A new superbug that is resistant to the antibiotic of last resort has been spotted in the United States. Twice.

US researchers reported Thursday that the mcr-1 gene has been found in E. coli bacteria retrieved from a woman from Pennsylvania. Separately, the US Department of Agriculture reported that the gene had been found in a sample of intestine from a pig. It did not provide further details, though a source told STAT the pig was raised in Texas.

The mcr-1 gene was first discovered last fall by Chinese scientists. In the weeks that followed, a cascade of scientific articles emerged from a range of countries reporting on finding the gene in animals, commercial meat, and occasionally in people.

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Researchers who track the battle of bacteria vs. antibiotics have predicted it was only a matter of time before the mcr-1 gene would be found in the US as well. Still, the news of its discovery was greeted with dismay.

“I do not like knowing that … [this] is now circulating here in the US,” said Dr. Beth Bell, director of the National Center for Emerging and Zoonotic Infectious Diseases at the Centers for Disease Control and Prevention.

“It is a very unfortunate example of what we’ve been saying about how dangerous this antibiotic resistance phenomenon really is.”

Mcr-1 is a gene that gives bacteria that carry it resistance to a drug called colistin, the last antibiotic that can cure infections caused by bacteria that have developed resistance to multiple antibiotics.

The gene is carried on a plasmid, a mobile piece of DNA. That means it can be swapped from one bacterium to another within a family such as E. coli — and to other bacterial families, as well.

The nightmare scenario is that this gene will find its way into bacteria that are resistant to all other antibiotics, creating a pan-resistant bug that causes infections that simply cannot be treated.

That has not yet been seen in the US, Bell said. But having mcr-1 present here could lead to that eventuality.

“We now have in the United States the last, final piece of that puzzle that’s needed to have a pan-resistant bacteria circulating in the US,” she said. “That’s not very heartening.”

The E. coli strain that infected the woman from Pennsylvania was not pan-resistant; it was susceptible to some antibiotics, said Patrick McGann, who with colleagues from the Walter Reed Army Institute of Research in Silver Springs, Md., reported on the case Thursday in the journal Antimicrobial Agents and Chemotherapy.

Military hospitals send samples of bacteria that are considered multidrug resistant to the institute for further testing. Just this month, the institute’s laboratories started screening E. coli samples for the mcr-1 gene.

Within three weeks, they had a hit.

“I was stunned to see it so quickly. I didn’t think we would see it at all,” McGann told STAT in an interview. Other laboratories in the US had been looking but hadn’t seen mcr-1, he said.

Although the World Health Organization designated colistin critically important for human medicine in 2012, the drug is widely used in food animal production in some parts of the world, including China. It is not used in animal husbandry in the US, McGann said.

When swarms of bacteria are exposed to an antibiotic, the ones susceptible to it will die. But some will always survive. It’s the bacterial equivalent of survival of the fittest. Increased exposure to the drug gives the bugs that pose the biggest risk to humans a chance to survive and thrive.

McGann could not say much about the woman in Pennsylvania because of medical privacy laws. But he noted that the strain of E. coli she was carrying was rare — not previously reported in the US.

“How did it get to Pennsylvania? How did it end up in this infection is anybody’s guess,” he said. “But I suspect that when we look more at the epidemiological evidence, that we’re going to start seeing that … this might not be a home-grown problem, so to speak.”

Disease detectives from the state health department and the CDC are interviewing the woman and her family to try to puzzle out how she might have picked up this dangerous E. coli bacteria.

And Bell said that surveillance is being ramped up nationwide to look for bacteria carrying mcr-1 in a more “comprehensive and systematic way.”

That is critical, said Lance Price, director of the Antibiotic Resistance Action Center at George Washington University. Allowing mcr-1 to become established in the US, either in the human population or among food animals, would be a disaster, he said.

“We need to try to squash this.”

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