Chrysothrix sp., a Lichen from Paposo Fog Oasis: Antibacterial Potential to Combat Multidrug-Resistant ESKAPE-E...

Lichens are mutualistic symbiosis between a fungus (mycobiont) and an alga or cyanobacteria (photobiont), forming metabolically versatile holobionts capable of producing diverse secondary metabolites that facilitate their survival in extreme environments. Chrysothrix species, commonly known as "gold dust lichens," are characterized by their vivid yellow thalli and their production of pulvinic acid derivatives, although their bioactive potential remains poorly explored. In this study, using organic chemistry techniques, we characterized the methanolic extract of Chrysothrix sp.-collected from the Paposo Fog Oasis in northern Chile, a unique coastal ecosystem sustained by persistent fog ("camanchacas") within the Atacama Desert-and identified calycin as its major secondary metabolite through chromatographic purification and single-crystal X-ray diffraction. Antibacterial assays revealed that both the crude methanolic extract and purified calycin exhibited selective inhibitory activity against multidrug-resistant Gram-positive ESKAPE-E pathogens. Minimum Inhibitory Concentrations (MICs) ranged from 125 to 500 μg/mL, with the strongest effects observed against Enterococcus faecium (MDR, VRE) and Staphylococcus aureus (MDR, MRSA). No meaningful activity was detected against Gram-negative bacteria, consistent with the known permeability barrier conferred by the outer membrane. This work provides the first evidence of antibacterial activity for calycin isolated from Chrysothrix sp., highlighting the relevance of pulvinic acid derivatives as promising scaffolds for antimicrobial development. These findings highlight the potential of lichen-derived agents and alternative sources of antibacterial agents to address the global challenge of antimicrobial resistance.