Epidemiology and Infection

Lessons from a large outbreak of Escherichia coli O157[ratio]H7 infections: insights into the infectious dose and method of widespread contamination of hamburger patties

J. TUTTLE a1c1, T. GOMEZ a2, M. P. DOYLE a3, J. G. WELLS a1, T. ZHAO a3, R. V. TAUXE  and P. M. GRIFFIN a1
a1 Foodborne and Diarrheal Diseases Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, Mailstop A-38, Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA 30333
a2 Veterinary Services, Animal and Plant Health Inspection Service, United States Department of Agriculture, Atlanta, GA 30333
a3 Center for Food Safety and Quality Enhancement, University of Georgia, Georgia Station, Griffin, GA 30223


Between November 1992 and February 1993, a large outbreak of Escherichia coli O157[ratio]H7 infections occurred in the western USA and was associated with eating ground beef patties at restaurants of one fast-food chain. Restaurants that were epidemiologically linked with cases served patties produced on two consecutive dates; cultures of recalled ground beef patties produced on those dates yielded E. coli O157[ratio]H7 strains indistinguishable from those isolated from patients, confirming the vehicle of illness. Seventy-six ground beef patty samples were cultured quantitatively for E. coli O157[ratio]H7. The median most probable number of organisms was 1·5 per gram (range, <0·3–15) or 67·5 organisms per patty (range, <13·5–675). Correlation of the presence of E. coli O157[ratio]H7 with other bacterial indicators yielded a significant association between coliform count and the presence of E. coli O157[ratio]H7 (P=0·04). A meat traceback to investigate possible sources of contamination revealed cattle were probably initially colonized with E. coli O157[ratio]H7, and that their slaughter caused surface contamination of meat, which once combined with meat from other sources, resulted in a large number of contaminated ground beef patties. Microbiological testing of meat from lots consumed by persons who became ill was suggestive of an infectious dose for E. coli O157[ratio]H7 of fewer than 700 organisms. These findings present a strong argument for enforcing zero tolerance for this organism in processed food and for markedly decreasing contamination of raw ground beef. Process controls that incorporate microbiological testing of meat may assist these efforts.

(Accepted August 3 1998)

c1 Author for correspondence.