OSAKA — Seven elderly patients at a hospital in Sakai, Osaka Prefecture, died between May 7 and Saturday after contracting serratia bacteria — apparently inside the facility, officials said Monday.

Four men and three women at Mimihara General Hospital died of infections caused by the bacteria, Sakai city and hospital officials told a press conference.

The patients were admitted to the hospital between December and mid-June and apparently contracted the bacteria after being hospitalized, they said.

Eight more patients at the Mimihara hospital have been infected with the serratia bacteria and they have been treated there and at other hospitals. The bacteria have been detected in the blood and phlegm of the patients, according to the city officials and the hospital.

The serratia organism exists in the air or in the large intestines and is usually not very harmful. It can, however, cause fever or harm the lungs — sometimes fatally — in people whose resistance to sickness is low. Those infected could suffer from septicemia and pneumonia.

On Thursday, the hospital learned of the serratia infection from the results of blood tests conducted on three inpatients who had fever since late June. One of the three died last Wednesday.

The hospital reported the infection to the Sakai public health center Friday, prompting the center to inspect the hospital that day. The inspectors found 12 more patients had contracted the bacteria since May, five of whom have died.

On Saturday, another patient infected with the bacteria died, bringing the death toll to seven. The municipal government has set up an investigative team to clarify the route of infection. Police also began questioning city officials Monday about the outbreak.

The hospital officials said tubes for intravenous drips and soap for washing hands could be the source of the infection, citing reports on similar cases overseas.

Nobuaki Ikeda, director of the Mimihara hospital, said it is important to clarify facts concerning the infection, such as the route through which the bacteria spread.

He pledged to publicize all the information as it becomes available while apologizing for the patients’ deaths.

Osaka Prefectural Police said they plan to question hospital officials about why it took so long for the hospital to report the infection, since the first patient died in May.

In July and August 1999, five patients at Sumida Chuo Hospital in Tokyo’s Sumida Ward died of infections caused by the serratia bacteria. That was the first known large-scale serratia infection in a hospital in Japan.