Synergistic antimycobacterial activities of sesquiterpene lactones from Laurus spp

Objectives: The aim of this study was to determine the antimycobacterial potential of laurel oil, its fractions and its two sesquiterpene lactones against several mycobacterial strains and clinical isolates, and to establish the possibility of occurrence of some synergistic effects between those lactones using a modification of the fluorometric Alamar Blue microassay (FMABA). Methods: The in vitro antimycobacterial activity of whole oil and its fractions and pure active compounds were determined by FMABA. A bioassay-guided fractionation of the traditional preparation of laurel oil from Madeira Islands was performed, yielding pure compounds chemically identified by standard procedures. Synergism of pure compounds was established by X/Y quotient analysis adapted to FMABA. Results: Sesquiterpene lactones, costunolide and dehydrocostuslactone, were the compounds responsible for the antimycobacterial activity against Mycobacterium tuberculosis H37Rv with MICs of 6.25 and 12.5 mg/L, respectively. Antimycobacterial activity against drug-resistant M. tuberculosis clinical isolates was better for the mixture than for pure compounds. Conclusions: Both lactones presented synergistic activity, i.e. analysis of relative fluorescence units presented an X/Y value < 0.5 at a concentration of 1/8 MIC of each compound in the combination. Establishment of synergism by FMABA represents another application of the microplate Alamar Blue assay.