Unless health care facilities act now they could miss the narrow window of opportunity to stop a potentially fatal group of bacteria in their tracks, federal health officials said Tuesday.

Bacteria known as CRE, for carbapenem-resistant Enterobacteriaceae are spreading rapidly through facilities, triggering outbreaks.

As their name implies, the bacteria are resistant to the carbapenems, a potent class of antibiotics long considered the drugs of last resort. Pharmaceutical companies have no new antibiotics under development, leading some patient advocates to say the era of miracle drugs, which began in the 1940s, may be reaching the end of its run.

"CRE are nightmare bacteria," said Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention, adding "they have high mortality rates and can spread their resistance to other bacteria."

Speaking at a news briefing Tuesday, he said the microbes kill 50 percent of infected patients.

In 2001, according to the CDC, 1.2 percent of Enterobacteriaceae were resistant to the carbapenems. By 2011, 4.2 percent could thwart the drugs.

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About 18 percent of long-term, chronic-care facilities providing treatment involving catheters and ventilators have experienced clusters of infection. An estimated 4 percent of acute-care hospitals also have been affected, according to a CDC report, which examined CRE infections in 2012.

The microbes are not as widespread in hospitals as MRSA or Clostridium difficile, Frieden said, but they have the potential to be just as pervasive.

Dr. Denise Verley, director of infectious diseases at Nassau University Medical Center, said the severity of infection depends on patients' overall health.

People with compromised immunity, she said, are more vulnerable than healthy people. The microbes pose no threat to robust health care workers.

"I don't think people should think that if they got this they're going to die," Verley said.

Yet, in an outbreak involving 17 patients at the National Institutes of Health two years ago, six people died -- including a 16-year-old boy.

The bacteria are not species new to humans, but those that normally dwell in the large intestine -- Klebsiella, for instance, or E. coli. What makes them dangerous is repeated exposure to antibiotics.

Bacteria, Frieden said, are naturally endowed with mini packages of DNA known as plasmids -- so-called jumping genes -- that allow one species in the gut to confer resistance to other species.

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The process ensures that all microbes in the bowel are genetically armed to repel antibiotics.

Infections spread when patients inadvertently ingest the microbes.

Outbreaks have occurred in New York City, said Dr. Arjun Srinivasan, the CDC's expert in health care-associated infections.

He could not name the facilities, but emphasized that six states will soon require hospitals to track and report CRE. New York is not among them.

"I haven't heard of any hospitals in our association having a severe problem with it," Janine Logan, spokeswoman for the Nassau-Suffolk Hospital Council said Tuesday. "There are very strict and comprehensive infection control measures that hospitals follow.

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"And in addition, there are state and federal regulations that have to be followed regarding infection control practices."