CROSS-CONTAMINATION OF LETTUCE BY ESCHERICHIA COLI O157:H7 VIA CONTAMINATED GROUND BEEF

Authors: Wachtel, Marian; McEvoy, James; Luo, Yaguang - Sunny; Williams Campbell, Anisha; Solomon, Morse

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2002
Publication Date: 7/7/2003
Citation: Wachtel, M.R., McEvoy, J.L., Luo, Y., Williams Campbell, A.M., Solomon, M.B. 2003. Cross-contamination of lettuce by Escherichia coli O157:H7 via contaminated ground beef. Journal of Food Protection. 66(7):1176-1183.

Interpretive Summary: Several recent E. coli O157:H7 outbreaks have occurred due to the consumption of raw produce that had presumably been cross-contaminated from meat in a food preparation setting. However, little information exists detailing how such cross-contamination can happen. In our study, the cross-contamination of E. coli O157:H7 from ground beef to hands, cutting-boards and lettuce was examined. This foodborne pathogen can cause severe cramping and bloody diarrhea in healthy individuals and even worse problems including death in children under 5, the elderly (over 55 to 60) and individuals with weakened immune systems. E. coli was capable of growth in ground beef patties when left overnight at room temperature, and was readily transferred to hands, cutting boards and lettuce. Warm water rinses of cutting boards were not effective at removing bacteria. Contact of lettuce with a contaminated cutting board resulted in transfer of E. coli from the board to the lettuce. Bacteria were transferred to the 25th consecutive leaf applied to a cutting board containing only a very few E. coli bacteria. Our results have documented under controlled, experimental conditions, that significant contamination of lettuce, and therefore possibly other fresh produce, can occur by cross-contamination from other sources in a kitchen or other food preparation setting. These results will be helpful to commercial food handlers and consumers, since they reinforce the need for adherence to strict food safety practices during food preparation, storage and handling.

Technical Abstract: An E. coli O157:H7 lettuce outbreak strain was used to quantitate pathogen survival in ground beef and transfer to hands, cutting board surfaces, and lettuce. Overnight storage of inoculated beef resulted in no pathogen growth, while room temperature storage allowed multiplication. Hamburger patty formation allowed bacterial transfer 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 number of recoverable pathogens by ~ 1 log CFU. A 15 sec water rinse failed to remove significant numbers of pathogens from cutting boards, whether applied immediately after contamination, or following overnight room temperature storage. Three lettuce leaves were applied successively to a single contaminated cutting board area immediately after contamination, and after overnight room temperature storage of contaminated boards. Another set of leaves was pressed onto boards immediately following contamination, then stored overnight at 4 degrees C before quantitation. Greater numbers of pathogens were transferred to the first pressed leaf compared to the second or third leaves. There was no difference in the number of pathogens recovered 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 pressed successively onto one area on a board containing 1.25 X 100 CFU E. coli. Pathogens were transferred to 46% of the leaves, including the 25th exposed leaf.