A newly identified gene that renders bacteria resistant to polymyxin antibiotics—drugs often used as the last line of defense against infections—has the potential to be shared between different types of bacteria. The finding raises concern that the transferable gene could make its way into infectious bacteria that are already highly resistant to drugs, thereby creating strains of bacteria immune to every drug in doctors’ arsenal.

The gene, dubbed mcr-1, exists on a tiny, circular piece of DNA called a plasmid. These genetic elements, common among bacteria, are mobile; bacteria can make copies of them and share them with whatever bacteria happens to be nearby. Though scientists have previously discovered genes for polymyxin resistance, those genes were embedded in bacterial genomes, thus were not likely to easily spread.

In a study, published Wednesday in the Lancet Infectious Diseases, researchers in China report first finding the mcr-1-containing plasmid in Escherichia coli bacteria isolated from a pig. The plasmid was solely responsible for the bacteria being resistant to colistin, a type of polymyxin commonly used in animals in China, but less so in the US and Europe. On further examination, the researchers also found mcr-1-containing plasmids in E. coli and Klebsiella pneumoniae isolated from a small number of patients in hospitals in Guangdong and Zhejiang provinces.

Lab tests showed that the plasmid could easily move between E. coli strains, transforming polymyxin-sensitive strains into polymyxin-resistant ones. But, the plasmid had a harder time moving between different bacterial species. The researchers had to coax the plasmid into other species by zapping bacterial cells with an electrical pulse to make their membranes more permeable—a technique called electroporation. The finding suggests that the plasmid may not easily move between species naturally.

Still, the authors cautioned that if mcr-1 containing plasmids spread to multi-drug resistant bacterial strains, it could make them “truly pan-drug resistant,” and resulting infections would be untreatable.

“This is indeed an important and concerning development as some multi-drug resistant bacteria are only sensitive to this type of antibiotic,” Judith Johnson, professor of pathology at the University of Florida, said in a statement.

But, other experts, such as Guy Loneragan, professor of food safety and public health at Texas Tech University, caution that it’s still unclear how likely it is that the plasmid will spread. And, just because this is the first report of mcr-1, it doesn't necessarily mean it's new.

What the study clearly shows, Loneragan said, is “that antibiotic resistance is a global issue and we need to develop and share (nationally and internationally) practices that reduce the need for antibiotic use in people and animals."

Lancet Infectious Diseases, 2015. DOI: 10.1016/S1473-3099(15)00424-7 (About DOIs).