MCR-1

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". , it has been detected in more than 30 countries on 5 continents in less than a year.
Description.
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. 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. Although the same mechanism has been observed before with enzymes like "eptA", "mcr-1" is the first polymyxin resistance gene known to be capable of horizontal transfer between different strains of a bacterial species.
Discovery and geographical spread.
The gene was first discovered in "E. coli" (strain SHP45) from a pig in China April 2011 and published in November 2015. 
It was identified by independent researchers in human samples from Malaysia, China, England, Scotland, and the United States.
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, 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.
 mcr-1 has been detected in more than 30 countries on 5 continents in less than a year, and it appears to be spreading in hospitals in China. 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.
Origins.
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.
Inhibition.
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. For example, ethylenediaminetetraacetic acid, a metal-chelating agent, was shown to inhibit MCR-1 as it is a zinc-dependent enzyme. Substrate analogues, such as ethanolamine and glucose, were also shown to inhibit MCR-1. 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.
Other "mcr" genes.
, 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. 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".
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.