Antibacterial compounds from the ethyl acetate extract of Platostoma rotundifolium (Briq.) A. J. Paton (Lamiaceae)

Abstract

Platostoma rotundifolium is widely used in traditional Burundian medicine against microbial diseases. Preliminary tests have shown antibacterial activity of the plant extracts against bacteria.The most active ethyl acetate extract was submitted to bioguided fractionation to isolate and identify the major antibacterial compounds. The aerial parts of P. rotundifolium were harvested in Burundi and authenticated by specialists of the National Herbarium of Belgium where a voucher specimen has been deposited. Minimum inhibitory (MIC) and bactericidal concentrations (MBC) of the plant extracts and fractions were determined by microdilution methods [1] against reference strains (Escherichia coli and methicillin-sensible (MSSA) and resistant (MRSA) Staphylococcus aureus), as well as methicillin-resistant S. aureus clinical isolates. The column chromatography fractionation was bioguided by bioautography on thin-layer chromatoplates [2]. Isolated compounds were characterized by spectroscopy and comparison with reference spectra [3]. At least three active spots were visible on the TLC-bioautograms. The fractionation yielded two compounds, the triterpene ursolic acid and a diterpene. Ursolic acid was active with the same values for MIC and MBC, which implies a bactericidal effect, on E. coli ATCC25922 (31 mg/ml), MSSA ATCC6538 (8 mg/ml), MRSA ATCC33591 (16 mg/ml), MRSA C98506 (16 mg/ml) and MRSA C100459 (31 mg/ml); whereas the diterpene has no effect on the tested strains. To our knowledge, this is the first time that ursolic acid is isolated from the genus Platostoma. Ursolic acid has significant activity against all tested strains and thus constitutes one of the molecules responsible for the antibacterial activity of the plant, which may support its use in traditional Burundian medicine.