Background: From March through August 1993, outbreaks of Escherichia coli O157:H7 occurred at 4 separate Oregon and Washington steak and salad bar restaurants affiliated with a single national chain.
Objective: To determine the cause of outbreaks of E coli O157:H7 at 4 chain restaurants.
Methods: Independent case-control studies were performed for each outbreak. Available E coli O157:H7 isolates were subtyped by pulse-field gel electrophoresis and by phage typing.
Results: Infection was not associated with beef consumption at any of the restaurants. Implicated foods varied by restaurant but all were items served at the salad bar. Among the salad bar items, no single item was implicated in all outbreaks, and no single item seemed to explain most of the cases at any individual restaurant. Molecular subtyping of bacterial isolates indicated that the first 2 outbreaks, which occurred concurrently, were caused by the same strain, the third outbreak was caused by a unique strain, and the fourth was multiclonal.
Conclusions: Independent events of cross-contamination from beef within the restaurant kitchens, where meats and multiple salad bar items were prepared, were the likely cause of these outbreaks. Meat can be a source of E coli O157:H7 infection even if it is later cooked properly, underscoring the need for meticulous food handling at all stages of preparation.