Performance of Rapid Polymyxin™ NP and Rapid Polymyxin™ Acinetobacter for the detection of polymyxin resistance in carbapenem-resistant Acinetobacter baumannii and Enterobacterales

Challenges in the Detection of Polymyxin Resistance: From Today to the Future

Antimicrobial resistance is known to be one of the greatest global threats to human health, and is one of the main causes of death worldwide. In this scenario, polymyxins are last-resort antibiotics to treat infections caused by multidrug-resistant bacteria. Currently, the reference test to evaluate the susceptibility of isolates to polymyxins is the broth microdilution method; however, this technique has numerous complications and challenges for use in laboratory routines. Several phenotypic methods have been reported as being promising for implementation in routine diagnostics, including the BMD commercial test, rapid polymyxin NP test, polymyxin elution test, culture medium with polymyxins, and the Polymyxin Drop Test, which require materials for use in routines and must be easy to perform. Furthermore, Sensititre®, molecular tests, MALDI-TOF MS, and Raman spectroscopy present reliable results, but the equipment is not found in most microbiology laboratories. In this context, this review discusses the main laboratory methodologies that allow the detection of resistance to polymyxins, elucidating the challenges and perspectives.

Comparison of Six Phenotypic Assays with Reference Methods for Assessing Colistin Resistance in Clinical Isolates of Carbapenemase-Producing Enterobacterales: Challenges and Opportunities

The global escalation of severe infections due to carbapenemase-producing Enterobacterales (CPE) isolates has prompted increased usage of parenteral colistin. Considering the reported difficulties in assessing their susceptibility to colistin, the purpose of the study was to perform a comparative evaluation of six phenotypic assays—the colistin broth disc elution (CBDE), Vitek 2 Compact (bioMérieux SA, Marcy l’Etoile, France), the Micronaut MIC-Strip Colistin (Merlin Diagnostika GMBH, Bornheim-Hensel, Germany), the gradient diffusion strip Etest (bioMérieux SA, Marcy l’Etoile, France), ChromID Colistin R Agar (COLR) (bioMérieux SA, Marcy l’Etoile, France), and the Rapid Polymyxin NP Test (ELITechGroup, Signes, France)—versus the reference method of broth microdilution (BMD). All false resistance results were further assessed using population analysis profiling (PAP). Ninety-two nonrepetitive clinical CPE strains collected from two hospitals were evaluated. The BMD confirmed 36 (39.13%) isolates susceptible to colistin. According to the BMD, the Micronaut MIC-Strip Colistin, the CBDE, and the COLR medium exhibited category agreement (CA) of 100%. In comparison with the BMD, the highest very major discrepancy (VMD) was noted for Etest (n = 15), and the only false resistance results were recorded for the Rapid Polymyxin NP Test (n = 3). Only the PAP method and the Rapid Polymyxin NP Test were able to detect heteroresistant isolates (n = 2). Thus, there is an urgent need to further optimize the diagnosis strategies for colistin resistance.

Analysis of colistin resistance in carbapenem-resistant Enterobacterales and XDR Klebsiella pneumoniae

Introduction:

Increasing occurrence of infections caused by multidrug-resistant Gram-negative bacteria resulted in colistin being the last agent for treatment. Apart from plasmid-mediated mcr genes, mutations involving several genes like mgrB, phoP/phoQ, pmrA, pmrB, pmrC, and crrABC genes, are leading causes of colistin resistance. Four colistin susceptibility testing methods were compared against broth microdilution (BMD) and determined the presence of the mcr1-5 gene.

Methodology:

A total of 100 carbapenem-resistant Enterobacterales isolates were tested for colistin susceptibility by commercial broth microdilution (cBMD), E-test, VITEK-2, and rapid polymyxin NP assay (RPNP) and compared with BMD. The presence of the mcr1-5 gene was determined by modified RPNP and PCR. Two non-mcr colistin-resistant XDR isolates were subjected to whole-genome sequencing using Illumina MiSeq sequencing platform.

Results:

Among 100 carbapenem-resistant Enterobacterales isolates, 15% were resistant to colistin. Essential agreement, categorical agreement, major error, and very major error for cBMD/E-test/VITEK-2/RPNP were 96%/73%/82%/NA; 99%/86%/88%/91%, 1.2%/9.4%/11.8%/8.2% and 0%/40%/13.3%/13.3%, respectively. Only one Klebsiella pneumoniae isolate harbored the mcr-1 gene, observed by both methods. Whole-genome sequencing of two non-mcr XDR Klebsiella pneumoniae showed multiple mutations in 10 genes responsible for lipopolysaccharide biosynthesis.

