In vitro susceptibility of Mycobacterium tuberculosis to extracts of Eucalyptus camaldulensis and Eucalyptus torelliana and isolated compounds

Context: Eucalyptus camaldulensis Dehnh. (Myrtaceae) and Eucalyptus torelliana F. Muell are used in Nigerian traditional medicine for the treatment of cough associated with tuberculosis (TB) and other respiratory infections. Objective: Hexane, chloroform, methanol extracts, and isolated compounds of E. camaldulensis and E. torelliana were screened for activity against Mycobacterium tuberculosis H37Rv (MtbH37Rv) to authenticate the traditional use of these plants. Materials and methods: The microplate Alamar blue assay (MABA) method was used to investigate the anti-M. tuberculosis activities. Bioassay-guided fractionation of the hexane extract of E. torelliana leaf was performed, and isolated compounds were characterized by MS, 1D- and 2D-NMR. Results: The extracts inhibited the growth of MtbH37Rv [minimum inhibitory concentration (MIC) 4-64 mu g/mL]. Spectroscopic characterization led to the identification of two compounds, hydroxymyristic acid methylester (1) and a substituted pyrenyl ester, a sterol (2). Compounds 1 and 2 had MIC of 49.45 and 46.99 mu g/mL; IC(50) > 100 and 38.21 mu g/mL; selectivity index (SI) > 2.02 and 0.81, respectively, and a minimum bactericidal concentration (MBC) of 62.50 mu g/mL. Discussion and conclusions: The anti-TB activities of these plants on M. tuberculosis H37Rv support their use in traditional medicine for the treatment of coughs associated with TB and reveals the presence of anti-Mtb active compounds in the plants. These findings not only demonstrate a new potential area of therapeutic value of E. camaldulensis and E. torelliana, but also illustrate the role of esters as anti-Mtb active principles in ethnobotanical preparations and as lead compounds in the development of new and effective anti-Mtb drugs.