Antibacterial constituents from Plectranthus ciliatus

Plectranthus ciliatus E.Mey is an herb belonging to the family Lamiaceae and is used as an ornamental in gardens and homes. This species has not been very well studied phytochemically, although the genus has been reported to be a rich source of diterpenoids particularly of the abietane, labdane and kaurane class [1–3]. A total of 14 known compounds were isolated as part of the phytochemical assessment of this plant, six of which belonging to the kaurane diterpenoid class. The known labdane manool, phytol and phytyl acetate, two methoxylated flavones and two simple polyhydroxylated aromatic compounds have also been characterised from P. ciliatus. Bioassay-guided isolation led to the characterisation of the kaurane diterpene, kaurenic acid. This compound exhibited moderate antibacterial activity against a panel of Staphylococcus aureus and rapidly growing mycobacteria strains, determined using the broth microtitre dilution assay, with minimum inhibitory concentration values ranging between 8–32µg/ml. The antibacterial activity of this compound is reported here for the first time against methicillin-resistant and multidrug resistant strains of Staphylococcus aureus.

Acknowledgement: The University of London School of Pharmacy and EPSRC are thanked for a post-doctoral scholarship to M. Stavri

References: [1] Adler, A.C. et al. (1984) Helv Chim Acta 1003–1011. [2] Gabetta, B. et al. (1989) Phytochemistry 859–862. [3] Gaspar-Marques, C. et al. (2003) J Nat Prod 491–496.