Rapid Detection Method Developed for Polymyxin-Resistant Enterobacteria

The standard reference technique for determining susceptibility to polymyxins is broth microdilution, which requires fastidious attention and at least 24 hours to perform.

Microbiologists at the University of Fribourg (Switzerland) developed the rapid polymyxin Nordmann/Poirel (NP) 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. Acid formation associated with carbohydrate metabolism in Enterobacteriaceae can be observed through the color change of a pH indicator. This test is rapid taking less than two hours and easy to perform.

To evaluate the performance of the rapid polymyxin NP test, the team used 200 isolates collected from clinical samples worldwide. This collection included 135 Enterobacteriaceae isolates resistant to polymyxin: five isolates of intrinsically polymyxin-resistant species and 130 isolates of various enterobacterial species with acquired resistance to polymyxins. The rapid polymyxin NP test uses two reagents and solutions: stock solutions of polymyxins and rapid polymyxin NP solution.

The sensitivity and specificity of the rapid NP test were 99.3% and 95.4%, respectively, compared with the standard broth microdilution method. By reading the color change of the wells every hour, the scientists determined that final results were obtained two hours after incubation when the tray was incubated at 35 ± 2 °C under an ambient atmosphere. However, positive results with frank color change were obtained as early as one hour after incubation for Klebsiella spp. and Escherichia coli isolates. Half of the Enterobacter spp. isolates gave positive results within 60 minutes of incubation and the other half within 120 minutes.

The authors concluded that the new test is inexpensive, easy to perform, sensitive, specific, and can be completed in less than two hours. It could be useful in countries facing endemic spread of carbapenemase producers and for which polymyxins are last-resort drugs. The study was published in the June 2016 issue of the journal Emerging Infectious Diseases.

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