CHENGDU -- Medical experts have confirmed that 57 pupils of a primary school who were sent to hospital after eating lunch at their school on the first day of the new semester in southwest China, were suffering from food poisoning caused by a diarrhea-causing bacteria.

Health authorities in Chengdu, capital of Sichuan Province, announced on Wednesday that clinical diagnoses show that 33 others have symptoms of food poisoning. Experts say they found shigella bacteria in the food. The 33 pupils will undergo lab tests.

As of 6 p.m. Tuesday, 606 pupils, including the 57 confirmed cases and the 33 clinical cases, had undergone medical checks at the local hospital. At press time, 516 pupils were still under medical observation, said Liu Jun, deputy director of the Chengdu Health Bureau.

Liu said, 1,134 pupils and 140 teachers from Chongzhou City Experimental Primary School had lunch at the school dining hall on September 1, the first day back to school after the summer vacation.

Some of the pupils and teachers began suffering headaches, fever, vomiting, stomach-ache and diarrhea on Friday and visited doctors for treatment. The number of affected pupils began increasing on Sunday.

Several teachers who developed symptoms have been discharged from hospital after being treated, according to the Chengdu city disease control center.

All the affected pupils are in a stable condition and no deaths have been reported, said Liu.

It remains unclear whether the food was contaminated by the food seller or during its preparation at the school, said Fu Xiaolu, director of the Disease Prevention and Control Center of Chengdu.

It generally takes a week for a person to recover after being infected with the bacteria, according to experts.

Disease control and sanitary inspection experts led by officials from the Sichuan provincial health department are investigating and helping with the treatment of sick pupils.

The school suspended classes on Monday. All the school buildings and areas inside and outside the school have been disinfected.

School sources said they would resume classes when most of the affected pupils have recovered.