Abstract and Introduction
Abstract
For identification of polymyxin resistance in Enterobacteriaceae, we developed a rapid test that detects glucose metabolization associated with bacterial growth in the presence of a defined concentration of colistin or polymyxin B. Formation of acid metabolites is evidenced by a color change (orange to yellow) of a pH indicator (red phenol). To evaluate the test, we used bacterial colonies of 135 isolates expressing various mechanisms of colistin resistance (intrinsic, chromosomally encoded, and plasmid-mediated MCR-1) and 65 colistin-susceptible isolates. Sensitivity and specificity were 99.3% and 95.4%, respectively, compared with the standard broth microdilution method. This new test is inexpensive, easy to perform, sensitive, specific, and can be completed in <2 hours. It could be useful in countries facing endemic spread of carbapenemase producers and for which polymyxins are last-resort drugs.
Introduction
Among the most clinically significant multidrug-resistant bacteria are carbapenemase-producing Enterobacteriaceae. Because these bacteria usually remain susceptible to polymyxins, an old class of antimicrobial drugs almost abandoned in the 1970s because of their potential toxicity, interest in polymyxins (colistin and polymyxin B) has been renewed worldwide.[1,2] However, the increasing use of colistin explains why acquired colistin resistance may now be added to the carbapenem resistance trait in Enterobacteriaceae.[3]
The standard reference technique for determining susceptibility to polymyxins is broth microdilution, which requires fastidious attention and a long time (24 h) to perform.[4] Other techniques for determining susceptibility to polymyxins (disk diffusion and Etest) have been proposed and also provide results in 18–24 h. Because of poor diffusion of polymyxin molecules in agar, rates of false susceptibility are high (up to 32%).[4,5]
Acquired resistance to colistin in Enterobacteriaceae results mostly from modification of lipopolysaccharide.[6] Addition of phosphoethanolamine, 4-amino-l-arabinose cationic groups, or both to lipopolysaccharide decreases polymyxin binding to the bacterial outer membrane. Addition of these groups may be associated with chromosome-encoded mechanisms (mutations in PmrAB or PhoPQ two-component systems or alterations of the mgrB gene).[6] A recent report revealed that addition of phosphoethanolamine may also be plasmid mediated through the mcr-1 gene, which confers the first known plasmid-mediated resistance to colistin in isolates from humans and animals.[7] More recently, the mcr-1 gene was identified in several plasmid backbones, mostly in Escherichia coli.[8–10] There is therefore a need for a test that enables rapid detection of polymyxin resistance in Enterobacteriaceae and that may contribute to its containment.
We developed a test (the rapid polymyxin NP [Nordmann/Poirel] test) that detects bacterial growth in the presence of a defined concentration of a polymyxin. Bacterial growth detection (or absence) is based on carbohydrate metabolism.[11] Acid formation associated with carbohydrate metabolism in Enterobacteriaceae can be observed through the color change of a pH indicator. This test is rapid (<2 h) and easy to perform.
Emerging Infectious Diseases. 2016;22(6):1038-1043. © 2016 Centers for Disease Control and Prevention (CDC)
