In vitro evaluation of the interaction between methanol extract of lichen (<i>Ramalina farinacea</i>) and tetracycline against clinical isolates of <i>Staphylococcus aureus</i>

Abstract

Antimicrobial interaction studies between methanol extract of lichen (Ramalina farinacea (I) ach. (Fam: Ramalinacea) and tetracycline, against clinical isolates of Staphylococcus aureus was evaluated. Preliminary antimicrobial properties of the extract were evaluated. Antimicrobial interaction studies between the methanol extract of lichen in combination with tetracycline against strains of S. aureus I, J and K was carried out using checkerboard method. The preliminary antimicrobial screening revealed that the extract was effective against S. aureus J and K. The interaction of lichen methanol extract with tetracycline against S. aureus strain I at ratios 8:2, 7:3, 6:4 and 4:6 was not significantly different, while ratios 9:1 and 3:7 showed antagonism. However, the combination ratios of methanol extract and the antibiotic against S. aureus J shows synergism (8:2, 7:3, 6:4, 5:5, 4:6 and 1:9), while ratios 9:1, 3:7 and 2:8 exhibit indifference. Interaction of the methanol extract and tetracycline against strain K at ratios 8:2, 6:4, 4:6 and 1:9 showed synergism, while ratios 9:1, 7:3, 5:5, 3:7 and 1:9 were not significantly different in their interactions. This shows that, in the treatment of infections caused by strains I, J and K of S. aureus, the combination of the methanol extract of lichen (R. farinacea) and tetracycline can be used.

Key words: Antimicrobial, checkerboard, combination ratio, minimum inhibitory concentration, strains.