Deadly drug-resistant bacteria can be difficult or even impossible to remove from the tiny crevices of a reusable medical scope doctors use to remove gallstones and detect cancer in half a million Americans a year, the Food and Drug Administration warned Thursday.

The FDA alert on contaminated duodenoscopes comes after news reports that two patients in California died after contracting forms of a drug-resistant form of bacteria known as Carbapenem-resistant enterobacteriaceae, or CRE. The FDA said at least 135 people have probably gotten the bacteria from scopes nationwide, but health officials in Los Angeles County said the outbreak has affected more than 180 patients there.

CRE, which is sometimes called a "superbug" because it is resistant to multiple antibiotics, has been a public health concern in Minnesota as well. More than 370 cases have been reported in the state since 2011, though state Health Department officials on Thursday said none is related to duodenoscopes.

Doctors on Thursday said the FDA alert is a pointed reminder of the need to be vigilant to prevent and detect transmission of CRE, but that the public must be assured that the risk of getting sick from the bacteria remains less significant than the harm that could come if gallstone and cancer patients forgo necessary procedures.

"We're taking this very seriously," said Dr. Martin Freeman, chief of gastroenterology and hepatology at the University of Minnesota Medical Center, which does about 1,200 such procedures a year. "I don't think the public should be alarmed to the point of being afraid to have one of these. The appropriate steps are being taken, and I think it's getting the attention it deserves."

The United States has seen half a dozen confirmed CRE outbreaks related to endoscopes in recent years, including in Seattle and Chicago. But Thursday's FDA alert came one day after the Los Angeles Times reported two patients died at UCLA's Ronald Reagan Medical Center after contracting the bacteria following scoping procedures there.

The duodenoscope is a complex type of reusable endoscope that is fed down the esophagus and into the bile duct, where it can be used to drain fluids and remove gallstones that have migrated out of the gallbladder. The device can be used with other tools to snip samples for cancer testing or even place small stents to hold a gallbladder open.

The FDA warned duodenoscopes may harbor infectious bacteria even after cleaning because of the difficult design, which includes a long hollow tube through which to run a surgical tool and a moving part on the end of the scope called an elevator. Residual bodily fluids and organic debris may collect in the tiny crevices in the device and potentially transmit bacteria, including CRE bugs that are immune to multiple forms of antibiotics.

"Although the complex design of duodenoscopes improves the efficiency and effectiveness of [the therapy], it causes challenges for cleaning and high-level disinfection," the FDA alert said. "Some part of the scopes may be extremely difficult to access and effective cleaning of all areas of the duodenoscope may not be possible."

Dr. Bret Petersen, a gastroenterologist at the Mayo Clinic who collaborated with several medical groups that wrote quality-control guidelines for endoscopes in 2011, noted that transmission of CRE remains rare. Of the 500,000 such scoping procedures performed in the U.S. annually, about 135 patients are believed to have been infected with CRE between 2013 and 2014.

"We don't know entirely the scope of the problem, but we think it's limited," Petersen said. "Patients shouldn't be fearful of the procedure because of the risk of infection, because there may be consequences of not having a procedure that they need."

Patients also shouldn't be afraid to ask their doctors about what steps are being taken to minimize the risk of infection, including whether the hospital has had recent cases of CRE, he said.

Typically hospitals clean duodenoscopes in-house. Petersen said that includes doing an immediate "pre-wash" scrub of the device at the bedside just after the procedure, followed by manual cleaning with a scrub brush by a technician in a reprocessing room, and finished with treatments in an automated machine using one or more cleaning agents. The scopes are then dried with an alcohol flush.

Officials with the Minnesota Health Department run a voluntary surveillance program monitoring for CRE infections in the state, including any related to duodenoscopes. In 2012, the department sent a letter to all health care facilities highlighting best practices for preventing transmission.

The state recorded 103 reports of CRE transmission in 2013, and 135 in 2014 — but none appeared related to duodenoscopes.

"It is important for health care facilities to know and completely understand their equipment and ensure that it is properly cleaned," department spokesman Doug Schultz said in an e-mail. "We work with health care facilities routinely and on an ongoing-basis to improve their infection control practices and proper cleaning and use of equipment are part of those conversations."

Twitter: @_JoeCarlson