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

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