748 Cutibacterium acnes supports the epidermal barrier by promoting epidermal lipid synthesis

Lipids are essential for the permeability barrier and to maintain skin microbiota homeostasis. To investigate if bacteria influence keratinocyte lipid synthesis, we screened conditioned media from several human commensal bacterial species and strains for the capacity to alter lipid accumulation in Normal Human Epidermal Keratinocytes (NHEK). We observed a 3-fold increase (p<0.0001) in response to strains of C. acnes but no change in response to the commensal species of coagulase-negative Staphylococci. Increased lipid staining by Bodipy was also seen in the epidermis of mice challenged with live C. acnes and in human acne lesions. Differential Mobility Spectrometry-based shotgun lipidomic analysis of NHEKs or mouse epidermis exposed to C. acnes showed a large increase in triglycerides and enrichment of ceramides, cholesterol, and free fatty acids. Liquid Chromatography-Mass Spectrometry and biochemical isolation of culture medium conditioned by C. acnes revealed the lipid response of NHEKs was mediated by propionic acid (PA) produced by C. acnes. RNA-seq analysis with validation by qPCR showed that C. acnes or PA induced expression of lipid synthesis genes including glycerol-3-phosphate-acyltransferase-3 (GPAT3), an important gene involved in the triglyceride metabolism. Silencing of GPAT3 inhibited the capacity of C. acnes to induce lipid accumulation in NHEK. Pharmacological inhibition or RNA silencing of PPARa, but not PPARb and PPARg, blocked this increase in lipid production. After exposure of NHEK to C. acnes or PA, epithelial barrier permeability improved and extracted lipids inhibited C. acnes growth. These results show how a metabolite from a commensal bacterial species can support skin barrier function and enhance innate immune defense.