Evaluation of Antimicrobial and Quorum Sensing Inhibitory Activities of Umbilicaria deusta Extract against Staphylococcus aureushylococcus aureus

There is growing interest in the potential antivirulence properties of lichens, which are recognized as a rich source of biologically active secondary metabolites. Staphylococcus aureus is a major pathogen of both human and veterinary importance, responsible for persistent infections such as bovine mastitis and skin infections in animals. This study examines the biological activity of an acetone extract from Umbilicaria deusta, on its antimicrobial effects and its ability to inhibit quorum sensing (QS) in S. aureus. Lichen thalli were collected and identified using ITS rDNA sequencing, and extracted by maceration. Antimicrobial activity was evaluated against S. aureus ATCC 25923 using microdilution assays, while QS inhibition was evaluated in USA300-derived agr P2-GFP and P3-GFP reporter strains. The U. deusta extract did not exhibit detectable antimicrobial or growth-inhibitory effects at the tested concentrations. However, the extract presented a clear reduction in QS activity in both reporter systems without affecting bacterial growth, with approximately 60% inhibition. These results indicate that U. deusta is unlikely to act as a conventional antimicrobial against S. aureus but may represent a promising source of antivirulence compounds targeting QS pathways in clinically relevant MRSA strains.