Investigation of Salmonella Enteritidis Harboring the Mcr-1 Resistance Gene — Connecticut, 2017

BACKGROUND:  In December 2016, routine surveillance identified the first mcr-1 gene in a Salmonella Enteritidis (SE) isolate in the United States. The isolate was from a stool specimen collected in May 2016 from a Connecticut patient experiencing diarrhea. Mcr-1 is located on a plasmid potentially capable of spreading among bacteria and confers resistance to colistin, a last-line antibiotic used to treat multidrug-resistant infections. We assessed possible routes of exposure to mcr-1, determined whether mcr-1 spread to the patient’s close contacts, and evaluated prevalence of mcr-1 among similar SE isolates.

METHODS:  We interviewed the patient and 4 close contacts about symptoms, travel, and exposure to foods, animals, and health care settings. Rectal swabs collected in January 2017 from the patient and contacts were cultured for Salmonella and tested for mcr-1 by polymerase chain reaction. Genomes of 170 U.S. SE isolates collected during October 2015–December 2016 with the same pulse-field gel electrophoresis (PFGE) pattern were screened for mcr-1.

RESULTS:  The patient had travelled abroad during the salmonellosis incubation period, and had no substantial exposure to health care settings before her illness. Rectal swabs from the patient, 2 travel companions who had diarrhea during travel, and 2 asymptomatic household contacts tested negative for Salmonella and mcr-1. Mcr-1 was not identified in the sequences of any of the 170 PFGE-matching SE isolates.

CONCLUSIONS:  The patient might have acquired mcr-1 abroad, adding to evidence that acquisition of this gene is associated with international travel. Continued screening of bacterial genomes for mcr-1 and investigations using these methods might delay the emergence of untreatable infections by identifying individuals at risk of spreading the gene and allowing for implementation of containment measures.