If you’ve been following the news about antibiotic resistant bacteria, you may have heard about a particularly alarming plasmid: MCR-1. A plasmid is a circle of DNA that is relatively small and mobile – an easy way for genetic information to spread between bacteria. MCR-1 raises concern because it provides bacteria resistance against the last-resort antibiotic colistin. The CDC reports MCR-1 was found in both patients and livestock in the United States this summer.

David Weiss, director of Emory’s Antibiotic Resistance Center, and colleagues have a short letter in The Lancet Infectious Diseases showing that MCR-1 also confers resistance to an antimicrobial enzyme produced by our bodies called lysozyme. MCR-1-containing strains were 5 to 20 times less susceptible to lysozyme, they report.

This suggests that the pressure of fighting the host immune system may select for MCR-1 to stick around, even in the absence of colistin use, the authors say.

While the findings are straightforward in bacterial culture, Weiss cautions that there is not yet evidence showing that this mechanism occurs in live hosts. For those that really want to get alarmed, he also calls attention to a recent Nature Microbiology paper describing a hybrid plasmid with both MCR-1 and resistance to carbapenem, another antibiotic.

MMG graduate student Edgar Sherman and postdoc David Hufnagel are co-authors on the letter. They write:

“Resistance to the host’s innate immune defences could drive plasmid maintenance in strains carrying mcr-1, mcr-2, or other transferable resistance plasmids, leading to propagation of colistin resistance. Therefore, mammals might currently or in the future serve as reservoirs for bacteria harbouring transmissible colistin resistance regardless of polymyxin exposure.”

Maryn McKenna has a Q+A with Rutgers microbiologist Barry Kreiswirth discussing the uncertainty about the hidden reservoir of MCR-1 in bacteria around the world.