An enterohemorrhaghic Escherichia coli O157:H7 (EHEC) outbreak in 1993 was epidemioiogically linked to commercial real mayonnaise. This study evaluated EHEC contamination risk during commercial mayonnaise and mayonnaise dressing production, and EHEC behavior in low-pH dressings. Two potential contamination sources, pasteurized liquid eggs and wet environmental areas, were surveyed for 4 months in three processing plants. One hundred eighty-eight egg lots and 114 environmental swabs were collected and analyzed for EHEC by enrichment and direct plating methods. All plant samples were EHEC negative. Commercial mayonnaise plants which use pasteurized eggs and employ effective good manufacturing practices (GMP) sanitation programs are unlikely EHEC harborage and contamination sources. Five commercial real-mayonnaise-based and reduced-calorie and/or fat mayonnaise dressings were inoculated with ≥6 log10 colony-forming units (CFU)/g EHEC contamination levels and stored at 25°C. The products contained a wide range of acetic acid, NaCl, and preservative levels, while pH varied from 3.21 to 3.94. Products below pH 3.6 rapidly inactivated EHEC, producing ≥7 log10 CFU/g decreases in ≤1 to ≤3 days. High EHEC lethality was also observed in the pH 3.94, egg white-mayonnaise dressing. Intact packages of commercial mayonnaise and mayonnaise dressings pose negligible EHEC contamination and health hazard risks. As with any food, consumers and food-service workers must use stringent hygienic practices to prevent microbial pathogen contamination during preparation, handling, and storage of mayonnaise-ingredient recipes such as chilled perishable salads and salad bar dressings.

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