Antimicrobial activities of some thymol derivatives from the roots of Inula hupehensis

In a search for some naturally occurring antimicrobial compounds found in medicinal plants, herbs and spice extracts, four thymol derivatives were isolated from the roots of Inula hupehensis (a medicinal plant) and identified as the new compound 9-O-β-d-glucopyranosyl-9-hydroxythymol (1), together with 8,9-epoxy-10-isobutyryloxythymyl isobutyrate (2), 8-hydroxy-9, 10-diisobutyloxythymol (3) and 8,9,10-trihydroxythymol (4). These thymol derivatives, with a semi-synthetic compound 9,10-dihydroxy-8-methoxythymol (5), were tested for their antimicrobial activities against three bacteria and six plant pathogenic fungi. Within the series of thymol derivatives tested, compound 3 was the most active, particularly displaying moderate antibacterial activities against Staphylococcus aureus, Methicillin-resistant S. aureus and Escherichia coli with MICs of 62.3, 62.8, and 250 μg/ml. Moreover, this compound exhibited inhibitory activities against three plant pathogenic fungi: Rhizoctonia solani, Phytophthora melonis and Peronophythora litchi, with EC50 values of 157, 180 and 141 μg/ml. To our knowledge, this is the first report that these five thymol derivatives show inhibitory activity against plant pathogenic fungi.