The more researchers look for colistin-resistant bacteria, the more they find

Last year, researchers identified a gene that confers resistance to "last-resort" antibiotic colistin. They found it in several E. coli isolates in China, and it didn't take long for other researchers around the world to find the same gene, mcr-1, in stored samples once they started looking for it. Researchers have now found mcr-1 in isolates from 32 countries.

The US wasn't among the initial list of countries finding mcr-1, but it didn't take long for that to change. In May, Walter Reed Army Institute of Research scientists reported finding mcr-1 in E. coli cultured from the urine of a Pennsylvania woman with symptoms of a urinary tract infection. Then in July, researchers from JMI Laboratories reported finding the gene in an E. coli sample collected in May 2015 in New York. So, this gene has already been in the US for more than a year, and could continue to be found in stored US samples as more labs start looking for it.

Also in July, researchers from Belgium published an article in Eurosurveillance reporting their identification of another gene that confers colistin resistance; they dubbed it mcr-2. They studied isolates collected in 2011 and 2012 from calves and piglets with diarrhea. Out of 105 E. coli isolates, they found mcr-1 in 13. They then analyzed a selection of the remaining samples and identified mcr-2; returning to the 92 samples that did not have mcr-1, they found that 12 of them carried mcr-2.

Like mcr-1, mcr-2 has been found on a plasmid, a piece of DNA that can move easily between different bacteria -- though mcr-2 might even disseminate more quickly than mcr-1. This plasmid mobility makes it easier for bacteria to acquire resistance to multiple drugs. So far, the isolates with mcr-1 haven't carried resistance to all other antibiotic classes. But the ease with which mcr-1 can transfer makes it increasingly likely that we'll see it acquired by bacteria that are already resistant to the first- and second-line antibiotics.

As is often the case, the best explanation of these developments in antimicrobial resistance comes from Maryn McKenna at Germination. She also tacks on this alarming news:

And in a development that medicine has been braced for, Italian researchers say today that they have found yet another variant of the superbug gene, mcr-1.2, in a child hospitalized with leukemia in Florence. That variant, like mcr-1 and the newly named mcr-2, creates resistance to the very last-resort antibiotic colistin.

But what especially rang their alarm bells is that they found the gene in a strain of bacteria, Klebsiella pneumoniae, that was already resistant to the almost-last-resort antibiotic class, carbapenems, and to several other classes as well. Those bacteria are known by the acronym KPC, and since the early 2000s, they have spread through hospitals around the world. So the Italian discovery signals two things that medicine has feared: that MCR has landed in bacteria that are already good at spreading through healthcare, and that it has begun the process of stacking up in bacteria, alongside other resistance DNA, on the way to creating what could be a truly untreatable bug.

Better surveillance is important to continue to learn more about how widespread the problem is. To slow the emergence and spread of antibiotic resistance, though, the most important thing to do is to halt the routine use of antibiotics in livestock production.

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