On the mode of action of 3-phenylindole towards Aspergillus niger

3-Phenylindole is an antimicrobial compound active towards many fungi and gram-positive bacteria. At 5 μg/ml it inhibits growth of Aspergillus niger. Higher concentrations (50 μg/ml) also suppress spore germination; they do not kill the fungus. Dry weight of the fungus still increases for 1 or 2 days after fungicide treatment. The toxicant has no effect on O₂ uptake even at higher concentrations (100 μg/ml). The compound markedly affects composition of the lipid fraction of A. niger inducing a decrease in phospholipid concentration with a coincident increase in free fatty acids. Sterol pattern and sterol concentration were not affected. Antifungal activity was reversed by phospholipids added to the medium. 3-Phenylindole induced a slight leakage of ³²P-labeled compounds from the treated cells under growth conditions but not under nongrowth conditions. A strain of A. niger resistant to 3-phenylindole had the same phospholipid and sterol pattern as the wild type, but the level of both components was higher (40–60%). The 3-phenylindole-resistant strain showed resistance to triarimol and pimaricin. The wild type and the resistant strain both took up 3-phenylindole quite rapidly and accumulated it in the mycelium. 3-Phenylindole possibly interferes with phospholipid function in cell membranes, although the specific site of action has not yet been elucidated.