The majority of the cases are in Wisconsin, with Minnesota reporting six and Illinois one.
The U.S. Agriculture Department is investigating the Minnesota and Wisconsin cases. Samples of ground beef obtained from the homes of two Minnesota victims tested positive for E. coli bacteria.
Health officials have not yet said where the ground beef was purchased or processed.
Nineteen of those sickened required hospitalization, with one victim suffering from hemolytic uremic syndrome, which can lead to kidney failure.
Epidemiologists said two strains of E. coli are involved -- one affecting 33 Wisconsin residents and the other infecting 19.
The outbreak first was reported Aug. 20 and the second, Sept. 9.
E. coli is characterized by diarrhea, nausea and stomach cramps. Health officials say it can be avoided by cooking meat thoroughly or choosing irradiated products.
News of the outbreak comes just days after the USDA announced a series of new measures aimed at reducing the incidence of contamination as part of a Food Safety and Inspection Service review prompted by scientific data indicating the pathogen is more common than previously believed.
Among the USDA actions was elimination of current exemptions from FSIS microbiological testing.