CDC In the U.S. today, the most common type of highly drug-resistant germs known as CREs are the Klebsiella pneumonia bacteria like those shown here. Nearly untreatable, they're being detected in a growing number of health care settings.

A sharp jump in the number of rare but potentially deadly types of a superbug resistant to nearly all last-resort antibiotics has prompted government health officials to renew warnings for U.S. hospitals, nursing homes and other health care settings.

The move comes just as researchers in Israel are reporting that people who carry dangerous CRE -- Carbapenem-resistant enterobacteriaceae -- can take more than a year before they test negative for the bacteria, making it more difficult to control and raising the risk of wider spread.

Reports of unusual forms of CRE have nearly doubled in the U.S., the Centers for Disease Control and Prevention reported this month. Of 37 cases of rare forms of CRE, including the alarming NDM -- New Delhi metallo-beta-lactamase -- 15 have been reported since last July.

“This increase highlights the need for U.S. health care providers to act aggressively to prevent the emergence and spread of these unusual CRE organisms,” the CDC said in a health advisory.

CREs are part of a family of drug-resistant germs that have shown up in growing numbers of U.S. health care settings. They’re named for their ability to elude carbapenem antibiotics, the big guns in the medical arsenal. They usually strike people who are already ill and require devices such as ventilators or catheters or who have been taking antibiotics for a long time. But they can infect any patient.

Twenty-nine of the unusual CRE cases have been NDM, up from the first case detected in the U.S. in 2010, said the CDC's Dr. Alex Kallen, a medical epidemiologist and outbreak response coordinator in the agency’s Healthcare Quality Promotion division. It's especially worrying because it confers resistance to multiple drugs and is easily transmitted to other types of bacteria.

The others were even rarer types of CRE, including VIMs, IMPs and OXA-48s, all of which produce enzymes that render most antibiotics virtually useless.

The agency called for stricter isolation and hygiene precautions, increased screening of patients potentially colonized with CRE and better communication within and between hospitals and other health care settings where the bugs can become intractable -- and deadly. CRE infections have a mortality rate of up to 40 percent, much higher than other health care infections, such as those caused by MRSA or C. difficile.

“Our main objective is to slow or stop the spread in places where we can identify them,” said Kallen. “Right now, the therapeutic options are very limited.”

Health officials have been worried about them for more than a decade, particularly the KPCs, or carbapenemase-producing Klebsiella pneumonia, which have now been reported in 42 U.S. states and Puerto Rico, the CDC reports.

Nine states have reported NDMs and at least two have reported other rare forms that also block antibiotic effectiveness, including those known as VIMs, or Verona integron-encoded metallo-beta-lactamase, and IMPs. So far, they’ve been associated mostly with people who’ve been hospitalized in countries outside the U.S.

The bugs were in the news last summer after reports of a CRE strain of Klebsiella pneumoniae roared through the National Institutes of Health Clinical Center near Washington, D.C., killing seven people, including a 16-year-old boy.

In Colorado last summer, NDM-producing CRE was detected in eight patients, the largest outbreak in the U.S. to date, according to a CDC report this month. It was found largely because the University of Colorado Hospital already has stringent surveillance protocols in place, said Dr. Michelle Barron, director of infection control and prevention. Since then, the hospital has probably tested 500 or 600 patients with unusual resistance patterns, she told NBC News.

None of the eight patients in the original outbreak died. The evidence showed that patients who were colonized with the germs, but not actually sick, contributed to the spread.

That’s a point underscored by the study by Israeli doctors published Wednesday in the American Journal of Infection Control. They studied medical records of adult patients hospitalized between January 2009 and December 2010 at Shaare Zedek Medical Center, a 700-bed, university-affiliated hospital in Jerusalem.

In 97 patients with positive CRE cultures, it took a mean time of 387 days to log a negative test -- and nearly 40 percent remained positive after a year, according to Dr. Amon Yinnon, one of the study authors.

“The major concern is that an undiagnosed carrier may be admitted to hospital for totally unrelated reasons, and subsequently and unwittingly pass his CRE to other patients,” Yinnon said in an email to NBC News.

Patients who were hospitalized repeatedly were at higher risk of remaining colonized with CRE, the study found.

CDC officials hope to increase awareness of the growing problem among the general public as well as the health care providers before it gets out of control.

“I can’t predict the future, of course, but there is a concern that we can see more of these as they spread,” Kallen said. “This can become a community bug.”

Related stories: