The in vitro antifungal activity of ophiobolins was determined in a 96-well microtiter plate bioassay by measuring the
absorbance of the fungal cultures at 620 nm. The wells contained SPEC medium supplemented with ophiobolin A or B and inoculated with the appropriate sporangiospore suspension (105 spores mL–1). The drug concentrations applied were 100, 50, 25, 12.5, 6.25, 3.175 and 1.5875 μg mL–1, respectively. The plates were incubated for 72 h at 24 or 37 °C depending on the culturing requirements of the strains. Absorbances were measured using an ASYS Jupiter HD microplate reader (ASYS Hitech) every 24 h. Each test plate contained a sterile control (containing medium alone), a growth control (containing inoculated medium without the drugs) and a drug-free control (containing inoculated medium and methanol in the appropriate dilution without the ophiobolins). The uninoculated medium was used as the background Selleckchem Palbociclib for the calibration of the spectrophotometry. Absorbance of the untreated control cultures was referred to 100% of growth in each case. To decide whether the antifungal effect was fungistatic or fungicidic, 10 μL of each suspension in the microdilution plates was dropped onto YEG plates. After incubation for 24 h, the plates were checked visually. If colony formation was observed, the antifungal effect was considered to be fungistatic; otherwise, it was
fungicidic. Each experiment was repeated three times. For morphological examinations, the Mucor circinelloides strain ATCC 1216b was cultured http://www.selleckchem.com/products/apo866-fk866.html on a solid and in a liquid YEG medium containing different concentrations of the drug (1.6, 3.2, 6.25 or 12.5 μg mL–1) at 24 °C. If the fungus was cultured on
a solid medium, microscopy was performed after incubation for 24 h. In the case of the liquid cultures, ophiobolin A was added to the fungus either at the time of spore inoculation (0 h) or 4 h postinoculation, and cells were examined microscopically 5 h after the addition of the inhibitor (5 or 9 h postinoculation, respectively). Treated cells were stained CHIR-99021 mw using the annexin V-fluorescein isothiocyanate (FITC) Apoptosis Detection Kit (Sigma) according to the manufacturer’s instructions. For nuclear staining, cells were resuspended in 1 mL of 0.1 μg mL–1 4′-6-diamidino-2-phenylindole (DAPI) staining solution and were allowed to stain for 30 min at room temperature. Stained spores were collected, washed twice with distilled water (dw), and resuspended in 50 μL dw. Microscopic examinations were performed with a Zeiss Jenalumar fluorescence microscope using an excitation filter U 205 g, a barrier filter G-244 and a 510 nm dichroic splitter. The susceptibility to ophiobolins A and B of 17 fungal isolates representing six different genera (Micromucor, Mortierella, Mucor, Rhizomucor, Rhizopus and Gilbertella) was tested and their MIC values were determined (Table 1).