The source of the salmonella outbreak that has sickened 399 people in 42 states since September may be peanut butter, Minnesota health officials said Friday. The Washington Post reports that the officials found salmonella in a five-pound container of King Nut brand creamy peanut butter. The product is sold to "long-term care facilities, hospitals, schools, universities, restaurants, delis, cafeterias and bakeries, but is not sold retail in grocery stores."

King Nut, of Solon, Ohio, said in a statement yesterday that it was recalling all of its peanut butter manufactured by Peanut Corporation of America, of Lynchburg, Va., including the Parnell's Pride brand.

Labs are now trying to confirm whether the strain, or serotype, found in the peanut butter—Salmonella typhimurium, the most common type found in the U.S.—is the one infecting people around the country.

A moderator of ProMEDmail, an e-mail alert that monitors emerging infectious diseases around the world, suggested that the peanut butter was as likely a cause as any. "It appears that the 'smoking' peanut butter has been found," wrote Larry Lutwick, the director of infectious diseases at the VA Health Care System in Brooklyn and a professor of medicine at the State University of New York, Downstate, also in Brooklyn. However, he cautioned, "confirmation is awaited."

Peanut butter was the source of a 2006-07 outbreak that sickened more than 600 people in 47 states. In 1996, it caused an outbreak in Australia. Slate's Daniel Engber wrote about how salmonella gets into peanut butter in 2007.

The best way to keep salmonella out of peanut butter is strict hygiene at plants. But scientists have studied how to make sure any salmonella that does manage to get into peanut butter is killed, Lutwick notes in his post. The good news: Fewer bacteria survive if the peanut butter is heated to 21 degrees Celsius (about 70 degrees Fahrenheit) in the manufacturing plant than do at 5 degrees C (41 degrees F), according to a 2000 study. The bad news: A 2006 study found that heat treatments "are inadequate to consistently destroy Salmonella in highly contaminated peanut butter and that the pasteurization process cannot be improved significantly by longer treatment or higher temperature," according to Lutwick.

Map of current salmonella outbreak by the CDC. Click here for a larger version.