Tracking the mcr gene
The mcr-1, mcr-2 and mcr-3 genes cause resistance to colistin. Colistin is considered a last-resort antibiotic because it can be used to treat patients with infections that have already developed resistance to other antibiotics, but it can also 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 the ability to become resistant can be shared, making other bacteria resistant to colistin, including the “nightmare bacteria” carbapenem-resistant Enterobacteriaceae (CRE).
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 Chinaexternal icon in November 2015, followed by Belgium’s reportexternal icon on the first mcr-2 in July 2016. In June 2017, an article published in mBioexternal icon, an open access journal published by the American Society for Microbiology, described finding the first mcr-3. The descriptions -1, -2, and -3 indicate different DNA sequences.
Following China’s report in 2015, CDC and federal partners began searching for mcr genes in the U.S. The gene was first identified in the U.S. in a patient in Pennsylvania, found by the Department of Defense, and in two pigs—one in South Carolina and the other in Illinois—found during a special study by the National Antimicrobial Resistance Monitoring System for Enteric Bacteria. CDC released reports describing patients from Pennsylvania and Connecticut, and the public health investigation that followed. CDC continues to work with partners to search for mcr genes, and shares 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. In 2016, CDC established the Antibiotic Resistance Laboratory Network to help detect new forms of antibiotic resistance, like mcr. Additionally, the Antibiotic Resistance Isolate Bank provides information on resistance to inform innovation in diagnostics and drug development. As new resistance continues to emerge, CDC will continue to work with state and local public health agencies to investigate cases and prevent infections from spreading.
The following map shows where the mcr gene has been reported in U.S. human and food animal sources as of Nov. 2, 2018. The map is updated regularly.