European scientists have discovered another gene that makes bacteria resistant to the antibiotic of last resort, one that could spread more easily among other kinds of bacteria.
The gene, called mcr-2, was found in E. coli bacteria from pigs in Belgium, the scientists reported Thursday in the journal Eurosurveillance. It is similar to the gene mcr-1, which was first identified in China last fall and has now been seen in 30 countries across five continents, including the United States.
Both genes confer resistance to a drug called colistin, an antibiotic used to cure infections that have already developed resistance to other antibiotics.
Colistin resistance has been seen before, but both mcr-1 and mcr-2 pose a particularly worrisome threat. The genes are carried on plasmids, mobile pieces of DNA that can be swapped from one bacterium to another, even from different families. That means they can wind up in bacteria that infect people.
While mcr-1 and mcr-2 have the same clinical effect, mcr-2 might be able to be passed among different bacteria more easily than mcr-1, the new study found.
“They showed that this has a higher transfer frequency, so this thing has the potential to disseminate more quickly than mcr-1,” said Lance Price, who leads the Antibiotic Resistance Action Center at George Washington University.
For the report, scientists studying E. coli samples from piglets and calves in Belgium found bacteria from the pigs that were resistant to colistin but that did not contain mcr-1. Genetic sequencing led them to the new gene.
The mcr-2 gene was more prevalent than mcr-1 in the samples of E. coli, but the scientists noted they were working with a small number of samples.
That another gene or another version of a gene also makes superbugs resistant to colistin is not unexpected, said Barry Kreiswirth of the Public Health Research Institute at Rutgers. Some genes that confer resistance to other antibiotics have multiple variants as well.
“There are more of these guys out there, including different flavors,” Kreiswirth said. “This is what you expect when people start looking.”
Mcr-1 has already been found in bacteria retrieved from people, including a woman in the United States. The fear is that such a gene could find its way into bacteria that are already resistant to other antibiotics, creating a pan-resistant bug that cannot be treated.
The emergence of superbugs has been blamed on the overuse of antibiotics in both people and in livestock. Colistin is not used in animal husbandry in the United States, but is in some parts of the world, including China.
US officials have said they are expanding surveillance efforts to hunt for mcr-1, and the authors of the new report called for the “immediate inclusion” of mcr-2 in such efforts around the world.
Price noted that many reports of colistin-resistant bacteria are coming out of Europe because researchers there are doing genetic analysis of bacteria more frequently than elsewhere. He said US scientists should be doing the same.
“We need more active surveillance looking for these bugs so we can get ahead of these things,” he said.