Effect of hypoxic conditions on in vitro susceptibility testing of amphotericin B, itraconazole and micafungin against Aspergillus and Candida

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Abstract
Objectives: The aim of this study was to investigate the effect of hypoxic conditions on in vitro susceptibility testing of amphotericin B, itraconazole and micafungin against Aspergillus (four species) and Candida (six species). Methods: In vitro susceptibility tests were set up according to NCCLS M27-A2 and M38-A recommendations, but incubation atmospheres were either air plus 5% CO2, 1% oxygen/5% CO2/94% nitrogen or 0.25% oxygen/5% CO2/94.75% nitrogen. Results: In all Aspergillus species, the MIC of amphotericin B was reduced but the MFC remained unaltered with reduced oxygen. The MICs and MFCs of itraconazole and micafungin were unaltered in hypoxic conditions but interpretation of the MIC was much simpler for micafungin with 1% and 0.25% oxygen. Against Candida, conditions modelling hypoxia had little effect on the MICs and MFCs of any of the agents. Conclusions: This simple adaptation of susceptibility testing may have important consequences for understanding how antifungal drugs work and for endpoint reading.