A New Trichlorinated Xanthone and Compounds Isolated from Cladonia skottsbergii with Antimicrobial Properties

Background/Objectives: The global rise in multidrug-resistant (MDR) bacteria, particularly methicillin-resistant and methicillin-susceptible Staphylococcus aureus (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 Cladonia skottsbergii. Methods: Specimens of C. skottsbergii 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 (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa). Cytotoxicity was assessed using A549 (non-small cell lung cancer) and Vero E6 (non-tumorigenic) cell lines. Results: Three compounds were isolated: clarosione (1), a newly identified trichlorinated xanthone, and two known metabolites, (S)-usnic acid (2) and perlatolic acid (3). Compounds 2 and 3 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. Conclusions: In the current study, a new compound, clarosione (1) was discovered. This enhances our understanding of the constituents of C. skottsbergii and its potential antibacterial properties. Lichen-derived compounds may serve as lead candidates for further development, and further study is warranted.