MCR-1

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.

The mechanism of resistance of the MCR gene is a phosphatidylethanolamine 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]

The gene was first discovered in E. coli (strain SHP45) from a pig in China April 2011 and published in November 2015.[5][6] It was identified by independent researchers in human samples from Malaysia, China,[1] England,[7][8] Scotland,[9] and the United States.[10]

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,[11] 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.[12]

As of February 2017 mcr-1 has been detected in more than 30 countries on 5 continents in less than a year,[13] and it appears to be spreading in hospitals in China.[14] 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.[15][16]

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 antibiotics.[17] For example, ethylenediaminetetraacetic acid, a metal-chelating agent, 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.[18] The use of a combined antibiotics regime has shown to be able to overcome the resistance that is caused by mcr-1, although the mechanism of action may not be directly targeting the MCR-1 protein.[19]

As of May 2019, nine mobilized colistin resistance genes termed mcr-1 through mcr-9 has been identified. They are homologous to each other, and work in similar ways.[20] 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.[21]

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.[20][22][18]

• New Delhi metallo-beta-lactamase 1