A marine-derived fatty acid targets the cell membrane of Gram-positive bacteria

Abstract

The rapid evolution of antibiotic resistance is shrinking our stockpile of commercially available antibiotics. Therefore, new antimicrobials with novel mechanisms of action (MoAs) are desperately needed. The free fatty acid, (Z)-13-methyltetra-4-decenoic acid ((Z)-4C-14:1), isolated from a marine sediment bacterium Olleya marilimosa, displays strong inhibition of Gram-positive pathogens with minimal cytotoxic effects to mammalian cells. Here, we applied a combination of experimental approaches to identify the mechanism by which (Z)-4C-14:1 kills the Gram-positive bacterium Bacillus subtilis. Using quick and cost-effective bacterial cytological profiling (BCP), we established the cytological signatures of 17 antibiotics representing 6 general classes of antibiotic MoAs. We used BCP to demonstrate that while (Z)-4C-14:1-treated B. subtilis cells display unique morphological features compared to other antibiotic classes, (Z)-4C-14:1 mode of action shares substantial overlap with the fast-acting antibiotic colistin specifically in Gram-positive cells and daptomycin, both of which target bacterial permeability by destroying the cell membrane and causing extensive cell surface alterations. To further determine if the cell membrane of B. subtilis was the target of (Z)-4C-14:1, we used cell membrane- and peptidoglycan-specific diagnostic stains to investigate bacterial permeability. Our results indicate that (Z)-4C-14:1 destabilizes the cell membrane by pore formation in Gram-positive bacteria and emphasize the importance of mining the marine environment for naturally occurring antibiotics.