More residents are expected to be infected with E. coli bacteria in an outbreak that has already sickened at least 23 people across the St. Louis region and may be linked to produce at area groceries.

St. Louis County health officials confirmed that the source of the E. coli O157 strain was foodborne but said that the investigation was ongoing and that not all of the affected people had been interviewed. More cases are expected because the incubation period for E. coli can be as long as 10 days.

Scientists with the U.S. Centers for Disease Control and Prevention, who investigate multistate food-borne disease outbreaks, are expected to arrive in St. Louis over the weekend. Though groceries have not been asked to pull any food, Schnucks voluntarily replaced or removed some produce in salad bars and shelves.

"Once we heard that the health department had declared an outbreak, we took some proactive steps with our food safety team to switch products out that recent history told us could be potential sources," said Schnucks spokeswoman Lori Willis.

The last E. coli outbreak linked to produce occurred in May 2010 and involved romaine lettuce in five states not including Missouri or Illinois, according to the CDC. Another multistate outbreak in 2006 was linked to spinach.

Schnucks on Clayton Road in Richmond Heights voluntarily pulled strawberries, lettuce and croutons out of its salad bar on Wednesday, according to deli manager Mike Reardon.

"It was just a precautionary move," Reardon said, adding, "As far as I know, there haven’t been any problems with anything we’ve sold."

Another Schnucks store, Culinaria in downtown St. Louis, put a sign up on empty shelves that read in part, "Due to a voluntary recall on pre-packed lettuce, we will not be able to produce these pre-made salads. Be assured quality is our main concern. All of the lettuce on the salad bar is fresh and not involved with the recall."

Willis said prepacked lettuce was not necessarily a concern, but the smaller Culinaria store has different methods of stocking their salads. She added that no products had been recalled.