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This award-winning blog supplements the articles in Hospital Infection Control & Prevention.

Post Antibiotic Era? New E. coli Plasmid Defeats Last-Line Drug

By Gary Evans, AHC Media Senior Staff Writer

In another grim milestone toward a dreaded “Post-Antibiotic Era,” researchers have found that horizontal genetic transfer of a novel plasmid mcr-1, which confers resistance to the last-line drug colistin in Escherichia coli, has now appeared in the U.S.

The possible emergence of untreatable E. coli – a common cause of urinary tract and other infections in the community – is enough to rudely awake a medical epidemiologist in the middle of the night. It is a sobering development that the CDC has been concerned about for some time.

First reported in China, the mcr-1 plasmid that can transfer colistin resistance to E. coli has now been found in a U.S. patient. In response to the reports from China, Walter Reed National Military Medical Center in Bethesda, MD started testing for colistin resistance in all extended-spectrum β-lactamase (ESBL)-producing E.coli clinical isolates submitted to the clinical microbiology laboratory as of May 16. The testing revealed mcr-1 in an E. coli isolate cultured from the urine of a 49-year-old female who presented to a clinic in Pennsylvania on April 26, 2016 with symptoms indicative of a UTI.

The isolate was forwarded to Walter Reed, where susceptibility testing revealed it had an MIC to colistin of 4μg/ml (all others had MICs ≤ 0.25 μ/ml). The colistin MIC -- a standard measure of antibiotic resistance -- was confirmed by microbroth dilution and mcr-1 was detected by real-time PCR.

“To the best of our knowledge, this is the first report of mcr-1 in the U.S.,” the researchers reported.

Interestingly, the patient reported no travel history in the prior five months. Continued surveillance to determine the true U.S. prevalence of this gene in the U.S. is critical, the authors stressed. The mcr-1 gene has been found primarily in E. coli but has also been identified in other species of Enterobacteriaceae from human, animal, food and environmental samples on every continent, the researchers report.

Colistin is a last-line drug for good reason. Clinicians have avoided using it because it may clear an infection but damage the patient’s kidneys. In one case reported in Hospital Infection Control and Prevention, a patient chose to have his leg amputated below the knee -- removing the site of infection -- rather than continue to take colistin and face going on dialysis. Thus it's only now being used in cases where nothing else works, but if this mcr-1 plasmid spreads the formulary could finally be completely empty for certain infections. The patient outcome was not clear from the original report, but according to some press reports the woman survived.

Look for more information on this landmark development in the next issue of HIC.