P. acnes (1 × 10⁷ CFU per ml) was incubated with 0-100 μg ml⁻¹ of lauric acid in 5% DMSO in PBS for 5 hours under anaerobic conditions. After incubation, P. acnes suspension was diluted 1:10-1:10⁶ with PBS, and 5 μl of the dilutions was spotted on a Brucella broth agar plate supplemented with 5% defibrinated sheep blood and hemin and vitamin K. After liquid in the P. acnes suspension was absorbed into the agar, the plate was incubated under anaerobic conditions to quantify CFU of P. acnes. Data represent mean±SE of three individual experiments (**P<0.005 by Student’s t-test). UD: undetectable.

(a) P. acnes (1 × 10⁶ CFU per ml), (b) S. aureus, ATCC 35556 (1 × 10⁶ CFU per ml), and (c) S. epidermidis, ATCC 12228 (1 × 10⁶ CFU per ml) were incubated with lauric acid (solid circles) and BPO (open circles), in 5% DMSO under anaerobic conditions at 37 °C for 72, 24, and 48 hours, respectively. After incubation, OD₆₀₀ of each sample was measured by a microplate reader to determine bacterial growth. Data represent mean±SE of three individual experiments (*P<0.05, **P<0.005, ***P<0.0005 by Student’s t-test).

The immortalized human SZ95 sebocytes (1 × 10⁵ cells) were incubated with the indicated concentrations of lauric acid in Sebmed supplemented with 1% fetal bovine serum, 5 ng ml⁻¹ EGF at 37 °C for 18 hours. As a background, Triton X-100 [0.1% (v v⁻¹)] was added to achieve 100% of cell cytotoxicity. After incubation, cell viability of sebocytes was determined with p-nitrophenyl phosphate, and the cytotoxicity of a neutralizing mixture was calculated as described in Materials and Methods. Data represent mean±SE of five individual experiments.

Ears of ICR mice were injected intradermally with 1 × 10⁷ CFU per 20 μl of P. acnes (left ear), or 20 μl of PBS (right ear) and observed by hematoxylin and eosin (H&E) staining (a, b), transmission electron microscopy, (c-e), and fluorescence immunohistochemistry (f, g) 24 hours after P. acnes injection. (a, b) Increase in ear thickness and infiltrated inflammatory cells (arrows) surrounding the injection site of P. acnes (arrowhead) were observed at an H&E-stained frozen section of the P. acnes-injected ear (b), but not the PBS-injected ear (a). Scale bar = 200 μm. (c-e) Colonized and/or phagocytized P. acnes were observed in macrophage-like cells (c and d; ×8,000 and ×24,000 magnifications, respectively), but not observed in PBS-injected control ear (e; ×24,000 magnification) (e). Scale bar = 4 μm. (f, g) The sections were stained with antimouse CD11b IgG, a conventional macrophage marker, and TRITC-streptavidin conjugate (red), followed by 4′-6-Diamidino-2-phenylindole (blue). Infiltration of numerous CD11b-positive macrophages was observed in the P. acnes-injected ear (f), but not in PBS-injected ear (g). Broken lines indicate the outlines of ear sections. Data are representative of four separate experiments with similar results. Scale bar = 200 μm.

Left ears of ICR mice were intradermally injected with P. acnes (1 × 10⁷ CFU per 20 μl in PBS). Right ears of the same mice were injected with 20 μl of PBS. Subsequently, the P. acnes- and PBS-injected sites were intradermally injected with lauric acid (2 μg per 20 μl in 5% DMSO in PBS). As a control, an equal volume (20 μl) of 5% DMSO in PBS was injected into both ears. (a) The increase in ear thickness was measured using a micro caliper before and 24 hours after the bacterial injection. (b) The P. acnes-injected ear was punched with an 8 mm biopsy punch 24 hours after P. acnes injection and homogenized in 200 μl of sterile PBS with a tissue grinder. CFUs of P. acnes were enumerated by plating serial dilutions of the homogenate on an agar plate. Data represent mean±SE of four individual experiments (*P<0.05, ***P<0.0005 by Student’s t-test). (c-d) Ear injected with lauric acid only (c), ear injected with both P. acnes and lauric acid (d), and ear injected with both P. acnes and vehicle (5% DMSO in PBS) (e) were cross-sectioned, stained with H&E. Increase in ear thickness and infiltrated inflammatory cells (arrows) surrounding the injected site of P. acnes (arrowhead) were observed in an H&E-stained frozen section of P. acnes injected ear (e), and were decreased in the presence of lauric acid (d). Data are representative of four separate experiments with similar results. Scale bar = 200 μm.

Left ears of ICR mice were intradermally injected with 1 × 10⁷ CFU per 20 μl of P. acnes. Right ears received an equal amount of PBS serving as a control. Lauric acid (150 μg in 5% acetone mixed with 15 mg of Vaseline) and 5% acetone mixed with 15 mg of Vaseline (vehicle) were epicutaneously applied on the left and right ears, respectively. (a) The increase in ear thickness was measured using a micro caliper before and 24 hours after the bacterial injection. The increase in ear thickness of P. acnes-injected ear was normalized to a PBS-injected control. (b) Ears with P. acnes injection were punched with an 8 mm biopsy punch 24 hours after bacterial injection and homogenized in 200 μl of sterile PBS. CFUs were enumerated by plating serial dilutions of the homogenate on an agar plate. The data represent mean±SE of six individual experiments (**P<0.005, ***P<0.0005 by Student’s t-test). (c) To examine in vivo cytotoxic effect of epicutaneous application of lauric acid on the keratinocytes, ear sections were detected by TUNEL assays and stained with rabbit anti-K10 (a differentiated keratinocyte marker) IgG, followed by goat anti-rabbit IgG-TRITC conjugate (red). Nuclei were counterstained with 4′-6-Diamidino-2-phenylindole (blue). No apoptotic differentiated keratinocytes (arrows) were detected on lauric acid-treated skins. Few apoptotic cells (light blue arrowheads) occurred naturally in dermis were detected in both vehicle- and lauric acid-treated skins. Broken lines indicate the outline of the surface of epidermis. Data are representative of six separate experiments with similar results. Scale bar = 200 μm.