Conclusions:

The performance of cBMD was excellent, whereas the E-test was unacceptable. VITEK-2 and RPNP performed better but remained unreliable due to high error rates. Multiple mutations in the target proteins involving lipopolysaccharide formation, modification, and regulation were seen, resulting in colistin resistance.

Colistin Susceptibility Testing by Rapid Colistin Disk Elution Test Among Enterobacteriaceae in Low-Resource Setting

We modified rapid polymyxin Nordmann-Poirel (RPNP) test, called rapid colistin disk elution (RCDE) test, for detecting colistin resistance in Gram-negative bacilli and evaluated its performance compared with colistin broth disk elution (CBDE) test recommended by Clinical and Laboratory Standards Institute (CLSI). The RCDE test was performed by using a 10-μg colistin disk in 2.7 mL volume (final colistin concentration of 3.7 μg/mL) of either cation-adjusted Mueller-Hinton broth or phenol red broth base media with bacterial inoculum of 1-μL loop, and 1-4 and 16-20 hr incubation for Enterobacteriaceae and Acinetobacter baumannii isolates, respectively. Both tests were evaluated in 236 Enterobacteriaceae and 49 A. baumannii isolates using broth microdilution as reference method. Among the Enterobacteriaceae isolates, categorical agreement and very major error (VME or false intermediate susceptibility) rate were 98.3% and 5.4%, respectively, for the RCDE test, compared with 97.9% and 7.1%, respectively, for the CBDE test. Both tests had major error (ME or false resistance) rate of 0.6%. For the A. baumannii isolates, the RCDE and CBDE tests gave high VME rates of 8.3% and 16.7%, respectively. The RCDE test showed good performance comparable with the CBDE test but is cheaper and more rapid (3 hr) and convenient, thus suggesting as an alternative for detecting colistin resistance among Enterobacteriaceae in low-income countries.

RapidResa Polymyxin Acinetobacter NP® Test for Rapid Detection of Polymyxin Resistance in Acinetobacter baumannii

A homemade and culture-based test, relying on the visual detection of the reduction of the resazurin reagent (a cell viability indicator), has been developed for the rapid detection of polymyxin resistance in Acinetobacter baumannii. Here, we evaluated the industrial version of this test, the RapidResa Polymyxin Acinetobacter NP^® test (Liofilchem, Italy). A well-characterized panel of 68 clinical A. baumannii strains (36 polymyxin-susceptible, 26 polymyxin-resistant A. baumannii, and 6 colistin-heteroresistant isolates) of worldwide origin was tested. All the colistin-susceptible A. baumannii isolates gave negative results according to the RapidResa Polymyxin Acinetobacter NP^® test, except for a single isolate that gave a false-positive result. Out of the 26 colistin-resistant A. baumannii strains, 25 were correctly identified as colistin resistant using the RapidResa Polymyxin Acinetobacter NP^® test. Only a single colistin-resistant A. baumannii strain gave a false-negative result. Additionally, the six colistin-heteroresistant isolates tested gave positive results. Altogether, the sensitivity and the specificity of the test were found to be 96% and 97%, respectively. The turn-around-time of this easy-to-perform test was 3-4h, which showed excellent reliability for identification of polymyxin resistance in A. baumannii. The RapidResa Polymyxin Acinetobacter NP^® test allows a rapid differentiation between polymyxin-susceptible and -resistant A. baumannii isolates, which may contribute to optimization of the use of polymyxins for treating infections due to multidrug-resistant A. baumannii.

Evaluation of the MICRONAUT MIC-strip colistin assay for colistin susceptibility testing of carbapenem-resistant Acinetobacter baumannii and Enterobacterales

We sought to evaluate the MICRONAUT MIC-Strip colistin (MMS) assay, a commercial broth microdilution (BMD) panel, using 273 carbapenem-resistant Acinetobacter baumannii and Enterobacterales isolates. BMD was used as gold standard. MMS had 98.5% sensitivity and 99.5% specificity. All 37 isolates with MICs close to the breakpoint exhibited complete agreement.