A type of bacteria that forms when food is left out at unsafe temperatures caused Chipotle Mexican Grill's latest foodborne illness outbreak.

Nearly 700 people reported gastrointestinal problems, including nausea, diarrhea and fever, after eating at the Chipotle restaurant in Powell, Ohio, between July 26 and July 30. The Centers for Disease Control and Prevention's tests named clostridium perfringens the culprit of the outbreak.

The bacteria is one of the most common causes of food poisoning in the U.S. It forms when food is left out at unsafe temperatures.

The incident deals another blow to Chipotle, which has spent years trying to convince diners and investors it has improved its food safety practices.

An E. coli outbreak in 2015 sickened hundreds of people and plunged Chipotle into crisis. The fast-casual restaurant rolled out promotions and new menu items such as queso to try to win customers back. But diners continued to shun the restaurant.

Chipotle hired former Taco Bell CEO Brian Niccol in February to replace founder Steve Ells as CEO. Since taking the helm, Niccol has unveiled a strategy to woo consumers back to the troubled burrito chain with a focus on new menu items and faster mobile and online orders.

Its turnaround efforts appeared to be working. Shares of Chipotle have surged 74 percent this year. They slid 4 percent Thursday.

Niccol said Chipotle will start retraining all restaurant employees on food safety and wellness protocols next week.

The tests do not confirm the exact number of people who fell ill. Twenty samples were sent to the Ohio Department of Health and returned negative, a spokeswoman for the Delaware General Health District said. Six samples were sent to the CDC for more advanced testing and five returned positive for the toxin, she said.

The number of samples submitted are in-line with the Ohio Department of Health's guidelines.