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31st May 2016  Content supplied by: US Dept of Health and Human Services

Resistant Gene Detected Against Colistin – the Antibiotic of Last Resort

The first colistin-resistant E. coli has just been detected in a person in the United States.  This is of major concern as colistin is the antibiotic of last resort that is used when the major antibiotics, including ampicillin, streptomycin, sulfisoxazole, and tetracycline, have failed and this strain is resistant to these as well.


Samples from the patient were genetically sequenced to confirm the presence of the mcr-1 gene that scientists in China had identified last November as conferring colistin resistance.  Scientists around the world have been searching for bacteria with the mcr-1 gene.  Screening is done by exposing samples to colistin at a concentration that would kill sensitive bacteria and allow any bacteria carrying mcr-1 to survive.


The US National Antimicrobial Resistance Monitoring System (NARMS) is searching for the presence of colistin-resistant bacteria is the US.  Out of 949 animal samples screened so far, one strain of colistin-resistant E. coli was found in a pig intestinal sample. The DNA sequence of this isolate revealed that the strain contained the mcr-1 gene on a plasmid.


As the mcr-1 gene exists on a plasmid, a small piece of DNA that is not a part of a bacterium’s chromosome. Plasmids are capable of moving from one bacterium to another, spreading antibiotic resistance between bacterial species.


In addition, the US Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration used whole genome sequencing technology to search for the mcr-1 gene in SalmonellaE. coli and Klebsiella taken from human and retail meat sources.  As of April 2016, more than 44,000 Salmonella and 9,000 E. coli Shigella isolates from NARMS as well as the National Center for Biotechnology Information genomic database did not show the presence of the mcr-1 gene.


The emergence of colistin resistance is puzzling as it is rarely used in human medicine compared to other antibiotics.  It is often used to treat multi-drug resistant infections and its use is increasing.  However, it is not used in animals in the US, which is often the source for the development of antibiotic resistance.


In order to monitor the emergence of resistant organisms in near real time, the CDC is setting up a network of labs across the US to detect new forms of antibiotic resistance.  This will generate better data for stronger infection control among patients to prevent and combat future resistance threats.



Date Published: 31st May 2016

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