Comparison between Aspergillus fumigatus conidia and hyphae susceptibilities to amphotericin B, itraconazole, and voriconazole by use of the mold rapid susceptibility assay.

Although hyphae are the morphological form observed in tissue during invasive Aspergillus fumigatus infections, antifungal susceptibility testing for A. fumigatus utilizes conidial inocula. Previous studies have yielded conflicting results as to whether conidia adequately reflect antifungal susceptibility of hyphae, but the ease of handling and quantification of conidia have prompted their use in such assays. The mold rapid susceptibility assay, which utilizes a conidial inoculum (cRSA), was adapted as a novel method to assess the utility of conidial versus hyphal inocula (hRSA) to further evaluate the susceptibility of A. fumigatus conidia and hyphae to amphotericin B (AMB), itraconazole (ITC), and voriconazole (VRC). Conidial inocula were prepared as previously described for the cRSA and minimum inhibitory values (MIC) were determined. For the hRSA, microtiter test wells lacking antifungal drug were inoculated with a standardized conidial inoculum and incubated for 12 h at 35-37 degrees C to allow formation of hyphae. Following addition of antifungal drug and 48 h incubation at 35-37 degrees C, hRSA antifungal minimum inhibitory concentration (MIC) values were determined by analysing the pattern of residual glucose levels in hRSA test wells. hRSA MIC values of each strain were influenced by hyphal inoculum size, with increasing hyphal inoculum size corresponding to increased AMB, ITC and VRC MIC values. Comparisons between the hRSA and cRSA MIC values demonstrated insignificant differences in conidial and hyphal susceptibility to drug, thus justifying the use of either fungal form in RSA-based susceptibility testing of A. fumigatus isolates. The RSA may be adapted for use of similar testing of other invasive molds that predominate as hyphal forms in tissue.