Skin mycobiota-mediated antagonism against Staphylococcus aureus through a modified fatty acid.

Microbiota promote host health by inhibiting pathogen colonization, yet how host-resident fungi or mycobiota contribute to this process remains unclear. The human skin mycobiota is uniquely stable compared with other body sites and dominated by skin-adapted yeasts of the genus Malassezia. We observe that colonization of human skin by Malassezia sympodialis significantly reduces subsequent colonization by the prominent bacterial pathogen Staphylococcus aureus. In vitro, M. sympodialis generates a hydroxyl palmitic acid isomer from environmental sources that has potent bactericidal activity against S. aureus in the context of skin-relevant stressors and is sufficient to impair S. aureus skin colonization. Leveraging experimental evolution to pinpoint mechanisms of S. aureus adaptation in response to antagonism by Malassezia, we identified multiple mutations in the stringent response regulator Rel that promote survival against M. sympodialis and provide a competitive advantage on human skin when M. sympodialis is present. Similar Rel alleles have been reported in S. aureus clinical isolates, and natural Rel variants are sufficient for tolerance to M. sympodialis antagonism. Partial stringent response activation underlies tolerance to clinical antibiotics, with both laboratory-evolved and natural Rel variants conferring multidrug tolerance in a manner that is dependent on the alternative sigma factor SigB. These findings demonstrate the ability of the mycobiota to mediate pathogen colonization resistance through generation of a hydroxy palmitic acid isomer, identify new mechanisms of bacterial adaptation in response to microbiota antagonism, and reveal the potential for microbiota-driven evolution to shape pathogen antibiotic susceptibility.