Colistin-Resistant mcr-1 E. coli Confirmed in the United States

Researchers found the multidrug resistant bacteria in a woman who sought treatment for UTI.

Researchers working at Walter Reed Institute of Research have found mcr-1 E. coli in a bacteria cultured from a Pennsylvanian who sought treatment for a urinary tract infection. This is the first confirmed appearance of the colistin-resistant strain of the bacteria in a human in the United States.

Published reports say that the patient, a 49-year-old woman from Pennsylvania, has since recovered.

Colistin belongs to a class of drugs called carbapenems that are often used as a last resort to treat patients with multidrug resistant infections. Infectious disease specialists fear that the recent discovery signals the arrival of a truly multidrug-resistant bacteria. Thomas R. Frieden, MD, MPH, director of the Centers for Disease Control and Prevention, said during a press conference Thursday that Colistin is the last antibiotic that can treat “nightmare bacteria.”

“The more we look at drug resistance, the more concerned we are,” he said. “Without better stewardship and identification of outbreaks, we’ll lose these miracle drugs. The medicine cabinet is empty for some patients. It is the end of the road for antibiotics unless we act urgently. We risk being in a post-antibiotic world.”

Researchers at Walter Reed published their findings online in Antimicrobial Agents and Chemotherapy.

“To date, a further 20 ESBL-producing E. coli from patients at the [Walter Reed National Military Medical Center] have tested negative for mcr-1, and are colistin sensitive. However, as testing has only been ongoing for 3 weeks, it remains unclear what the true prevalence of mcr-1 is in the population,” wrote McGann et al. “The association between mcr-1 and IncF plasmids is concerning as these plasmids are vehicles for the dissemination of antibiotic resistance and virulence genes among the Enterobacteriaceae. Continued surveillance to determine the true frequency for this gene in the USA is critical.”

The clinic where the woman presented with symptoms of a UTI cultured E. coli MRSN 388634 from her urine and sent the sample to Walter Reed, where susceptibility testing indicated an ESBL phenotype. The isolate was included in the first 6 ESBL-producing E. coli selected for colistin susceptibility testing, and it was the only isolate to have a MIC of colistin of 4μg/ml (all others had MICs ≤ 0.25 μ/ml).

Colistin MIC was confirmed by microbroth dilution and mcr-1 detected by real-time PCR.

Chinese researchers announced the discovery of mcr-1 in November.2 The Department of Health and Human Services (HHS) said in a statement that strains have since been found in Europe and Canada, and in a sample taken from a pig intestine in the US.3 Researchers at Walter Reed said the patient had not traveled in the past 5 months.

HHS added that “cooking all meat, poultry and fish to its proper internal temperature kills bacteria, viruses and other foodborne pathogens, whether or not they are antibiotic-resistant.”


  1. McCann P, Snesrud E, Maybank R, et al. Escherichia coli harboring mcr-1 and blaCTX-M on a novel IncF plasmid: First report of mcr-1 in the USA. Antimicrob Agents Chemother. 2016. doi: doi: 10.1128/AAC.01103-16.
  2. Liu YY, Wang Y, Walsh TR, et al. Emergence of plasmid-mediated colistin resistance mechanism MCR-1 in animals and human beings in China: a microbiological and molecular biological study. Lancet Infect Dis. 2016;16:161-168.
  3. United States Department of Health and Human Services. Proactive Efforts by U.S. Federal Agencies Enable Early Detection of New Antibiotic Resistance. Accessed May 26, 2016.