"The medicine cabinet is empty for some patients," Frieden said.
The CDC said it was working with the state health department in Pennsylvania to talk to the patient and her family to see how she may have been infected.
They'll also test others in the area who may have been in contact to see if they are carrying the bacteria - which may not necessarily cause illness or any symptoms at all.
"An urgent public health response is underway to contain and prevent potential spread of mcr-1," Walter Reed said in a statement.
Dr. David Hyun of the Pew Charitable Trusts, who follows the issue of drug-resistant bacteria, said details will be important. "I am very interested in finding out how did this patient do," he told NBC News. "What kind of treatment did she receive?"
There have been reports in other countries of patients with bacteria carrying mcr-1, but not many details of how they were cared for or whether other antibiotics cured their infections.
Colistin, used to treat carbapenem-resistant Enterobacteriaceae or CRE, is an older antibiotic with some tough side-effects such as kidney damage. That's why it's only used as a last resort.
Hyun said in several of the international cases, people have been infected with CRE that carried the mcr-1 gene. That would leave them with few, if any, option for treatment. "If we are finding it in other countries, chances are that it's already happened in the United States as well," he said.
Bacteria develop resistance to drugs quickly. Even before penicillin was introduced in 1943, staphylococcus germs had genes that would have made them resistant to its effects.
Just nine years after tetracycline was introduced in 1950, a resistant strain of Shigella evolved. Methicillin-resistant Staphylococcus aureus (MRSA) evolved just two years after methicillin hit the market in 1960. The last new antibiotic to be introduced was ceftaroline, in 2010. It took just a year for the first staph germ to evolve that resisted its effects.
The CDC says more than two million people are infected by drug-resistant germs each year, and 23,000 die of their infections. The biggest killer by far in the U.S. is diarrhea-causing C. difficile.
Near-untreatable cases of diarrhea, sepsis, pneumonia and gonorrhea are infecting millions more globally, the World Health Organization says.