Determination of antibacterial activity of 6β-hydroxybetunolic acid and interaction with oxacillin

Abstract

Various strides have been undertaken with successful results in the treatment and cure of significant number of bacterial infections. However, many bacteria are becoming resistant to some of the prevailing drugs due to either the misuse or the prolonged use of the available antibiotics increasing the necessity for discovering new antibiotics to combat resistant microorganisms. Plants and their secondary metabolites can be considered as good sources of scaffolds to provide structurally diverse bioactive compounds as potential therapeutic agents and combination of them with standard antibiotic to obtain synergistic effect. The objective of this study was to determine the antibacterial activity of one of a lupeol type triterpenoid (6β-hydroxybetunolic acid) isolated from the bark of Schumacheria castaneifolia Vahl. which is an endemic plant to Sri Lanka and to determine the synergistic effect with Oxacillin. Antimicrobial activity of 6β-hydroxybetunolic acid was evaluated utilizing micro broth dilution assay in 96 well plates against two standard strains of Staphylococcus aureus, 4 strains of clinically isolated Methicillin resistant S. aureus, standard strains of Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa, carbepenemas producing Kebsiella pneumonia and carbepenemas non-producing Kebsiella pneumonia and four strains of clinically isolated Acinetobacter sp. Synergistic effect of the combination of 6β-hydroxybetunolic acid and Oxacillin was tested against Standard strain of S. aureus and MRSA using checker board method. Results revealed that 6β-hydroxybetunolic acid shows significant antibacterial activity only against the Gram positive strains; MIC values of S. aureus (ATCC 29213), S. aureus (ATCC 29213), E. faecalis (ATCC 29212) and four MRSA strains were 32, 16, 32, 32, 32, 32, 16 ppm respectively. However MIC value of Oxacilin against S. aureus (ATCC 29213) was 0.25 ppm. 6β-hydroxybetunolic acid has synergistic effect with Oxacillin against S. aureus and additive effect against all the tested MRSA. These results concluded that the antibacterial activity of 6β-hydroxybetunolic acid is predominantly depending on the cell wall difference of the bacteria.