A superbug that’s resistant to the absolutely last-ditch antibiotic colistin has been reported in seagulls on two continents—pinpointing one way, though almost certainly not the only way, that this dangerous drug resistance is moving around the world.
Since last November, when researchers in England and China announced the discovery of bacteria able to survive colistin, there has been an explosion of people looking for that resistance, and finding it. Scientists have published almost 100 reports of colistin resistance—known as MCR and conferred by a gene that’s been dubbed mcr-1—in almost two dozen countries.
It has been found in human patients, including a woman in the United States in May; in livestock, which get the drug on intensive farms, and are probably the original source of the problem; and even in pets.
Now, in letters to the Journal of Antimicrobial Chemotherapy, two research teams in Lithuania and Argentina report that they trapped birds and swabbed their butts, or scooped up seagull droppings, and found the resistance-conferring gene in E. coli being carried by two species: herring gulls in Lithuania (Larus argentatus) and kelp gulls in Argentina (Larus dominicanus).
Both teams think the birds probably picked up the resistant E. coli by eating garbage, which may have contained sewage or medical waste. (The organisms in the South American gulls also contained another important type of antibiotic resistance, known for short as ESBL.)
This isn’t the first time that gulls have been identified as possible carriers of antibiotic-resistant bacteria. In 2011, French researchers found multi-drug resistant E. coli in seagull droppings in Miami Beach, and those researchers and others earlier found resistant bacteria in gulls in Portugal, France, Russia, and Greenland.
The point in all those stories, as well as in the new reports, is that gulls migrate, from hundreds to thousands of miles depending on the species—so they could serve as a vehicle for carrying resistant bacteria somewhere new.
Gulls migrate, from hundreds to thousands of miles, so they could serve as a vehicle for carrying resistant bacteria somewhere new.
“The lifestyle of gulls allows them to carry and disseminate pathogenic and resistant microorganisms despite country borders,” the Lithuanian researchers say in their report. “Water contaminated by feces of birds should be foreseen as an important risk factor for transmission of resistant bacteria.”
The undetected movement of bacteria is especially important in the case of MCR, because the discovery of colistin resistance is truly alarming. Colistin is an old drug that medicine consigned to the back of the shelf in the 1950s because it is toxic, and only recently started using again because so many other antibiotics have been undermined by overuse in medicine and agriculture.
The gene that creates colistin resistance is on what is called a plasmid, a loop of DNA that isn’t bound up in chromosomes but can move easily between bacteria. That has scientists worried that the gene could move into disease organisms that already possess resistance to other antibiotics, creating a superbug that would be completely untreatable.
So far, mcr-1 has been found in the United States three times: in two stored samples from slaughtered pigs that were stashed in a U.S. Department of Agriculture database, and in a 49-year-old woman in Pennsylvania, not identified by name, who went to a clinic for help with a urinary tract infection.
At a meeting Tuesday afternoon in Washington, D.C., of the Presidential Advisory Council on Combating Antibiotic-Resistant Bacteria, federal officials relayed that the woman has recovered from her infection, but still continues to carry the highly resistant bacterium in her system. Dr. Beth Bell, director of the National Center for Emerging and Zoonotic Diseases at the Centers for Disease Control and Prevention, also said that 99 of the woman’s family members and close contacts have been checked, and none of them are carrying bacteria containing mcr-1, reinforcing the mystery of how the resistant bacteria reached her.
Bell and representatives of the U.S. Department of Agriculture said the gene remains rare in the U.S.: The CDC has checked more than 55,000 stored samples collected from patients, animals, and food, and the USDA is checking 2,000 additional samples that it has stored. So far, that search has revealed only the two samples from pigs that were slaughtered in Illinois and South Carolina.
The officials commenting Tuesday agreed that there may be no way of tracing the path that MCR took to reach the U.S.—the bacteria may have spread from another person, or on food—and that the key thing now is to build surveillance systems that alert health planners as it moves.
“The good news is we found it,” observed Dr. Martin Blaser, a professor of medicine and microbiology at NYU Medical Center and chair of the Presidential council. “The bad news is, it’s here.”