In the present study, antimicrobial activity of a common Himalayan lichen viz. Bulbothrix setschwanensis (Zahlbr.) Hale extract in three common solvents (acetone, chloroform and methanol) was evaluated against six bacterial and seven fungal clinical strains. The acetone extract showed promising antimicrobial activity against S. aureus (1.56 mg/mL) and C. neoformans (6.25 mg/mL). Further, GC-MS analysis revealed 2,3-bis(2-methylpentanoyloxy)propyl 2-methylpentanoate and Ethyl 2-[(2R,3R,4aR,8aS)-3-hydroxy-2,3,4,4a,6,7,8,8a-octahydropyrano [3,2-b]pyran-2-yl]acetate as the predominant compounds. The combination of acetone extract with antibacterial drugs [kanamycin (KAN), rifampicin (RIF)] and antifungal drugs [amphotericin B (Amp B) and fluconazole (FLC)] showed lysis of S. aureus and C. neoformans at non-inhibitory concentration (FICI values were 0.31 for KAN, 0.18 for RIF, 0.37 for Amp B and 0.30 for FLC, respectively). Notably, the acetone extract confirmed cell wall damage of both S. aureus and C. neoformans cells and was clearly visualized under scanning electron microscopy (SEM), flow cytometry and confocal microscopy. Besides this, the three extracts also have less significant cytotoxic activity at MIC concentrations against mammalian cells (HEK-293 and HeLa). This study for the first time suggests that the chemical compounds present in the acetone extract of B. setschwanensis could be used against S. aureus and C. neoformans infections.
Keywords: Antibacterial; Antifungal; Checkerboard assay; Lichen; Mammalian cytotoxicity; Mechanistic study.
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