Two-Site Evaluation of the Colistin Broth Disk Elution Test To Determine Colistin In Vitro Activity against Gram-Negative Bacilli

Abstract

Limited methods for colistin MIC determination are available to clinical microbiology laboratories. The purpose of this study was to evaluate the accuracy of the colistin broth disk elution (CBDE) test compared to that of broth microdilution (BMD) for identifying colistin MICs. CBDE was compared to colistin BMD using a collection of Gram-negative bacilli tested at two U.S. microbiology laboratories. The isolates tested included 121 retrospective clinical isolates, 45 prospective clinical isolates, and 6 mcr-1-positive Escherichia coli isolates. CBDE was performed with four 10-ml cation-adjusted Mueller-Hinton broth tubes per isolate, to which 0, 1, 2, and 4 colistin 10-µg disks were added, generating final concentrations in the tubes of 0 (growth control), 1, 2, and 4 µg/ml, respectively. MICs were evaluated visually and interpreted using Clinical and Laboratory Standards Institute breakpoints. Site 2 also compared CBDE to the reference broth macrodilution (BMAD) method (n = 110 isolates). Overall, CBDE yielded a categorical agreement (CA) and essential agreement (EA) of 98% and 99%, respectively, compared to the results of colistin BMD. Very major errors occurred for mcr-1-producing strains, with MICs fluctuating from 2 to 4 µg/ml on repeat testing. The results for all other isolates were in CA with those of BMD. CBDE versus BMAD had an EA of 100% and a CA of 100%. Compared to currently used techniques, CBDE is an easy and practical method to perform colistin MIC testing. Some mcr-1-producing isolates yielded MICs of 2 µg/ml by CBDE and 4 µg/ml by BMD. As such, the results for isolates with colistin MICs of 2 µg/ml by CBDE should be confirmed by the reference BMD method, and isolates with MICs of ≥2 µg/ml should be evaluated for the presence of mcr genes.

Evaluation of the MicroScan Colistin Well and Gradient Diffusion Strips for Colistin Susceptibility Testing in Enterobacteriaceae

“…A study by Richter et al examined 106 carbapenemresistant K. pneumoniae isolates (28 with an MIC of Ն4 g/ml) and compared Sensititre GNX2F plates to broth macrodilution and found an essential agreement of 94.3% and a categorical agreement of 96.2% (41). Finally, the recently developed colistin broth disk elution test may be another option (42). Nevertheless, colistin testing is difficult for most labs to implement.…”

Evaluación de la elución de sensidiscos para la determinación de susceptibilidad a colistín en bacilos gramnegativos multi-resistentes

“…La elución de sensidiscos resultó ser confiable para la determinación de susceptibilidad a colistín en bacilos gramnegativos de la familia Enterobacteriaceae y Pseudomonas, ya que mostró una elevada sensibilidad y especificidad tal como se ha reportado previamente para bacilos gramnegativos multi-resistentes 4,10 . Sin embargo, éste no fue concluyente para A. baumannii, lo que se relaciona con su falsa resistencia frente a colistín 8 y con los errores muy mayores observados en nuestra investigación, por lo que se requiere hacer un estudio con un mayor número de cepas de este tipo.…”

“…Para la elución de sensidiscos, nos basamos en la metodología descrita por el Laboratorio de Medicina de la UCLA, California, E.U.A. 4 .…”

“…Esta comunicación tiene como objetivo informar acerca del uso del método de elución de sensidiscos 4 , como alternativa para la determinación de susceptibilidad a colistín en bacilos gramnegativos multi-resistentes de relevancia clínica.…”

A Novel Phenotypic Method To Screen for Plasmid-Mediated Colistin Resistance among Enterobacteriales

“…Colistin MICs were determined by broth microdilution using the Sensititre GNX2F panel (Thermo Fisher) and by the CBDE method, which performs comparably to reference broth dilution methods (see Table S1) (6). CBDE was performed by setting up four 10-ml cation-adjusted Mueller-Hinton broth (CA-MHB) tubes (Remel, Lenexa, KS) per isolate with 0, 1, 2, and 4 colistin disks (10 g) (BD, Sparks, MD) added to the tubes, generating a final concentration of 0 (growth control), 1, 2, and 4 g/ml (Fig.…”