-s water rinse failed to remove significant numbers of pathogens from cutting boards whether it was applied immediately after contamination or following overnight room-temperature storage. Three lettuce leaves were successively applied to a single contaminated cutting board area both immediately after contamination and after overnight room-temperature storage of contaminated boards. Another set of leaves was pressed onto boards immediately following contamination and was then stored overnight at 4 degrees C before pathogen enumeration. The numbers of pathogens transferred to the first pressed leaves were larger than those transferred to the second or third leaves. There were no significant differences in the numbers of pathogensrecovered from leaves pressed immediately after contamination whether pathogens were enumerated immediately or following overnight storage at 4 degrees C. However, fewer pathogens were transferred to leaves pressed to boards stored overnight at room temperature prior to contact with lettuce. Twenty-five lettuce pieces were successively pressed onto one area on a board containing 1.25 x 10(2) CFU of E. coli. Pathogens were transferred to 46% of the leaves, including the 25th exposed leaf.