In Vitroantibacterial Activity of Usnic Acid and Octyl Gallate against Resistant Enterococcus Strains

The genus Enterococcus is represented by some of the clinically important species and is well known for its antibiotic resistance, which participates in colonization and infection. Increasing resistance of enterococci has been evoked due to the excessive use of antibiotics not merely for therapeutic purposes or in indicated cases. One of the promising possibilities how to reduce the use of great amount of antibiotics is to utilize antimicrobial properties of natural substances. Usnic acid is a lichen compound possessing antibacterial activity against gram-positive bacteria including enterococci. Octyl gallate, gallic acid ester, has significant antibacterial and antifungal properties. This study was focused on the evaluation of resistant enterococci susceptibility to usnic acid and octyl gallate in comparison with control group of enterococci. Antibacterial activity of usnic acid and octyl gallate was defined as minimum inhibitory concentration (MIC value) and minimum bactericidal concentration (MBC value). Usnic acid inhibited all tested enterococci in concentration range 4.7-37.5 mg.L-1. MIC values of octyl gallate for all tested enterococci ranged between 37.5-150 mg.L-1. In contrast to octyl gallate, bactericidal activity of usnic acid was not confirmed. Antibacterial activities of tested compounds were almost equal among resistant enterococci and control group of enterococci exhibiting great potential of usnic acid and octyl gallate for treatment of enterococcal infections.