All staff at Chipotle's US Mexican Grill restaurants are to be retrained in food safety procedures after a series of food poisoning incidents.
The latest outbreak in Powell, Ohio was caused by a bacteria that occurs when food is left at unsafe temperatures.
Over 600 people that ate at the restaurant during four days in July suffered food poisoning.
Chipotle's shares have fallen from a 2015 peak of $749 (£590) before a food poisoning outbreak in Boston, to $501.
The US chain said then that it would change its cooking methods in an effort to address food safety concerns and said it was taking "aggressive actions".
Last month's outbreak was caused by the bacterium, clostridium perfringen, and occurred among customers who had eaten food that included tacos and burrito bowls.
However, it was not certain exactly which food caused the illness and testing is still being conducted by the US Centres for Disease Control.
Chipotle, which has more than 2,400 outlets worldwide, has faced a spate of other food-safety incidents in recent years.
In 2015, it temporarily closed 43 restaurants in Washington state and Oregon while authorities investigate an outbreak of E. coli.
And it is facing a federal criminal investigation into food-related illnesses at its restaurants, including norovirus, dating back three years.
In 2017, the firm also suffered a security breach involving customer payment data.
Earlier this month the US Department of Justice said it had arrested three Ukrainian citizen charged with using malware to attack more than 120 US companies, including Chipotle.
Commenting on the latest food poisoning, boss Brian Niccol said: "Chipotle Field Leadership will be retraining all restaurant employees nationwide beginning next week on food safety and wellness protocols."