Investigators on Friday added Pennsylvania to a growing list of states reporting outbreaks of food poisoning linked to a strain of E. coli associated with Chipotle Mexican Grill restaurants.

The state Department of Health confirmed that a Pennsylvania resident had become infected by the Shiga toxin-producing strain, which has sickened 52 people in nine states. The Pennsylvanian did not eat at a Chipotle restaurant, and the source of the infection has not been determined. The E. coli strain, however, matches the same DNA signature of other cases in this outbreak.

State officials did not provide the patient's hometown, saying they were prevented from releasing details under a confidentiality clause of the Disease Prevention and Control Law of 1955.

There are no other known cases of the E. coli strain reported in the state.

In addition to Pennsylvania, the Centers for Disease Control and Prevention on Friday added Maryland and Illinois to the list of states reporting illness. They joined California, Minnesota, New York, Ohio, Oregon, and Washington. Twenty people sickened in the outbreak had to be hospitalized, according to the CDC.

The source of the E. coli O26 outbreak is still unknown, though investigators suspect a fresh produce item that was shipped from one location to multiple restaurants.

People should contact a health care provider if they recently became ill with diarrhea after eating at Chipotle or any restaurant, officials said. Symptoms of the Shiga toxin-producing E. coli, including sometimes-bloody diarrhea and cramps, usually strike three to four days after ingesting the bacteria.

Two of the newly reported illnesses started in October, and five started in November, suggesting the outbreak was not as short-lived as previously thought. Prior to Friday's report, the onset of most cases of illness had been in late October.

E. coli is among a vast array of bacteria that live in the human gut and generally cause no problems. But some strains can cause serious symptoms and be life-threatening. They are spread by oral contact with fecal matter.

Chipotle on Friday said it had enhanced its food safety efforts, with steps that include testing all fresh produce before it is shipped to restaurants.

"While no cause has been identified, none of the ingredients that were in our restaurants at the time of this incident are still in our restaurants," Chipotle spokesman Chris Arnold said in an email.

A similar Shiga toxin-producing strain sickened at least 19 people in late November. The illness was mostly confined to the western states. Five people were hospitalized and two were treated for hemolytic uremic syndrome, a type of kidney failure. Investigators believe the bacteria was carried by a celery and onion blend used to make a rotisserie chicken salad sold at Costco. According to the CDC, nearly all those infected purchased or ate the chicken salad in the week before they were stricken.

Costco removed the chicken salad from its stores and Taylor Farms Pacific Inc., which produced the celery and onion blend, recalled several products containing celery, the CDC reported.

As fresh, unprocessed food grows in popularity, experts say, it raises the risk of foodborne illness, because illness-causing pathogens often are killed during cooking.

An estimated 48 million Americans get sick each year from foodborne diseases. Of these, 128,000 are hospitalized and 3,000 die, according to the CDC.

This report includes information from Reuters.