Rapid Polymyxin NP

Among the most clinically significant multidrug-resistant bacteria are carbapenemase-producing Enterobacteriaceae. Because these bacteria usually remain susceptible to polymyxins, interest in this old class of antibiotics has been recently renewed. However, the increasing use of colistin explains why Enterobacterial strains resistant to colistin are increasingly reported worldwide.

Currently available polymyxins susceptibility methods are fastidious, time-consuming (24 hours) and some methods are not reliable. They are poorly adapted to the clinical need and to the prevention of the dissemination of those multidrug resistant isolates.

ELITechGroup Microbiology releases its new Rapid Polymyxin NP test proposed as a rapid, reliable and cost-effective test to detect polymyxins (polymyxin E or Colistin, polymyxin B) resistant Enterobacteriaceae.

This test is based on the detection of the glucose metabolization related to bacterial growth in presence of a defined concentration of colistin. Formation of acid metabolites consecutive to the glucose metabolization is evidenced by a color change (orange to yellow) of a pH indicator.

The Rapid Polymyxin NP test is easy-to-perform, very sensitive and specific. It can detect in only a couple of hours polymyxin-resistant and –susceptible isolates from any enterobacterial species, regardless of the molecular mechanism of polymyxins resistance.

This test offers the possibility of detecting polymyxins resistance from bacterial cultures before any antimicrobial drug susceptibility testing results are obtained. Results are obtained at least 16 hours sooner with this test than with the reference broth microdilution method. It is as reliable as the reference dilution technique but much less cumbersome and is not based on diffusion of large polymyxin molecules in agar, which therefore prevents false susceptibility results.

• Reagents and material:

• Stability and storage:

Reagents are ready to use.

The kit and its content when stored at +2+8°C in their original state are stable until the expiry date indicated on the box.
RP Medium and RP NaCl vials are single-use reagents.
The RP TC vial is used as a control for inocula calibration and has to be kept until the last RP Medium vial of the kit has been inoculated.
The RP TC reagent has to be stored protected from light and at +2+8°C.

• Performance

The evaluation of the performance of the Rapid Polymyxin NP test was conducted in the Emerging Antibiotic Resistance Unit (INSERM, Faculty of Sciences, University of Fribourg, Switzerland), compared with the method for determining the Minimum inhibitory concentration (MIC) in liquid medium (microdilution Mueller-Hinton cation adjusted broth – according to the guidelines of the Clinical Laboratory Standard Institute (3,4). Described as the reference method).

The bacteria used in this study come from different, international, clinical samples and include the following species:

219 Enterobacteriacae strains were selected as the most representative species of Enterobacteriacae. These include 78 sensitive strains and 141 strains resistant to colistin. Of the resistant strains, diff erent molecular mechanisms of resistance to polymyxins have been observed (chromosomal, plasmidic, intrinsic or unknown).
The MICs of the strains and their resistance mechanisms are as follows:

The bacterial strains were reisolated in agar (Luria Bertani, Mueller Hinton, Columbia 5 % blood) for 18 – 24h in order to test the Rapid Polymyxin NP in parallel with the determination of the MIC via the CLSI liquid micro-dilution technique.
The Rapid Polymyxin NP test was read after 2h and 3h of incubation.
The percentage of clinical concordance of the Rapid Polymyxin NPtest is 97.7% compared to the MIC method in liquid medium. The sensitivity of the test is 99.3% and the specifi city is 94.9%.
There are 4 Major Discordances (MIC between 1 and 2 μg/mL) and a Very Major Discordance (VMD) (Klebsiella pneumoniae strain with an MIC of 8 μg/mL of which the resistance mechanism is unknown).
The percentage of clinical concordance, to within 1 dilution, is 99.1 %. 1 MD and 1 VMD remain.
As far as incubation time is concerned, all of the strains gave a readable result within 2h. Furthermore, the profi le of sensitive strains is stable even after 3h of incubation.

• Material required but not provided

Waste container for contaminated waste
Optional: Suspension turbidity detector (densitometer)
Pipettes and tips
Certified incubator at +36°C+/-2°C