Antibacterial activity and cytotoxicity of Pterocarpus erinaceus Poir extracts, fractions and isolated compounds

Abstract

Ethnopharmacological relevance: Pterocarpus erinaceus has been chosen based on ethnobotanical surveys carried out in the Tchamba district of the Republic of Togo.

Aim of the study: Investigation of the antibacterial as well as cytotoxic activities of whole extracts, fractions and compounds isolated from the leaves, trunk bark and roots of Pterocarpus erinaceus.

Materials and methods: Bio-guided fractionation of the raw extracts of plant parts and subsequent isolation of compounds from active fractions using normal phase open column chromatography. The broth microdilution method was used to evaluate the antibacterial activity, based on the determination of Minimal Inhibitory Concentrations (MICs) against several bacterial species representative of the most commonly encountered infectious diseases worldwide. The cytotoxicity of the raw extract and the most active fractions on a human non-cancerous cell (namely MRC-5) was estimated with a MTT assay. The chemical structure of the compounds isolated was elucidated using a combination of advanced Nuclear Magnetic Resonance (NMR) and Mass Spectrometry (MS).

Results: All extracts and fractions tested have shown good activities against Gram-positive bacteria (including Methicillin-Resistant Staphylococcus aureus, MRSA) and against Pseudomonas aeruginosa with MIC values ranging from 32µg/mL to 256µg/mL. In contrast, extracts were not toxic to MRC-5 cells. Four compounds have been isolated: Compound 1 (friedeline); Compound 2 (2,3 dihydroxypropyloctacosanoate); Compound 3 (a mixture of β-sitosterol, stigmasterol and campesterol); Compound 4 (β-sitosteryl-β-D-glucopyranoside) and shown to be active against some of the bacteria tested. They were active with MIC equal to 4µg/mL against strains of S. aureus (including MRSA). To the best of our knowledge, all of them except friedeline have never been reported in this plant species.

Conclusion: P. erinaceus is confirmed as a plant harboring promising antibacterial activity with activities against serious human pathogens at very low concentrations. Some of the compounds isolated are also active at concentrations as low as 4µg/mL and therefore, may provide new leads for the development of antibacterial agents.

Keywords: Antibacterial activity; Bio-guided isolation; Pterocarpus erinaceus; Togo.