Ultrastructural investigations of Candida albicans after in vitro treatment with the new fungicidal agent rilopirox.

Candida albicans was maintained in various culture media and incubated with different concentrations of the antifungal agent rilopirox. After fixation, dehydration and embedding in Spurr's medium, the cells were analysed at the ultrastructural level to investigate morphological aspects of the antifungal mode of action of this new hydroxpyridone compound. All untreated or sham-treated control cells exhibited a normal ultrastructural appearance. The cells were surrounded by a multilayered cell wall of typical structure, and the plasmalemma was in close contact with the cell wall. Also, the cell organelles of the protoplast corresponded well with the findings of other authors. After treatment with rilopirox, a variety of ultrastructural changes were seen, and the extent of damage was dependent on the specific culture condition, drug concentration and incubation time. After only 6 h and 1-10 micrograms ml-1 rilopirox, the plasmalemma exhibited elongated invaginations, the number and size of the lipid droplets had increased and greatly enlarged mitochondria containing electron-dense deposits became visible. The vacuolar system was strongly expanded and occupied nearly the whole cell. Exposure to higher concentrations of the antifungal agent and prolonged incubation times resulted in complete cytoplasmic autolysis and membrane breakdown, while the fungal cell wall remained unaffected. After treatment with 0.5% rilopirox suspension gel on agar cultures, the extent of cellular damage was clearly enhanced and included all cell types of a treated yeast colony, i.e. single blastospores and pseudohyphae.