This story is part of a partnership between MedPage Today and VICE News.

A bacteria strain that is resistant to what doctors consider the antibiotic of last resort has been reported in a patient for the first time in the United States, prompting concern among public health officials who fear the dawning of a post-antibiotic era.

Although lab tests confirmed the patient's infection would be able to be treated with the second-to-last resort class of antibiotics, the discovery of this strain of bacteria in the US is a warning to public health officials to act before it's too late.

"It is a warning on the horizon," said Dr. Henry Chambers, an infectious diseases expert at the University of California San Francisco and member of the Infectious Diseases Society of America. "For now, we're talking one isolate [strain of] e. Coli. This particular isolate could use some other drug. But it's a harbinger, if you will."

Here's what you need to know:

What Was the Case?

After a 49-year-old Pennsylvania woman went to a clinic with symptoms of a urinary tract infection on April 26, doctors and lab technicians learned that the e. Coli bacteria that was causing her infection carried a gene called mcr-1, researchers at the Walter Reed Army Institute of Research in Silver Spring, Maryland, reported in a case study. Mcr-1 is a gene that indicates the bacteria is resistant to colostin, a potent antibiotic that doctors turn to only as a last-ditch effort.

Doctors used colistin frequently in the 1970s and 1980s, but it caused very serious side effects and could be toxic to the liver, said epidemiologist Dr. William Schaffner, chair of preventive medicine at Vanderbilt University Medical Center. As a result, when researchers discovered a new, less toxic class of antibiotics, colistin was shelved. But that was then.

"We're pulling it off the shelf now because some bugs are so resistant to so many antibiotics, the only thing left is colistin," Schaffner said.

It's especially important to treat something called CRE, a family of germs that are resistant to a class of antibiotics called carbapenems. Centers for Disease Control and Prevention director Dr. Thomas Frieden calls this a "nightmare bacteria."

But the Pennsylvania patient didn't have a CRE. Lab tests confirmed that the strain of e. Coli was also resistant to several other classes of bacteria, but that carbapenems could still be used to treat the patient's infection.

Although that's a comfort, there's still concern because it's not clear where the patient contracted the bacteria with the mcr-1 gene. She hadn't traveled outside the US in five months.

"We know now the more we look, the more we'll find," Frieden said of bacteria harboring the mcr-1 gene. "The more we look at drug resistance, the more concerned we are."

Why is mcr-1 significant?

The mcr-1 gene can be transferred to other bacteria, which is why public health officials are worried. The gene isn't found in the chromosomal material that bacteria need to survive. It's found in something called a plasmid, which means the bacteria can shed the gene without harming the core DNA found in the chromosome. That means the gene can jump to other bacteria.

"[This strain] is one gene away from being completely drug resistant," Chambers said. "Either the plasmid winds up in a strain that's resistant to all other antibiotics or the plasmid itself acquires more resistance genes and carries them to another strain."

The mcr-1 gene has been reported in other countries, including the United Kingdom in 2008.

"It's not a rampant problem, but it is a similar circumstance," Chambers said, explaining that it's still considered a rare strain of e. Coli. "Now the gene is circulating in the human population of organisms. That's the worry."

Why is Antibiotic Resistance a Big Deal?

The discovery of antibiotics less than a century ago was a turning point in public health that has saved countless lives. Although antibiotic resistance develops naturally with normal bacterial mutation, humans are speeding it up by using antibiotics improperly.

Now, 2 million people a year in the US develop antibiotic-resistant infections, and 23,000 of them die of those infections, according to the CDC.

"We risk being in a post antibiotic world," Frieden said before telling audience members about the mcr-1 finding. "That wouldn't just be for infections that you think of as bad infections, pneumonia, urinary tract infections, that's bad enough. That could be for the 600,000 Americans a year who need cancer treatment for whom we just assume we'll be able to treat infections. We may lose that ability."

What Can We Do?

Schaffner said physicians and patients can start by limiting their antibiotic use. If a patient has a virus, for instance, an antibiotic won't work, so doctors shouldn't prescribe antibiotics even if the patient insists. And when patients do need antibiotics, it's important to make sure they take the full course to kill off every last infection-causing germ. Otherwise the strong survive, mutate, and spread.

As a society, curbing antibiotic use in healthy animals used in human food production is another important step, he said.

And finally, we must find incentives to convince pharmaceutical companies to search for and develop new antibiotics. This has been problematic in recent years because pharma companies don't stand to make as much money as they would if they were to develop a drug to treat a chronic condition like diabetes or high cholesterol, which require patients to take pills daily for the rest of their lives.

Patients only take antibiotics for a week or so, and pharma companies are concerned that public health officials would push doctors to reserve new antibiotics for dire situations in order to minimize resistance, making development of new antibiotics a costly endeavor without the possibility of much profit.

"It's not very appealing to stockholders," Schaffner said, adding that IDSA is working with Congress to solve this problem. IDSA has been pushing for 10 new antibiotics by 2020, but experts have said there aren't enough prospective antibiotics in the drug development pipeline to meet that goal.

"The medicine cabinet is empty for some patients," Frieden said. "It is the end of the road for antibiotics unless we act urgently."

Follow Sydney Lupkin on Twitter: @slupkin