A rare strain of E. coli never before associated with foodborne illness in the United States has sickened 29 people in three states, public health officials said Friday. The outbreak has been tied to romaine lettuce served in restaurants, school cafeterias and deli and supermarket salad bars.

Freshway Foods, an Ohio company, recalled the lettuce on Thursday. It said the lettuce had been sold primarily to food service and wholesale customers. The recall did not involve any bagged or mixed lettuce sold in supermarket produce sections, the company said, although some supermarkets appeared to have used the romaine in salad bars.

Officials traced the illness to a bacterial strain known as E. coli O145, which is different from the more widely known E. coli O157:H7, which has been associated with outbreaks linked to ground beef, leafy greens and other foods.

“This is the first time this particular type of E. coli has been associated with a foodborne outbreak,” said Dr. Robert Tauxe, a deputy director of the Centers for Disease Control and Prevention.