Health officials have determined that a type of bacteria found in food left at unsafe temperatures is the cause of an outbreak of gastrointestinal illness that struck 647 people who ate last month at a Chipotle Mexican Grill restaurant in Ohio.

Between July 26 and July 30, customers of a Chipotle restaurant in Powell, Ohio, just north of Columbus, complained of food poisoning and diarrhea after eating tacos and burrito bowls there.

Local health officials say a bacterium known as Clostridium perfringens caused the outbreak. It is commonly found in beef, poultry, gravies and dried or pre-cooked foods that "are prepared in large quantities and kept warm for a long time before serving," according to the Centers for Disease Control and Prevention.

CDC says the bacteria affect nearly 1 million people each year. The infection is not contagious and people usually recover in a day or two, according to Reuters.

Chipotle announced Thursday that it would retrain its restaurant staff nationwide.

"Chipotle has a zero-tolerance policy for any violations of our stringent food safety standards and we are committed to doing all we can to ensure it does not happen again," CEO Brian Niccol said in a statement published by USA Today.

"Once we identified this incident, we acted quickly to close the Powell restaurant and implemented our food safety response protocols that include total replacement of all food inventory and complete cleaning and sanitization of the restaurant," he said.

The outbreak in Ohio is the latest of several in recent years that have hit the restaurant chain, sickening hundreds of customers.

In February, Chipotle hired Niccol, who had previously been with Taco Bell, to be its new CEO. Niccol was credited with turning around Taco Bell, and many hoped he could boost Chipotle's image.

Earlier this year, Niccol announced the closing of 65 underperforming Chipotle restaurants as part of a corporate reorganization.

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