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

The patient is well now, 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. The resistance can spread because it arises from loose genetic material that bacteria typically share with one another.

“Think of a puzzle,” said Dr. Beth Bell, of the Centers for Disease Control and Prevention. “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 a drug called 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.