Patterns of Antibiotic Resistant mcr-1 Gene Suggest Low Transmission Risk

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Person-to-person or environmental transmission is low in mcr-1 gene antibiotic resistant bacteria. <i> Photo Credit: By NIAID via Wikimedia Commons. </i>
Person-to-person or environmental transmission is low in mcr-1 gene antibiotic resistant bacteria. Photo Credit: By NIAID via Wikimedia Commons.

A report of a Connecticut child who visited the Caribbean and contracted Escherichia coli O157 with the mcr-1 antibiotic-resistant gene demonstrates that there is likely a low risk of person-to-person or environmental transmission of this illness, according to the Centers for Disease Control and Prevention's (CDC) Morbidity and Mortality Weekly Report.1

Amber M. Vasquez, MD, of the Epidemic Intelligence Service and the Division of Healthcare Quality Promotion at the National Center for Emerging and Zoonotic Infectious Diseases at the CDC and colleagues examined the stool from a pediatric patient who had visited the Caribbean for 2 weeks with friends and relatives. 

On June 12, 2016, the researchers noted that the patient had a fever and bloody diarrhea, 2 days before returning to the United States. The patient was treated with paromomycin from the start of symptoms. The researchers noted that while the infection did not require the patient to be hospitalized, there was 1 primary care visit and one short emergency department visit.

Dr Vasquez and colleagues reported that stool samples collected on June 16 tested positive for non-Shiga toxin–producing E coli O157 with the mcr-1 gene. They wrote that 2 additional stool cultures on June 18 and June 23 yielded the same result. Researchers used broth microdilution to determine the “colistin (also known as polymyxin E) minimum inhibitory concentration (MIC) of 2 mcg/ml, and polymyxin B MIC of 4 mcg/ml. The isolates also carried a plasmid blacmy-2 gene, which encodes AmpC, an enzyme that confers resistance to third generation cephalosporins; the isolates were susceptible to carbapenems.” Two additional stool tests on June 24 and July 1 showed no additional infection.

People who had showered or diapered the patient were considered at risk for infection. Six household contacts of the child submitted perirectal swab samples – all were negative for the mcr-1 gene bacteria, which were conducted by real-time polymerase chain reaction. In addition, none of the 16 samples from the household environment tested positive for bacteria for the mcr-1 gene. Dr Vasquez and colleagues noted that healthcare professionals did not have contact with the patient's bodily fluids and were not considered at risk.

“At this time, CDC recommends that Enterobacteriaceae isolates with a colistin or polymyxin B MIC ≥4 mcg/ml be tested for the presence of mcr-1; testing is available through CDC. Prompt reporting of mcr-1-carrying isolates to public health officials allows for a rapid response to identify transmission and limit further spread,” the researchers concluded.

This is the fourth case of mcr-1 gene being detected in the United States – others include Pennsylvania, New Jersey and New York. Worldwide, the resistant gene has also been found in South America, Europe, Africa, and Asia. Researchers said it was first identified in China.


  1. Vasquez AM, Montero N, Laughlin M, et al. Investigation of Escherichia coli Harboring the mcr-1 Resistance Gene - Connecticut, 2016MMWR Morb Mortal Wkly Rep. 2016;65(36):979-980. doi: 10.15585/mmwr.mm6536e3.
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