Antimicrobial agents from Heterotheca inuloides.

By bioassay directed fractionation, four sesquiterpenoids 1-4 were isolated as antimicrobial agents from the dried flowers of Heterotheca inuloides, a Mexican medicinal plant locally known as "arnica". 7-Hydroxy-3,4-dihydrocadalin (3) and 7-hydroxycadalin (4) exhibited potent antibacterial activity against Gram-positive bacteria with minimum inhibitory concentrations (MICs) ranging from 6.25 to 12.5 micrograms/ml. Notably, 7-hydroxy-3,4-dihydrocadalin showed bactericidal activity against methicillin-resistant Staphylococcus aureus (MRSA) with a minimum bactericidal concentration (MBC) of 12.5 micrograms/ml.