Antibiotic Resistance Gene mcr-1 Not Easily Spread

Lara C. Pullen, PhD

September 14, 2016

The plasmid-encoded mcr-1 gene confers bacterial resistance to the antibiotic colistin, which is used as a medication of last resort for multidrug-resistant infections. Two new reports published in the Morbidity and Mortality Weekly Report provide reassuring evidence that organisms containing mcr-1 are not easily transmitted to healthy contacts.

The mcr-1 gene was first reported in China in 2015, as described by Medscape Medical News. In May 2016, a patient in Pennsylvania was identified as the first patient in the United States to be colonized with mcr-1-positive Escherichia coli, as reported by Medscape Medical News. She had no known epidemiologic link to individuals or places with identified mcr-1.

Patient 1 Did Not Spread mcr-1

Kelly E. Kline, MPH, from the Pennsylvania Department of Health, Harrisburg, and colleagues collaborated with the Centers for Disease Control and Prevention to further evaluate the female patient who initially presented with a urinary tract infection. An isolate from the urine culture was positive for mcr-1.

The researchers evaluated transmission of mcr-1 by investigating the presence of mcr-1-positive bacteria in the home and community of the index patient. The study included analysis of patients who resided in the same unit where the index patient received care.

Altogether, the investigators screened 105 individuals for the presence of bacteria with the mcr-1 gene, but were unable to detect any such microorganisms.

The team continued to perform monthly screening of the index patient via perirectal swabs. Although the swabs collected in May and June were positive for bacteria with mcr-1, the swab collected on August 1 tested negative. The patient was not treated with antibiotics during this period.

Patient 4 Did Not Spread mcr-1

In July 2016, mcr-1-positive E coli was isolated from a pediatric patient in Connecticut. The patient presented with bloody diarrhea, but was otherwise healthy, with no prior surgeries or hospitalizations. "The patient traveled to the Caribbean for approximately 2 weeks to visit friends and relatives and developed fever and bloody diarrhea on June 12, 2 days before returning to the United States," the authors write.

The identification of mcr-1 was performed by the Pathogen Detection System at the National Center for Biotechnology Information.

Amber M. Vasquez, MD, from the Epidemic Intelligence Service, Centers for Disease Control and Prevention, Atlanta, Georgia, and colleagues report that the E coli O157 harboring mcr-1 was isolated from three separate stool samples: June 16, June 18, and June 23. The isolates were resistant to polymyxin and third-generation cephalosporins. They were susceptible to carbapenems.

Stool cultures from June 24 and July 1 tested negative for E coli O157.

The investigators screened six household contacts who were in close contact with the pediatric patient. They found no transmission beyond the index patient.

Previous mcr-1-Positive Isolates

These two cases represent the first and fourth cases of mcr-1-positive bacteria in the United States.

In July, public health officials identified a second case in New York. The third case was a patient in New Jersey and was reported in August.

The second case was identified as part of the worldwide SENTRY Antimicrobial Surveillance Program. The SENTRY program found that 1.9% of E coli isolated globally were resistant to colistin, of which only a few tested positive for mcr-1. The 19 resistant isolates came from Belgium, Brazil, Germany, Hong Kong, Italy, Malaysia, Poland, Russia, and Spain.

The analysis revealed that all of the colistin-resistant organisms identified by the SENTRY program, including the isolate from the patient in New York, were susceptible to other commonly used antibiotics.

Public Health Response

Public health officials remain concerned that bacteria with the mcr-1 gene could transfer that gene to bacteria that are already resistant to available antibiotics.

Healthcare workers should immediately report colistin-resistant bacteria to their local health department. Such prompt reporting should allow public health officials to rapidly respond to mcr-1-positive isolates by identifying transmission and limiting further spread.

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. Published online September 9, 2016. Kline full text, Vasquez full text

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