Antimicrobial activities of isoalantolactone, a major sesquiterpene lactone of Inula racemosa

Isoalantolactone, a major constituent of Inula racemosa (Compositae) was te sted for its antimicrobial action against five bacteria, six human and six plant pathogenic fungi. The lactone showed absolute toxicities at 500 mug/m L against 3 soil borne phytopathogenic fungi (Gaeumannomyces graminis var. tritici, Rhizoctonia cerealis and Phytophthora capsici) with the MICs deter mined to be 100, 100 and 300 mug/mL, respectively. At the MICs, isoalantola ctone exhibited its fungistatic nature of toxicity. The lowest fungicidal c oncentrations of the lactone to G. graminis var. tritici, R. cerealis and P . capsici were shown to be 150, 150 and 350 mug/mL, respectively. Moreover, isoalantolactone displayed weaker antibacterial activities against Bacillu s subtilis, Escherichia coli, Pseudomonas fluorecense, Sarcina lentus and S taphylococcus aureus with MICs of 125, 425, 150, 150 and 100 mug/mL, respec tively. The investigation disclosed the strong inhibition of isoalantolacto ne to the phytopathogenic fungi, raising a possibility that the lactone cou ld be considered as a starting point for the project aiming at the developm ent of new fungicide(s).