They were potentially infected with Carbapenem-Resistant Enterobacteriaceae, or CRE, during endoscopic procedures at Ronald Reagan UCLA Medical Center between October and January, said Dale Tate, a University of California, Los Angeles spokeswoman. Tests on a patient uncovered the outbreak.
Similar outbreaks of CRE have been reported around the nation. They are difficult to treat because some varieties are resistant to most known antibiotics. By one estimate, CRE can contribute to death in up to half of seriously infected patients, according to the national Centers for Disease Control and Prevention.
The bacteria may have been a "contributing factor" in the deaths of two patients, a UCLA statement said. Those who were exposed are being sent free home-testing kits that the university will analyze.
The bacteria can cause infections of the bladder or lungs, leading to coughing, fever or chills.
UCLA said Wednesday that infections may have been transmitted through two endoscopes used during the diagnosis and treatment of pancreatic and bile-duct problems.
"We notified all patients who had this type of procedure, and we were using seven different scopes. Only two of them were found to be infected. In an abundance of caution, we notified everybody," Tate said.
The two medical devices carried the bacteria even though they were sterilized according to the manufacturer's specifications, UCLA said.
"We removed the infected instruments, and we have heightened the sterilization process," Tate said.
National figures on the bacteria are not kept, but 47 states have seen cases, the CDC said.
Since 2012, there have been about a half-dozen outbreaks reaching as many as 150 patients, according to the Los Angeles Times, which first reported the UCLA outbreak.
One occurred in Illinois in 2013. Dozens of patients were exposed to CRE, with some cases apparently linked to a tainted endoscope used at a hospital.
A Seattle hospital, Virginia Mason Medical Center, reported in January that CRE linked to an endoscope sickened at least 35 patients, and 11 died, although it was unclear whether the infection played a role in their deaths.
Experts say the cases represent a disturbing surge.
"This bacteria is emerging in the U.S., and it's associated with a high mortality rate," Dr. Alex Kallen, an epidemiologist in CDC's Division of Healthcare Quality Promotion, told the Times. "We don't want this circulating anywhere in the community."