LOS ANGELES — A “superbug” outbreak suspected in the deaths of two Los Angeles hospital patients is raising disturbing questions about the design of a hard-to-clean medical instrument used on more than half a million people in the U.S. every year.

At least seven people – two of whom died – have been infected with a potentially lethal, antibiotic-resistant strain of bacteria after undergoing endoscopic procedures at Ronald Reagan UCLA Medical Center between October and January. And more than 170 other patients may have been exposed as well, university officials said.

UCLA said the infections may have been transmitted through at least two contaminated endoscopes that were used to diagnose and treat pancreatic and bile-duct problems.

Hospital officials said they immediately removed contaminated medical devices blamed for the outbreak and adopted more stringent sterilization techniques.

At a news conference Thursday afternoon, health officials sought to reassure the public that there is no broad danger.

“This outbreak is not a threat to public health,” said Dr. Benjamin Schwartz, deputy director of acute communicable disease control and prevention for the LA County Department of Public Health.

Infections of carbapenem-resistant Enterobacteriaceae, or CRE, have been reported at hospitals around the country, and some have been linked to a type of endoscope that is used to diagnose and treat pancreatic and bile-duct problems. The duodenoscope is a thin, flexible fiber-optic tube that is inserted down the throat to enable a doctor to examine an organ. It typically has a light and a miniature camera.

Doctors first discovered the problem in mid-December when a patient underwent an endoscopic procedure and developed an infection that couldn’t be treated with antibiotics.

An investigation was launched and doctors employed high-tech techniques to find other cases – a process that took several weeks, said Dr. Zachary Rubin, medical director of clinical epidemiology and infection prevention.

It was determined that CRE infections had been passed on from one “source case” patient to seven other patients between Oct. 3 and Jan. 28, Rubin said.

The two endoscopes were found to have “embedded” CRE infections even though they had been sterilized according to manufacturer instructions, said Dr. Robert Cherry, UCLA Health System’s chief medical and quality officer. Five other devices were clear.

As a precaution, the two instruments were pulled from service, more stringent sterilization procedures were adopted and anyone who had undergone procedures with any of the seven endoscopic devices were notified that there was a potential risk that they may have been infected, officials said.

The hospital has notified them through letters and phone calls and is offering free testing and treatment options.

Health inspectors visited UCLA after being notified and found “no breaks and no breaches” in its disinfection process, said Dr. Benjamin Schwartz of the county health department.

“You can very easily do everything right and still have some contamination,” said Dr. Deverick Anderson, an infectious-disease expert at Duke University. “We’re finding this is a problem, but it’s probably one that we don’t have a very good solution to right now.”

Lawrence Muscarella, a Philadelphia infection-control expert, said the recent incidents point to a design flaw that needs to be addressed.

On Thursday, the U.S. Food and Drug Administration issued an advisory warning doctors that even when a manufacturer’s cleaning instructions are followed, infectious germs may linger in the devices. Their complex design and tiny parts make complete disinfection extremely difficult, the advisory said.

In a statement, the FDA said it is trying to determine what more can be done to reduce such infections. But it said that pulling the device from the market would deprive hundreds of thousands of patients of “this beneficial and often life-saving procedure.”

“The FDA believes at this time that the continued availability of these devices is in the best interest of the public health,” the agency said.

More than 500,000 patients undergo procedures using duodenoscopes in the U.S. every year, according to the FDA.

CRE can cause infections of the bladder or lungs. Symptoms can include coughing, fever and chills.

Healthy people usually don’t get CRE infections. The infections typically occur in patients in hospitals, nursing homes and other health care settings.

By one estimate, CRE can contribute to death in up to half of seriously infected patients, according to the U.S. Centers for Disease Control and Prevention.

National figures on the bacteria are not kept, but 47 states have seen cases, the CDC said.