Cross-contamination of Lettuce with Escherichia coli O157:H7

Abstract

Contamination of produce by bacterial pathogens is an increasingly recognized problem. In March 1999, 72 patrons of a Nebraska restaurant were infected with enterohemorrhagic Escherichia coli (EHEC) O157:H7, and shredded iceberg lettuce was implicated as the food source. We simulated the restaurant's lettuce preparation procedure to determine the extent of possible EHEC cross-contamination and growth during handling. EHEC inoculation experiments were conducted to simulate the restaurant's cutting procedure and the subsequent storage of shredded lettuce in water in the refrigerator. All lettuce pieces were contaminated after 24 h of storage in inoculated water (2 x 10 9 CFU of EHEC per 3 liters of water) at room temperature or at 4°C; EHEC levels associated with lettuce increased by > 1.5 logs on the second day of storage at 4°C. All lettuce pieces were contaminated after 24 h of storage in water containing one inoculated lettuce piece (-105 CFU of EHEC per lettuce piece) at both temperatures. The mixing of one inoculated dry lettuce piece with a large volume of dry lettuce, followed by storage at 4°C or 25°C for 20 h resulted in 100% contamination of the leaves tested. Microcolonies were observed on lettuce stored at 25°C, while only single cells were seen on leaves stored at 4°C, suggesting that bacterial growth had occurred at room temperature. Three water washes did not significantly decrease the number of contaminated leaves. Washing with 2,000 mg of calcium hypochlorite per liter significantly reduced the number of contaminated pieces but did not eliminate contamination on large numbers of leaves. Temperature abuse during storage at 25°C for 20 h decreased the effectiveness of the calcium hypochlorite treatment, most likely because of bacterial growth during the storage period. These data indicate that storage of cut lettuce in water is not advisable and that strict attention must be paid to temperature control during the storage of cut lettuce.