The case raises the specter of superbugs that could cause untreatable infections, because the bacteria can easily transmit their resistance to other germs that are already resistant to additional antibiotics.

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U.S. military researchers have identified the first patient in the United States to be infected with bacteria resistant to an antibiotic that was the last resort against drug-resistant germs.

The patient — a 49-year-old woman in Pennsylvania — has recovered, but the case raises the specter of superbugs that could cause untreatable infections, because the bacteria can easily transmit their resistance to other germs that are already resistant to additional antibiotics.

“Think of a puzzle,” said Dr. Beth Bell, of the Centers for Disease Control and Prevention (CDC). “You need lots of different pieces to get a result that is resistant to everything. This is the last piece of that puzzle, unfortunately, in the United States. We have that genetic element that would allow for bacteria that are resistant to every antibiotic.”

The bacteria are resistant to colistin, an old antibiotic that in the United States is held in reserve to treat especially dangerous infections that are resistant to a class of drugs called carbapenems. If carbapenem-resistant bacteria, called CRE, also pick up resistance to colistin, they will be unstoppable.

“This is huge,” said Dr. Lance Price, a researcher at George Washington University. “We are one step away from CRE strains that cannot be treated with antibiotics. We now have all the pieces in place for it to be untreatable.”

The gene for resistance to colistin was first found in China, where the drug is used in pig and poultry farming. Researchers reported its discovery there in November. It has also been found in the intestine of one pig in the United States.

CRE is relatively rare, causing just 600 deaths a year, but by 2013, researchers had identified it in health-care facilities in 44 states. CDC director Dr. Tom Frieden often calls it the “nightmare superbug,” because it is resistant to all but one antibiotic: colistin.

“We risk being in a post-antibiotic world,” Frieden said in Washington on Thursday. “That wouldn’t just be urinary tract infections or pneumonia; that could be for the 600,000 patients a year who need cancer treatment.”

He added: “The medicine cabinet is empty for some patients.”

The colistin resistance in the United States came to light when the 49-year-old woman, who Bell said was “connected to the military,” was treated for a urinary infection at a military clinic in Pennsylvania. Initial tests found she was infected with E. coli bacteria, a common variety of germ seen in the gut that often makes its way to the bladder. But the tests showed this E. coli was resistant to antibiotics commonly used first for such infections. Because her urine culture had unusual results, the sample was sent to the Walter Reed National Military Medical Center, which identified the drug resistance. The bacteria, though resistant to colistin and some other antibiotics, were not resistant to carbapenems. Doctors there published a report on the case in a medical journal.

Bell said researchers did not know how the patient contracted the resistant bacteria. The microbes have been found in people in Asia and Europe, but the patient had not traveled during the past five months. It is possible she contracted the bacteria from food, or from contact with someone else who was infected, Bell said.

Public-health workers will interview the woman and will probably test her relatives and other close contacts for the bacteria, Bell said.

Infectious-disease experts at the University of Washington said the discovery of colistin-resistant bacteria in a person and in pigs shows that it’s probably already present, if not widespread, in the U.S.

“This one’s already occurring underneath our noses,” said Dr. John Lynch, an associate professor of medicine at UW Medicine and medical director of infection prevention at Harborview Medical Center.

Washington state doesn’t screen specifically for colistin-resistant bacteria, but Lynch said experts in the state are well aware of the issue. “If someone showed up here with one of those things, I’d be calling my colleagues,” he said.

Infectious-disease doctors have long warned that overuse of antibiotics in people and in animals put human health at risk by reducing the power of the drugs, some of modern medicine’s most prized jewels. About 2 million Americans fall ill from antibiotic-resistant bacteria every year and at least 23,000 die. The Obama administration has elevated the issue, laying out a strategy for how to bring the problem under control.

The CRE germs usually strike people receiving medical care in hospitals or nursing homes, including patients on breathing machines or dependent on catheters. Healthy people are rarely, if ever, affected. But the bugs attack broadly, and the infections they cause are not limited to people with severely compromised immune systems.