Inactivation of farR causes high rhodomyrtone resistance and increased pathogenicity in Staphylococcus aureus. rhodomyrtone resistance in Staphylococcus aureus

Rhodomyrtone (Rom) is an acylphloroglucinol antibiotic originally isolated from leaves of Rhodomyrtus tomentosa. Rom targets the bacterial membrane and is active against a wide range of Gram-positive bacteria. The exact mode of action is still obscure. Here we isolated and characterized a Rom-resistant mutant from the model strain Staphylococcus aureus HG001 (RomR) to learn more about the resistance mechanism. We showed that Rom-resistance is based on a single point mutation in the coding region of farR (regulator of fatty acid resistance) that causes an amino acid exchange from Cys116 to Arg116 in FarR, which affects FarR activity. Comparative transcriptome analysis shows that FarR is a global regulator controlling directly or indirectly more than 1000 genes. FarR represses for example the expression of its own gene (farR), its flanking gene farE (effector of fatty acid resistance), and other global regulators such as agr and sarA. All these genes were consequently upregulated in the RomR clone, the farR mutant. Particularly the upregulation of agr and sarA leads to increased expression of virulence genes rendering the RomR clone more cytotoxic and more pathogenic in a mouse infection model. The Rom-resistance is largely due to the de-repression of farE. FarE is described as an efflux pump for linoleic and arachidonic acids. Indeed, we see an increased release of lipids in the RomR clone compared to its parent strain HG001. If farE is deleted in the RomR clone, or, if native farR is expressed in the RomR strain, the corresponding clones become hypersensitive to Rom. We show here that the high Rom-resistance is mediated by overexpression of farE in the RomR clone, that FarR is a global regulator, and that the point mutation in farR (RomR clone) makes the clone hyper-virulent.