It’s convenient and popular, a healthy option for harried shoppers. But bagged lettuce suspected of causing a multi-state outbreak of E. coli  illness raises new questions about whether pre-cut produce is riskier than whole vegetables.

The outbreak, which involves romaine lettuce cut up and distributed in bags to 23 states and the District, is the latest in a string of recent food poisoning cases involving pre-shredded leafy greens.

Twenty-three people in four states have been sickened since March 1, with another seven probable cases, according to the Centers for Disease Control and Prevention. Of the confirmed cases, a dozen people were hospitalized and three developed a life-threatening type of kidney failure. No deaths have been reported.

The romaine in question was not sold directly to consumers in the produce section but was used by food service companies and supermarkets in salad bars and “grab and go” meals. Several of the victims were students at colleges in Michigan, Ohio and New York who apparently ate the infected lettuce in dining halls.

It is difficult to judge whether pre-cut produce has been linked to more outbreaks than whole vegetables because state and federal health officials don’t always specify whether the leafy greens associated with an outbreak were bagged or whole. But several multi-state outbreaks involving pre-cut produce in the last five years have raised concerns, most notably the 2006 outbreak of E. coli O157:H7 associated with Dole bagged spinach that sickened 238 people and caused five deaths.