Untreatable and increasingly resistant bacteria infected four per cent of patients in U.S. hospitals , the Centers for Disease Control and Prevention said in issuing a call to action for healthcare workers to stop further spread.

The bacteria, called Carbapenem-Resistant Enterobacteriaceae (CRE), kill up to half of patients who get bloodstream infections from them.

A scientist holds a plate testing for antibiotic resistance in Klebsiella bacteria with a mutation called NDM-1. Labs are on the lookout for the hard-to-treat hospital infections. (Suzanne Plunkett/Reuters)

Most CRE infections occur in people in hospital, long-term care facilities or nursing homes, where they remain uncommon, CDC said Tuesday.

"CRE are nightmare bacteria," CDC director Dr. Tom Frieden, told reporters.

The Enterobacteriaceae includes more than 70 species that normally live in the human digestive system, such as E. coli. If those bacteria get into wrong areas of the body such as the blood or bladder, severe infections can occur. Some of these infections are resistant to last-resort antibiotics called carbapenems, the CDC said.

In the agency's Vital Signs report, researchers said about four per cent of acute-care hospitals and 18 per cent of long-term care hospitals reported at least one of the infections to a national safety network in the U.S. during the first half of 2012.

One type of CRE, a resistant form of Klebsiella pneumoniae, showed a seven-fold increase in the last decade. In the U.S., northeastern states report the most cases of CRE. It was tracked from a single facility in 2001 to 42 states or more now, including some where it is now a "routine challenge."

The CDC gave six recommendations for healthcare workers to take to stop CRE from emerging in institutions and stopping its spread to the community:

• Grouping patients with CRE together and requesting immediate laboratory reports when patients do.
• Dedicating staff, rooms and equipment to the care of patients with CRE, whenever possible.
• Having facilities alert each other when patients with CRE transfer back and forth.
• Asking patients whether they have recently received care somewhere else, including another country.
• Using antibiotics wisely.
• Removing temporary medical devices such as urinary catheters as soon as possible.

"We only have a limited window of opportunity to stop spread," Frieden said.

It's most commonly spread through unclean hands.

While the infections are also uncommon in Canadian hospitals, cases have been documented. These include a Toronto patient with both E. coli and Klebsiella pneumoniae bacteria that contained the NDM-1 enzyme that disables antibiotics and led to an outbreak in 2011 at Montreal's Jewish General.

The CRE can be controlled using standard hospital cleaning practices, said Dr. Arjun Srinivasan, an infectious disease expert at the CDC, adding the key is to use them.

CDC's report did not include better-known superbugs like the staph infection MRSA or C. difficile.