Tracking the mcr gene

About mcr Genes, an Emerging Threat

The mcr-1, mcr-2 and mcr-3 genes cause resistance to colistin, a last-resort antibiotic used for treating resistant infections. Colistin is considered a last-resort antibiotic because—while it can be used to treat patients with infections that have already developed resistance to other antibiotics—it can have serious side effects.

The mcr gene is particularly worrisome because it is found on plasmids, small pieces of DNA that carry genetic instructions from one bacterium to another. This means that plasmids carrying resistant mcr genes can make other bacteria become resistant to colistin, including the “nightmare bacteria” carbapenem-resistant Enterobacteriaceae (CRE). The term mcr stands for “plasmid-mediated colistin resistance,” describing the gene’s ability to avoid the effects of colistin and share this ability with other bacteria. The descriptions -1, -2, and -3 indicate different DNA sequences.

The mcr gene has been found in food animals and in people since it was first reported in November 2015. The first mcr-1 gene was reported by China in November 2015, followed by Belgium’s report on the first mcr-2 in July 2016. In June 2017, an article published in mBio, an open access journal published by the American Society for Microbiology, described finding the first mcr-3. 

CDC’s Response to Contain mcr

Following China’s report in 2015, CDC and federal partners began searching for mcr genes in the United States. The gene was first identified in the United States by the Department of Defense in bacteria cultured from a Pennsylvania patient and in a special study by the National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS) in intestinal samples of two pigs, one in South Carolina and the other in Illinois. CDC released reports describing patients from Pennsylvania and Connecticut, and the public health investigation that followed. CDC continues to work with international, federal, state, and U.S. academic partners to search for the mcr genes, and displays these findings on the map below.

These discoveries emphasize the importance of a coordinated public health response, not only to detect new threats like mcr genes, but also to track, slow, and respond to the emergence of antibiotic resistance. CDC is also working to stand up a full, faster response to protect Americans from existing and emerging antibiotic resistant threats. In 2016, CDC’s Antibiotic Resistance Laboratory Network established support for seven regional labs that can detect new forms of antibiotic resistance, like mcr, and report these findings to CDC. Since July 2015, the CDC and FDA Antibiotic Resistance Isolate Bank offers a centralized repository of bacterial germs with well-characterized resistance profiles. A panel of resistant bacterial isolates with new or novel antibiotic resistance is available, which includes mcr-1 isolates. These panels can be used for the development of new diagnostics and therapeutics that can rapidly detect and treat infections caused by resistant bacteria.

New resistant bacteria continue to emerge with the potential to severely impact our ability to provide effective medical treatment. We cannot stop the development of mcr genes; bacteria inevitably find ways to avoid the effects of the drugs designed to kill them. However, CDC will continue to work with state and local public health agencies to investigate cases and prevent infections from spreading by following infection control measures for every patient, every time.

Map: Tracking mcr in the U.S.

The following map displays where the mcr gene has been reported in U.S. human and food animal sources as of Oct. 3, 2017. The map is updated monthly.

View other maps: CDC tracks and updates website maps of other emerging resistance to prevent spread and protect people.