Standardization of hyphal growth inhibition rate as a means of evaluating Microsporum spp. in vitro susceptibility to terbinafine, griseofulvin, and ciclopiroxolamine |
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Authors: | Biancalana Fernanda Simas Corrêa Lyra Luzia Moretti Maria Luiza Kamei Katsuhiko Schreiber Angélica Zaninelli |
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Institution: | School of Medical Sciences, State University of Campinas, S?o Paulo, Brazil. |
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Abstract: | Reference methods for antifungal susceptibility tests recommend the use of conidia as inoculum. However, some isolates produce
few conidia, while the invasive form of filamentous fungi in general is hyphae making susceptibility tests infeaseble. These
facts suggest that other than conidia broth dilution method is required for susceptibility tests. The aim of this study was
to clarify if the hyphal growth inhibition rate could be used as a method of determining the antifungal susceptibility of
genus Microsporum. For this reason, a method which traces hyphal tips automatically and measures their growth rate was standardized for Microsporum spp. Control growth curves and test growth curves obtained by real-time observation of the hyphae groups responses to different
concentrations of terbinafine, griseofulvin, and ciclopiroxolamine were used to compare with minimum inhibitory concentrations
(MICs) obtained by conidia broth microdilution method. A visible reduction in the growth inhibition rate was observed when
hyphal activity was evaluated using the third or fourth serial two-fold dilution below the MIC determined by broth microdilution
for terbinafine and ciclopiroxolamine. For griseofulvin, this reduction occurred after the fifth dilution below the MIC. This
study highlights the importance of the inoculum type used to determine the in vitro susceptibility of Microsporum strains. We conclude that measurement of hyphal growth inhibition, despite being time consuming, could be a suitable method
for evaluating antifungal susceptibility, particularly for fungi as Microsporum spp. that produce a small (or not at all) number of conidia. |
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