Lab testing has confirmed 26 cases of infection with another eight suspected in the St. Louis area, health officials said.
Health authorities said interviews with victims suggested salad bars at Schnucks supermarket locations were a common link in most of their food histories, the St. Louis Post-Dispatch reported Wednesday.
Schnucks voluntarily removed suspected items including lettuce, strawberries and Caesar dressing from its salad bars when the outbreak was confirmed Oct. 26, a company spokeswoman said.
"Those products that were in question were not created by Schnucks," spokeswoman Lori Willis said. "There's got to be a supplier involved in some way."
The source of the bacteria could be further up the food distribution chain, and the U.S. Food and Drug Administration will join the investigation as it expands to farms and distribution centers in other states, health officials said.