Harzianic Acid has multi-target antimicrobial activity against Gram-Positive Bacteria

The thermophilic fungus Oidiodendron flavum is a saprobe that is commonly isolated from soil. Here, we identified a Gram-positive bacteria-selective antimicrobial secondary metabolite from this fungal species, harzianic acid (HA). Using Bacillus subtilis strain 168 combined with several assays, we found that HA targeted the cell membrane, though only at high concentrations. To further study the antimicrobial activity of HA, we isolated an HA-resistant strain, Bacillus subtilis strain M9015, and discovered that the mutant has more translucent colonies, has cross resistance to rifampin, and harbors five mutations in the coding region of four distinct genes. Further analysis of these genes indicated that the mutation in atpE might be responsible for the translucency of the colony, and mutation in yusO for resistance to both HA and rifampin. We conclude that HA is a multi-target antimicrobial agent against Gram-positive bacteria.