CROSS-CONTAMINATION OF LETTUCE WITH ESCHERICHIA COLI 0157:H7

Authors: Wachtel, Marian; CHARKOWSKI, AMY - U. OF WISCONSIN, MADISON

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Contamination of fresh-cut produce with human pathogens is an increasing problem in this country. In March 1999, seventy-two patrons of a Nebraska restaurant were infected with Enterohemorrhagic Escherichia coli (EHEC) O157:H7, with shredded iceberg lettuce implicated as the food source. We simulated the restaurant's lettuce preparation procedure to identify critical control points for lettuce preparation. EHEC inoculation experiments and washing studies were conducted to simulate the restaurant's cutting and storage of shredded lettuce in water in the refrigerator. Our data indicate that water storage of cut lettuce is ill-advised because it can lead to cross-contamination of a large volume of produce. Dry-mixing also caused extensive cross- contamination of cut lettuce, and temperature abuse during storage may have led to bacterial growth on produce, compounding this problem. Washing lettuce with water was ineffective at removing bacteria from leaf surfaces. Washing leaves with high amounts of calcium hypochlorite reduced bacterial contamination on cut lettuce pieces. However, enough contamination remained on cut lettuce to cause human disease. These data represent important information for scientists working in the field, as well as for industry to document the extensive contamination possible from water storage and dry-mixing of contaminated produce, and the ineffectiveness of current washing procedures.

Technical Abstract: In March 1999, seventy-two patrons of a Nebraska restaurant were infected with Enterohemorrhagic Escherichia coli (EHEC) O157:H7, with shredded iceberg lettuce implicated as the food source. We simulated the restaurant's lettuce preparation procedure to identify critical control points for lettuce preparation. EHEC inoculation experiments were conducted to simulate the restaurant's cutting and storage of shredded lettuce in water in the refrigerator. All lettuce pieces were contaminated after 24 hours of storage in inoculated water at room temperature or 4 C; EHEC levels associated with lettuce increased > 1.5 logs on the second day of storage at 4 C. All lettuce pieces were contaminated after 24 hours of storage in water containing one inoculated lettuce piece, at both temperatures. Dry-mixing one inoculated lettuce piece with a large volume of lettuce, followed by storage at 4 C or 20 C for 20 hours resulted in 100% contamination of leaves tested. Microcolonies were observed on lettuce stored at 20 C, while only single cells were seen on 4 C-stored leaves, suggesting that bacterial growth had occurred at room temperature. Washing leaves with three water washes did not significantly decrease the number of contaminated leaves; washing with 2000 ppm calcium hypochlorite significantly reduced the number of contaminated pieces. Temperature abuse during storage decreased the effectiveness of the calcium hypochlorite treatment, most likely due to bacterial growth during the storage period. These data indicate that water storage of cut lettuce is ill-advised, and that strict attention must be made to temperature control during storage of cut lettuce.