Friday, December 18, 2015

The Lancet: Dissemination Of The MCR-1 Colistin Resistance Gene


It has only been a month since The Lancet published a study on the emergence of a plasmid-mediated MCR-1 gene in China (see Return of the Plasmids), which was quickly followed by a Eurosurveillance Dispatch (see MRC-1 In Denmark) showing this antibiotic resistance mechanism was already present in Europe.

Very much like the NDM-1 Carbepenum resistance gene - which came to prominence just over 5 years ago  (see NDM-1: A New Acronym To Memorize) - MCR-1 confers resistance to another class of antibiotics - Polymixins (Colistin). 

And like NDM-1, the MCR-1 gene is carried by plasmids - tiny snippets of DNA that can  transfer the gene  to other types of bacteria (see Study: Adaptation Of Plasmids To New Bacterial Species) in a shared environment. 

All of this become extremely important because Carbapenums and Polymixins are the two classes of antibiotics of last resort.  Should a bacteria acquire both forms of resistance, it would leave few - if any -  options for treatment.

While an alarming prospect - one that brings us just a bit closer to the stark `Post-Antibiotic Era' WHO Director Margaret Chan has described -  it isn't known just how well distributed this new found resistance gene is around the world, or how fast it is spreading.

Yesterday The Lancet published a correspondence detailing a retrospective look at fecal samples collected from Dutch travelers between Nov. 2012 and Nov. 2013 for another resistance study (The COMBAT study) that found disturbing evidence of MCR-1 acquisition from three continents.

A link and a short excerpt, but you'll want to read the entire letter.

Dissemination of the mcr-1 colistin resistance gene


Of the six travellers who acquired ESBL-producing E coli carrying the mcr-1 gene, two unrelated travellers visited Peru and Bolivia, two unrelated travellers visited China, one visited Tunisia, and one visited multiple countries in southeast Asia (Thailand, Vietnam, Laos, and Cambodia). The duration of travel ranged between 8 and 40 days (mean 21·3 days). 

None of the travellers had accessed medical care and none had used antimicrobial drugs during travel, while five had experienced traveller's diarrhoea. Analysis of subsequent fecal samples collected at 1, 3, 6, and 12 months after return to the Netherlands did not show ESBL-producing E coli, suggesting short-term colonisation with colistin resistant ESBL-producing E coli or loss of plasmids carrying ESBL and potentially mcr-1 genes.
(Continue . . .)

Colistin - which has been around since the late 1950s - isn't routinely used for humans anymore because of its nephrotoxic side effects, although it is increasingly viewed as an option of last resort against highly resistant infections.    

Unfortunately, it is cheap and readily accessible in much of the world, and has been widely used in agriculture - particularly in China and Southeast Asia - where this resistance gene appears to be most common. 

For more on all of this - Maryn McKenna - writing for her National Geographic blog Germination, wrote a terrific overview of the agricultural misuse of Colistin last month  called Apocalypse Pig: The Last Antibiotic Begins to Fail.

Highly recommended. 

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