Influence of different pre-treatment methods on isolation of extracts with strong antibacterial activity from lichen Usnea barbata using carbon dioxide as a solvent

Multiplication and spreading of multiresistant pathogenic bacteria that cause fatal infections in humans and animals have boosted research on implementation of newer technologies for producing natural substances with strong antibacterial activities. In this study, SFE from lichen Usnea barbata was investigated for different pre-treatment methods with respect to extracts’ yield, usnic acid recovery and antibacterial activity against more than thirty methicillin resistant staphylococci strains. Extraction of U. barbata with subcritical carbon dioxide was studied as well. Extraction yield, usnic acid content and antibacterial activity of the extracts were strongly affected by the pre-treatment method. Intensive pre-treatment methods such as flaking and cutting + grinding caused a considerable comminution of the lichen thallus and enabled high extraction yields. On the other hand, mild pre-treatment of the lichen such as impact + shearing enabled isolation of extracts with lower MIC values. Experimental results indicated that extraction with liquid carbon dioxide at 30 MPa and 25 °C was the most favorable condition at which a satisfactory high extraction yield was obtained (2.08%). Extract isolated with liquid carbon dioxide had a high usnic acid content in the extract (632 g/kg) and a very strong antibacterial activity (average MIC of 19.11 μg/mL). Antibacterial activity of all the extracts against tested strains was comparable to the pure usnic acid or even stronger. This study indicated a potential use of the supercritical extracts of U. barbata in pharmaceutical formulations for treatment of infections caused by staphylococci.