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

Between November 1992 and February 1993, a large outbreak of Eschelichia co li O157:H7 infections occurred in the western USA and was associated with e ating ground beef patties at restaurants of one fast-food chain. Restaurant s 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,H7 strains indistinguishable from those i solated from patients, confirming the vehicle of illness. Seventy-six groun d beef patty samples were cultured quantitatively for E. coli O157: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 presen ce of E. coil O157:H7 with other bacterial indicators yielded a significant association between coliform count and the presence of E. coil O157:H7 (P = 0.04). A meat traceback to investigate possible sources of contamination revealed cattle were probably initially colonized with E, coli O157:H7, and that their slaughter caused surface contamination of meat, which once comb ined with meat from other sources, resulted in a large number of contaminat ed ground beef patties. Microbiological testing of meat from lots consumed by persons who became ill was suggestive of an infectious dose for E. coil O157:H7 of fewer than 700 organisms. These findings present a strong argume nt for enforcing zero tolerance for this organism in processed food and for markedly decreasing contamination of raw ground beef. Process controls tha t incorporate microbiological testing of meat may assist these efforts.