Antibacterial and antifungal properties of ent-kaurenoic acid from Smallanthus sonchifolius

AIM: To screen for the antibacterial activity of ent-kaurenoic acid (1) from the dichloromethane extract of Smallanthus sonchifolius leaves against Staphylococcus aureus, Staphylococcus epidermidis, Bacillus subtilis, Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, and Pseudomonas aeruginosa, and for its antifungal activity against Candida albicans, Trichophyton rubrum, and Epidermophyton floccosum. METHODS: Compound 1 was isolated by silica gel chromatography and its structure was elucidated by NMR spectroscopy. For assaying the antibacterial and antifungal activities of 1, the disk diffusion method was used, while the minimum inhibitory concentrations (MICs) were determined by the broth dilution method. RESULTS: With the disk diffusion method, 1 was found to be active against all the Gram-positive organisms tested (S. aureus, S. epidermidis, B. subtilis) at the lowest concentration of 1 000 gmL-1, while it was active against the fungus T. rubrum at 10 000 gmL-1. No inhibitory activity was observed against C. albicans, E. floccosum and all the Gram-negative test strains. The activity indices (AI) of 1 were noted to be highest against S. aureus and lowest against T. rubrum. Statistically significant differences were found between the mean inhibition zones (IZ) of 1 and the standard drugs (ofloxacin and clotrimazole). The results of the broth dilution MIC determination revealed that 1 exhibited moderate activity against S. aureus and S. epidermidis with MIC values of 125 gmL-1 and 250 gmL-1, respectively; and weak activity against B. subtilis with a MIC of 1 000 gmL-1. The growth of T. rubrum in the MIC assay was not inhibited at the highest tested concentration of 1 (10 000 gmL-1). CONCLUSION: The minimum bactericidal concentration (MBC) indicated that the bactericidal activities of 1 occurred at concentrations higher than its growth inhibitory concentrations. Furthermore, the MBC: MIC ratio of 2: 1 clearly demonstrated the in vitro bactericidal effect of 1 against S. aureus and S. epidermidis.