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Comparison Between Etest and Broth Microdilution Methods for Testing Itraconazole-Resistant Aspergillus fumigatus Susceptibility to Antifungal Combinations

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Abstract

The checkerboard broth microdilution assay (BMD) is the most frequently used method for the in vitro evaluation of drug combinations. However, its use to evaluate the effect of antifungal drugs on filamentous fungi is sometimes associated with endpoint-reading difficulties, and different degrees of interaction are assigned to the same drug combination. We evaluated combinations of the azoles, itraconazole, posaconazole, and voriconazole, with the echinocandins, anidulafungin, caspofungin, and micafungin, against 15 itraconazole-resistant Aspergillus fumigatus clinical strains via the checkerboard BMD and Etest assay. Readings after 24 and 48 h, considering the two reading endpoints, the minimum inhibitory concentration (MIC) and minimum effective concentration (MEC), were performed for both methods. Our results showed that the correlation coefficients between the BMD and Etest methods were quite diverse to the drug combinations tested. The highest correlation coefficients of the Etest with the BMD assays (MEC and MIC reading) were the Etest-MIC reading at 24 h and the Etest-MEC reading at 48 h. Improvements in experimental conditions may increase the correlation between the two methods and ensure that Etest assay can be safely used in the evaluation of antifungal combinations against Aspergillus species.

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Acknowledgements

This study was supported by the Higher Education Personnel Training Coordination (CAPES), Brazil. Alves SH thanks the financial support provided by the Brazilian Agencies FAPERGS (Grant Proc. 2378-2251/14-8) and CNPq (Grant Proc. 470229/2012-8).

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Authors and Affiliations

  1. Postgraduate Program in Pharmaceutical Sciences, Health Sciences Center, Federal University of Santa Maria, Santa Maria, RS, 97105-900, Brazil

    Laura Bedin Denardi, Jéssica Tairine Keller, Fernanda Baldissera Piasentin & Sydney Hartz Alves

  2. Postgraduate Program in Pharmacology, Health Sciences Center, Federal University of Santa Maria (UFSM), Santa Maria, RS, Brazil

    Maria Isabel de Azevedo & Janio Morais Santurio

  3. Mycological Research Laboratory, Federal University of Santa Maria, UFSM, Santa Maria, RS, Brazil

    Laura Bedin Denardi, Jéssica Tairine Keller, Maria Isabel de Azevedo, Vanessa Oliveira, Fernanda Baldissera Piasentin, Janio Morais Santurio & Sydney Hartz Alves

  4. University of Health Sciences of Porto Alegre (UCSPA), Porto Alegre, RS, Brazil

    Cecília Bittencourt Severo

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  1. Laura Bedin Denardi
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Correspondence to Laura Bedin Denardi.

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Denardi, L.B., Keller, J.T., de Azevedo, M.I. et al. Comparison Between Etest and Broth Microdilution Methods for Testing Itraconazole-Resistant Aspergillus fumigatus Susceptibility to Antifungal Combinations. Mycopathologia 183, 359–370 (2018). https://doi.org/10.1007/s11046-017-0208-7

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