Colistin-Resistant Gene Noted for Second Time

May be first case in the United States.

For the second time, a clinical isolate of a bacterial pathogen has been detected in humans in the United States which carries the colistin resistance gene, mcr-1.  That would be concerning because plasmids often jump between different bacterial species, spreading any resistance genes they carry, according to research published this week  in Antimicrobial Agents and Chemotherapy, a journal of the American Society for Microbiology.

In the current study, the researchers, who coordinate the worldwide SENTRY Antimicrobial Surveillance Program, first tested 13,526 Escherichia coli and 7,480 Klebsiella pneumoniae clinical strains that had been collected systematically from hospitals in the Asia-Pacific region, Latin America, Europe, and North America in 2015. They found that 390—1.9%—were resistant to colistin, and that 19 of these isolates tested positive for mcr-1.

The 19 isolates originated in ten countries representing all of the above regions. However, only one, an isolate of E.coli that was originally recovered in 2015 from a patient in New York, came from the US. (Previously, a single isolate containing mcr-1-positive E. coli, was identified from a patient in Pennsylvania.)

The isolates that tested positive for mcr-1 were susceptible to several commonly used antimicrobial agents, including carbapenems, and recently approved antimicrobial agents that can be used against gram negative bacteria, generally, corresponding author, Mariana Castanheira, PhD, Director for Molecular and Microbiology, at JMI Laboratories, North Liberty, IA said in a prepared statement.  These and similar isolates are unlikely to cause hard to treat infections at this time.

A key question the authors are currently working to answer is whether the mcr-1 gene is plasmid-mediated in the isolates they have identified. Plasmid-mediated mcr-1 was first isolated from food animals and humans in China, in late 2015. Given this possibility, and the potential of plasmids to jump to other bacteria, especially to those that are already resistant to other antibiotics, as well as the gene’s global distribution, close monitoring of the gene is warranted, according to the authors.

“The fact that the gene has been detected in food livestock and raw meat is also concerning,” Dr Castanheira said. “The prospect of a mobile gene encoding resistance to colistin spreading among isolates resistant to most antimicrobial agents clinically available is threatening for the therapy of serious infection caused by isolates,” the investigators conclude.  Additional studies are ongoing.


1. Castanheira M, Griffin MA, Deshpande LM, et al. Detection of mcr- 1 among Escherichia coli clinical isolates collected worldwide as part of the SENTRY 2 Antimicrobial Surveillance Program during 2014 -2015. doi:10.1128/AAC.01267-16.