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.
