Testing of antifungal natural products: methodologies, comparability of results and assay choice

Antifungal testing of filamentous fungi generally suffers from incomparability of results. In order to illustrate this problem, the polyacetylene falcarindiol and the naphthoquinone juglone, two known antifungal natural products, were assayed in various dilution and diffusion bioassays against three selected microfungi, Botrytis cinerea, Cladosporium herbarum and Fusarium avenaceum. Broth microdilution, based on the 96-well microtitre plate format, can be scored by various direct and biochemical methods. As examples, direct observation with image analysis and the fluorescein diacetate scoring method are compared. Of the other methodologies used, results obtained by thin-layer bioautography and the radial growth rate method clearly deviated. Disk diffusion results, however, matched microdilution. In conclusion, microdilution offers the greatest potential of all bioassays to become the future general standard methodology. Copyright © 2000 John Wiley & Sons, Ltd.