Bacteria have developed a new way to resist a sweeping array of antibiotics, raising alarms about the spread of infections that might defy nearly all treatments.
Three Americans were recently diagnosed with the new infection, which they acquired during medical treatment in India and Pakistan. Media reports have dubbed the new infection a "superbug."
So how worried should we be?
While the infection is worth taking seriously, experts say, the public health implications are uncertain, and the finding has been widely misunderstood. Multi drug-resistant bacteria have been around for decades, they say. Furthermore, there are still two kinds of antibiotics that fight the new infection, which crops up only in hospitals not in communities.
And while the new bug is highly resistant to drugs, it is not spreading rapidly -- at least not in the United States -- and it is not particularly deadly.
In fact, it is not actually a bug at all. Instead, the drug resistance comes from a gene called NDM-1 that gets passed from one kind of bacteria to another.
"Calling it a superbug doesn't quite make sense," said Stephen Calderwood, chief of the infectious disease division at Massachusetts General Hospital in Boston, who recently treated someone with the infection. "It is highly resistant, but it doesn't make someone more sick. And as far as we know, it doesn't more easily go from one person to another."
Rather than sparking fears of an imminent epidemic, he added, the arrival of the gene points to the need for new kinds of antibiotics, tighter controls on existing antibiotics in some places, better international cooperation on health prevention, and more careful controls on medical tourism.
"If you put this into context, there are some worrisome features that need close attention. But with the data available to date, we don't know how worrisome it will be," Calderwood said. "Drug resistance is a big problem. It's underappreciated, and it needs a new investment. This latest bug is just one more example. It's not that dramatically different."
For decades, there has been an ongoing arms race between thee bacteria that cause illnesses and the drugs that are designed to kill them. Traditionally, when bacteria developed resistance to one drug, another drug that works in a slightly different way was able to do the job.
Worries escalated in the mid-1990s, when bacteria began to defy a variety of antibiotics at once. Many of these multi-drug resistant bacteria had acquired new genes, which produced enzymes that cut antibiotic molecules into pieces. That rendered the drugs powerless.
These genes have sparked even more concerns because they don't lie in the genomes of the bacteria themselves. Instead, they sit on small, circular pieces of DNA called plasmids, which can be passed between bacteria. On a single plasmid, there might be as many as 15 antibiotic-resistant genes, said Timothy Walsh, professor of medical microbiology at the University of Cardiff, in Wales. He discovered NDM-1.
What sets NDM-1 apart, Walsh said, is that it lies on a plasmid that seems to move especially freely between bacteria, even if they are completely unrelated. That means the gene could move from an E. coli bacterium that causes urinary tract infections to other types of bacteria that cause pneumonia, salmonella or cholera.
With some 100 trillion bacteria on and in our bodies at all times, plasmids with the NDM-1 gene have many opportunities to spread. That can happen inside a person's gut, in the soil, or within water that contains fecal contamination.
In a paper published in The Lancet last month, Walsh and colleagues reported clusters of the gene in Indian and Pakistan and in British people who had received medical treatment in those countries, suggesting that that's where the gene emerged.
In that part of the world, he said, antibiotics are available over the counter and used indiscriminately, millions of people drink contaminated water, and hygiene is often lacking. These conditions can cause drug resistance to both develop and spread.
"In the U.S., there's no need at the moment for people to hit the panic button," Walsh said. "On a global level, there's an awful lot to be worried about."