Even more worrisome in the Lancet report was evidence that mcr-1 had already leapt from pigs to humans. Out of 1322 patient samples from hospitals in China, the team found 16 containing mcr-1. And, of course, drug-resistant bacteria don’t respect national borders. As the team was writing up its report, it noticed other researchers had uploaded genomes of bacteria in Malaysia containing the mcr-1 gene sequence to an online database. “The possibility that mcr-1-positive E. coli have spread outside China and into other countries in southeastern Asia is deeply concerning,” the authors warned.

To be clear, these E. coli with mcr-1 found in China were still susceptible to antibiotics other than colistin, but if a bacterium with genes that make it resistant to every other drug then picks up mcr-1, you get the nightmare scenario: a pan-resistant bacteria. These drug- resistant infections usually happen in people who are already sick or have weakened immune systems.

The Lancet report immediately kicked off a flurry of activity in microbiology labs around the world. And soon enough, the reports started coming out. In December, scientists reported finding mcr-1 in Denmark. Then Germany and Vietnam and Spain and United States and on and on. It seems like every month is a new mcr-1 development, I said to James Spencer, a microbiologist University of Bristol who collaborated with the Chinese scientists on the original Lancet paper. “Every week,” he corrected me. By the time anyone had figured out mcr-1’s existence, it had already spread around the world.

The story of mcr-1’s silent spread is, by now, a familiar one. Over and over, scientists have identified genes conferring resistance to a class of antibiotics, only to find the gene had circled the globe. Another recent example is ndm-1, a gene found in 2009 that confers resistance to class of antibiotics called carbapenems. “It’s very rare to catch something at the very beginning,” says Alexander Kallen, a medical epidemiologist with the Centers for Disease Control and Prevention. Looking for resistance is a constant game of catch-up. You don’t notice anything until there is something to notice; by the time there is something to notice, something bad has already happened. And you have to have eyes everywhere: Resistance initially found on a Chinese pig farm could repercussions all over the world.

When Timothy Walsh, a microbiologist at Cardiff University first heard of mcr-1’s existence from his colleague Yang Wang of China Agricultural

University, he didn’t believe it. “It’s like the holy grail of resistance,” says Walsh. He was also skeptical because of the way colistin works. The antibiotic binds to molecules on the surface of bacteria—and modifying those molecules typically requires mutations in several different genes. Instead of getting lucky just once, that bacteria would have to get lucky several times to beat off colistin. In fact, other researchers had identified colistin-resistant bacteria before, which had multiple mutations in the DNA in their chromosomes.