A New Trichlorinated Xanthone and Compounds Isolated from with Antimicrobial Properties.

: The global rise in multidrug-resistant (MDR) bacteria, particularly methicillin-resistant and methicillin-susceptible (MRSA and MSSA), continues to pose a major public health challenge, including in Hawaii. This underscores the need to discover new antimicrobial agents from natural sources. Guided by teachings from a Buddhist master regarding the medicinal value of lichens, we investigated the endemic Hawaiian lichen . : Specimens of were collected from the Lotus Buddhist Monastery in Mountain View, Hawaii. A methanolic extract was prepared and purified using chromatographic techniques, and compound structures were elucidated through spectroscopic analyses and single-crystal X-ray diffraction. The antibacterial activity of the compounds was assessed against Gram-positive strains (MRSA, MSSA) and Gram-negative bacteria (, , ). Cytotoxicity was assessed using A549 (non-small cell lung cancer) and Vero E6 (non-tumorigenic) cell lines. : Three compounds were isolated: clarosione (), a newly identified trichlorinated xanthone, and two known metabolites, ()-usnic acid () and perlatolic acid (). Compounds and demonstrated strong inhibitory effects against MRSA and MSSA. Their minimum inhibitory concentrations (MICs) ranged from 2 to 4 µg/mL, compared with vancomycin (0.5-1 µg/mL). Cytotoxicity testing showed higher sensitivity in A549 cells than in Vero E6 cells, resulting in favorable selectivity indices for the active compounds. : In the current study, a new compound, clarosione () was discovered. This enhances our understanding of the constituents of and its potential antibacterial properties. Lichen-derived compounds may serve as lead candidates for further development, and further study is warranted.