Discovery of Bacterial Fatty Acid Synthase Inhibitors from aPhomaSpecies as Antimicrobial Agents Using a New Antisense-Based Strategy

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Abstract
Fatty acids are essential for bacterial growth and viability, with the type II fatty acid synthesis (FAS II) pathway being a potential antibacterial target. A new, selective, and highly sensitive whole cell-based antisense strategy has been designed to screen for natural product inhibitors of FabH/F of the FAS II pathway using a high-throughput two-plate agar-based differential sensitivity assay (FabF2p). An antisense assay along with the FASII enzyme prepared from Staphylococcus aureus was used for bioactivity-guided fractionation, leading to the isolation of phomallenic acids A−C (1−3) from a leaf litter fungus identified as Phoma sp. Compounds 1−3 exhibited minimum detection concentrations (MDC) of 0.63, 0.31, and 0.15 μg/mL in the FabF2P assay, IC50 values of 22, 3.4, and 0.77 μg/mL in the FASII enzyme assay, and minimum inhibitory concentrations (MIC) of 250, 7.8, and 3.9 μg/mL, respectively, against wild-type S. aureus. Phomallenic acid C (3), the analogue with the longest chain, exhibited the best overall activity within the phomallenic acids obtained and was superior to cerulenin and thiolactomycin, the two most studied and commonly used FabF inhibitors.