(Reuters) - A type of bacteria found in meat and pre-cooked food left at unsafe temperatures was responsible for sickening hundreds of people who ate at a Chipotle Mexican Grill CMG.N restaurant in Ohio, local health officials said on Thursday.

A logo of Chipotle Mexican Grill is seen on one of their bags in Manhattan, New York November 23, 2015. REUTERS/Andrew Kelly

The outbreak last month was the latest in a series of food safety lapses at the burrito chain, and health officials said it was caused by the clostridium perfringens bacterium, which often infects food that is prepared in large quantities and kept warm for a long time.

The resulting illnesses are fairly common and affect nearly 1 million people each year, according to the Centers for Disease Control and Prevention (CDC). People usually recover in a day or two and the infection cannot spread from person to person.

Customers had last month complained of food poisoning and diarrhea after eating tacos and burrito bowls at the Powell, Ohio Chipotle restaurant.

Health officials said 647 people had self-reported gastrointestinal symptoms after eating there between July 26 and July 30. A specific food has not been identified as the source of the illness, and the CDC is conducting further tests, they added.

Though not as severe as Chipotle’s food safety lapses in 2015 that sickened customers with E. coli and salmonella, the outbreak in Ohio is a headache for CEO Brian Niccol, who has been trying to repair Chipotle’s reputation.

“With all of Brian Niccol’s turnaround efforts, you really have to have food safety as a foundation of what you do,” Maxim Group analyst Stephen Anderson said.

Chipotle’s leadership will retrain restaurant employees nationwide about food safety and wellness protocols during working hours starting next week, company spokeswoman Laurie Schalow via email.

The burrito chain will not close any restaurants and will add a recurring online employee assessment for food safety standards. In 2016, Chipotle shut all its stores for a few hours for food safety training.

Chipotle shares reversed course to fall 3.8 percent on Thursday afternoon after hitting a two-year high earlier.