The popular burrito chain Chipotle Mexican Grill will briefly close all branches nationwide Monday to meet with employees over food safety issues, following outbreaks of the bacteria E. coli. Above, customers enter a Chipotle restaurant in Seattle, Nov. 20, 2015. Reuters/David Ryder

Besieged by outbreaks of foodborne illnesses, Chipotle Mexican Grill is closing all its restaurants temporarily Monday in order to meet with employees and discuss food safety. Rather than opening before noon, branches of the popular burrito chain will open at 3 p.m. local time, according to the company's website.

The food safety problems began last summer, with an outbreak of E. coli in Seattle in July that sickened five. In August, at least 234 people caught the stomach bug norovirus, blamed on a sick Chipotle worker, in Simi Valley, California, while a salmonella outbreak in August and September sickened 43 people in Minnesota and sent 14 of them to the hospital.

Overall, at least 500 people are believed to have gotten sick after eating at Chipotle during the second half of 2015, due to outbreaks of E. coli, salmonella or the highly contagious norovirus. In several cases, including one E. coli outbreak that began in October and affected a dozen states, the exact cause of the contamination remains unclear.

With Monday's nationwide meetings, the company plans to thank its employees for working to learn new food safety procedures and to answer questions from employees. In the meetings, managers will discuss other changes the chain is making for food safety, a spokesman told the New York Times in January when Chipotle announced the meetings.

Chipotle Mexican Grill Inc. (CMG) | FindTheCompany

The outbreaks have been disastrous for Chipotle's public image — and for its business. In the fourth quarter of 2015, visits to the restaurant dropped 5 percent and sales fell by 30 percent compared to the previous year.

In January, the company said it had received a federal subpoena in December related to a criminal investigation into the outbreak of norovirus in Simi Valley. The U.S. Attorney's Office for the Central District of California is carrying out the investigation jointly with the Office of Criminal Investigations at the Food and Drug Administration.

Meanwhile, the Centers for Disease Control and Prevention announced at the beginning of February that Chipotle's E. coli outbreak was over, and that it had not been able to determine the cause.