The mobilized colistin resistance (mcr) gene confers plasmid-mediated resistance to colistin, one of a number of last-resort antibiotics for treating Gram-negative infections. mcr-1, the original variant, is capable of horizontal transfer between different strains of a bacterial species. After discovery in November 2015 in E. coli (strain SHP45) from a pig in China it has been found in Escherichia coli, Salmonella enterica, Klebsiella pneumoniae, Enterobacter aerogenes, and Enterobacter cloacae. As of 2017^[update], it has been detected in more than 30 countries on 5 continents in less than a year.

The "mobilized colistin resistance" (mcr-1) gene confers plasmid-mediated resistance to colistin, a polymyxin and one of a number of last-resort antibiotics for treating infections.^[1][2] The gene is found in no less than ten species of the Enterobacteriaceae: Escherichia coli, Salmonella, Klebsiella pneumoniae, Enterobacter aerogenes, Enterobacter cloacae, Cronobacter sakazakii, Shigella sonnei, Kluyvera species, Citrobacter species, and Raoultella ornithinolytica.^{[citation needed]}

The mechanism of resistance of the MCR gene is a lipid A phosphoethanolamine transferase. The enzyme transfers a phosphoethanolamine residue to the lipid A present in the cell membrane of gram-negative bacteria. The altered lipid A has much lower affinity for colistin and related polymyxins resulting in reduced activity of the antimicrobial. This type of resistance is known as target modification.^[3] Although the same mechanism has been observed before with enzymes like eptA,^[4] mcr-1 is the first polymyxin resistance gene known to be capable of horizontal transfer between different strains of a bacterial species.^[1]

mcr-1 also provides resistance to host antimicrobial peptides. Bacteria carrying the gene were better at killing infected caterpillars.^[5]

The gene was first discovered in E. coli (strain SHP45) from a pig in China April 2011 and published in November 2015.^[6][7] It was identified by independent researchers in human samples from Malaysia, China,^[1] England,^[8][9] Scotland,^[10] and the United States.^[11]

In April 2016, a 49-year-old woman sought medical care at a Pennsylvania clinic for UTI symptoms. PCR of an E. coli isolate cultured from her urine revealed the mcr-1 gene for the first time in the United States,^[12] and the CDC sent an alert to health care facilities. In the following twelve months, four additional people were reported to have infections with mcr-1 carrying bacteria.^[13]

As of February 2017^[update] mcr-1 has been detected in more than 30 countries on 5 continents in less than a year,^[14] and it appears to be spreading in hospitals in China.^[15] The prevalence in five Chinese provinces between April 2011 and November 2014 was 15% in raw meat samples and 21% in food animals during 2011–14, and 1% in people hospitalized with infection.^[1]

Using genetic analysis, researchers believe that they have shown that the origins of the gene were on a Chinese pig farm where colistin was routinely used.^[16][17]

Given the importance of mcr-1 in enabling bacteria to acquire polymyxin resistance, MCR-1 (the protein that is encoded by mcr-1) is a current inhibition target for the development of new antibiotic adjuvants.^[18][19] For example, ethylenediaminetetraacetic acid (EDTA), a metal-chelating agent and common food additive, was shown to inhibit MCR-1 as it is a zinc-dependent enzyme.^[3] Substrate analogues, such as ethanolamine and glucose, were also shown to inhibit MCR-1.^[20] The use of a combined antibiotics regime has shown to be able to overcome the resistance that is caused by mcr-1, and the mechanism of action may be directly or indirectly targeting the MCR-1 protein.^[21] Recently, a couple of studies showed that peptide nucleic acids (PNAs), a class of antisense molecules, when conjugated with cell-penetrating peptides, may inhibit the translation of MCR to turn off polymyxin resistance.^[22][23]

As of April 2021^[update], ten mobilized colistin resistance genes termed mcr-1 through mcr-10 have been identified. They are homologous to each other, and work in similar ways.^[24] The mcr-2 gene is a rare variant of mcr-1 and is found only in Belgium. The less-related mcr-3, mcr-4, and mcr-5 were identified in E. coli and Salmonella.^[25]

On the phylogenic tree, the various clusters of mcr genes are scattered between immobile resistance genes of the same type, suggesting a history of multiple transfer to plasmids.^{[26][27][20][28]}

• New Delhi metallo-beta-lactamase 1