December 01, 2015
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Chinese surveillance detects transmissible colistin resistance gene in livestock, humans

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Enterobacteriaceae carrying a gene conferring transmissible polymyxin resistance have been identified and isolated from Chinese livestock and hospitalized patients, according to recent data.

“Until now, colistin resistance has occurred via chromosomal mutations and, although clonal outbreaks have been reported, the resistance is often unstable, imposes a fitness cost upon the bacterium and is incapable of spreading to other bacteria,” the researchers wrote. “The rapid dissemination of previous resistance mechanisms indicates that, with the advent of transmissible colistin resistance, progression of Enterobacteriaceae from extensive drug resistance to pan-drug resistance is inevitable and will ultimately become global.”

In response to the large increase of colistin resistance among Chinese food animals, researchers selected a random Escherichia coli strain (SHP45) for analysis and plasmid transfer experimentation. Recovered from a Shanghai pig farm in July 2013, the strain’s polymyxin resistance was traced to a previously unexamined gene, designated mcr-1. Studies of this mechanism were performed using sequence comparisons, homology modelling and electrospray ionization mass spectrometry. Researchers screened 902 E. coli and 420 Klebsiella pneumoniae isolates collected from two Chinese tertiary hospitals, 804 isolates from slaughtered pigs and 523 isolates from raw meat to determine the gene’s prevalence, and its ability to confer polymyxin resistance was tested in murinae.

They found that mcr-1 was able to transfer colistin resistance to an E. coli recipient at a frequency of 10-1 to 10-3 cells per recipient cell by conjugation. The gene could also be transferred via transformation to K. pneumoniae and Pseudomonas aeruginosa, conferring up to 16-fold increases in colistin’s minimum inhibitory concentrations. Among the collected E. coli samples, carriage of the mcr-1 gene was detected in 14.9% of raw meat, 20.6% of animals and 1.4% of inpatients with infection.

“In the absence of new agents effective against resistant gram-negative pathogens, the effect on human health by mobile colistin resistance cannot be underestimated,” the researchers wrote. “It is imperative that surveillance and molecular epidemiological studies on the distribution and dissemination of mcr-1 among gram-negative bacteria in both human and veterinary medicine are initiated, along with re-evaluation of the use of polymyxins in animals.” – by Dave Muoio

Disclosure: The researchers report no relevant financial disclosures.