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Evaluation of the Rapid Polymyxin NP Test for Polymyxin B Resistance Detection Using Enterobacter cloacae and
Shelby Simar1, Diane Sibley1, Deborah Ashcraft1
1Ochsner Clinic Foundation, New Orleans, Louisiana, USA.
Abstract:
Polymyxin resistance is an increasing problem worldwide. Currently, determining susceptibility to polymyxins is problematic and lengthy. Polymyxins diffuse poorly into agar, potentially giving inaccurate disk diffusion and Etest results. A rapid screening test (2 h) for the detection of polymyxin resistance in Enterobacteriaceae, developed by P. Nordmann and L. Poirel (rapid polymyxin NP test) in 2016, detects glucose metabolization in the presence of polymyxin E (PE) and PB via pH-induced color change. The sensitivity and specificity were 99.3 and 95.4%, respectively, with results obtained in ≤2 h. Our goal was to evaluate this test using PB against larger numbers of Enterobacter A total of 143 nonduplicate Enterobacter isolates (102 E. cloacae complex, 41 E. aerogenes) were tested, including 136 collected from Ochsner Health System patients from March to May 2016 and 7 previously determined PB-resistant E. cloacae isolates from JMI Laboratories. MICs were determined via broth microdilution. For the rapid polymyxin NP test, a color change from orange to yellow is positive; a weak/no color change is deemed negative after 4 h. Of 143 Enterobacter isolates, 25 were determined to be PB resistant by broth microdilution (MIC > 2 μg/ml), including all 7 JMI isolates. Of these 25, 7 were positive by the rapid polymyxin NP test (included 3/7 JMI isolates). All 118 isolates determined to be PB susceptible by broth microdilution were NP test negative. The sensitivity and specificity for the rapid polymyxin NP test were 25 and 100%, respectively, compared to broth microdilution. Although the rapid polymyxin NP test is a much faster method (2 to 4 h) for polymyxin resistance determination compared to broth microdilution (16 to 20 h), our study indicates that it may be subject to limitations when testing Enterobacter.
Insights
The rapid polymyxin NP test shows low sensitivity (25%) for detecting polymyxin B resistance in Enterobacter isolates, despite its speed. Further evaluation is needed for this specific bacterial group.
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
- Infectious Diseases
Background:
- Rising global polymyxin resistance necessitates rapid susceptibility testing methods.
- Traditional methods like disk diffusion and Etest are slow and can be inaccurate due to poor agar diffusion of polymyxins.
- The rapid polymyxin NP test offers a faster alternative for detecting polymyxin resistance in Enterobacteriaceae.
Purpose of the Study:
- To evaluate the performance of the rapid polymyxin NP test for detecting polymyxin B (PB) resistance in a large collection of Enterobacter isolates.
- To compare the results of the rapid polymyxin NP test with broth microdilution, the gold standard for determining polymyxin B minimum inhibitory concentrations (MICs).
Main Methods:
- 143 nonduplicate Enterobacter isolates (E. cloacae complex and E. aerogenes) were tested.
- Minimum inhibitory concentrations (MICs) for polymyxin B were determined using broth microdilution.
- The rapid polymyxin NP test was performed according to manufacturer instructions, with results interpreted after 4 hours based on color change.
Main Results:
- Broth microdilution identified 25 out of 143 isolates as polymyxin B resistant (MIC > 2 μg/ml).
- The rapid polymyxin NP test correctly identified only 7 of these 25 resistant isolates, yielding a sensitivity of 25%.
- All 118 polymyxin B susceptible isolates were correctly identified as negative by the NP test, resulting in 100% specificity.
Conclusions:
- The rapid polymyxin NP test demonstrated low sensitivity (25%) but high specificity (100%) for detecting polymyxin B resistance in the tested Enterobacter isolates.
- Despite being significantly faster than broth microdilution, the test's low sensitivity suggests limitations for reliable polymyxin B resistance screening in this bacterial genus.
- Further studies are warranted to understand and potentially improve the performance of rapid polymyxin NP testing in Enterobacter species.
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