BACKGROUND: In 2015, scientists reported the emergence of the plasmid-encoded mcr-1 gene, which confers resistance to the antibiotic colistin. Its discovery signaled the potential emergence of pan-resistant bacteria. In May 2016, mcr-1 positive Escherichia coli was first isolated from a specimen from a U.S. patient when a Pennsylvania woman was evaluated for a urinary tract infection. In collaboration with the Centers for Disease Control and Prevention, the Pennsylvania Department of Health launched an investigation.
METHODS: Highest-risk contacts in household and healthcare settings were identified for screening. Twenty persons at potentially higher risk included the patient’s medical facility roommate, household contacts, home health personnel, friends who assisted with activities of daily living, and a patient who developed an E. coli infection after receiving direct care from a caregiver who also assisted the index patient. All 20 contacts completed screening. The index patient was also subsequently screened monthly to monitor colonization status. Four medical facilities that had admitted the patient in 2016 were also assessed for transmission risk. Two higher-risk facilities were identified, and 78 of 98 identified healthcare workers completed screening. One facility also conducted a point prevalence study to determine if transmission was occurring among patients; seven of 18 patients completed screening. Prospective laboratory surveillance was conducted for 30 days at all four medical facilities. In total, 51 extended-spectrum β-lactamase (ESBL)-producing isolates underwent colistin susceptibility testing.
RESULTS: The index patient had no international travel in nearly a year, no exposure to livestock, and a limited role in meal preparation with store-bought groceries. Among the 105 persons screened and 51 ESBL-producing isolates tested, no bacteria with the mcr-1 gene were detected. The index patient’s monthly screenings on May 31 and June 26 were positive for the mcr-1 gene and negative on August 1 and 17.
CONCLUSIONS: No transmission was detected among the patient’s highest-risk contacts, suggesting the risk for transmission to otherwise healthy individuals might be relatively small, even if these individuals had substantial exposure to a colonized patient. Therefore, focused screening of contacts at highest risk of transmission is recommended. It is not known how the patient became colonized, especially in the absence of an epidemiologic link to known persons or places with identified mcr-1. Nonetheless, as more surveillance systems with broader testing are established, it is anticipated that mcr-1 will be identified with increasing frequency. This highlights the importance of comprehensive antimicrobial stewardship, especially given potential transmission of the mcr-1 gene among bacteria.