Abstract

An ethnobotanical survey of Tongan pharmacopoeia, conducted through semi-structured interviews with healers, informed the selection of twenty-six plants for antimicrobial bioassays. The parts of the plants recommended by the healers were collected and extractions were made with methanol and hexane in various concentrations, and screened against Staphylococcus aureus, Escherichia coli, and Candida albicans using microbial inhibition assay. Further, minimum inhibitory concentrations (MICs) of Syzygium corynocarpum (A.Gray) Müll. Stuttg. were determined against S. aureus and methicillin-resistant S. aureus (MRSA) using extractions from both the healer-recommended plant part—young leaves—and two other plant parts—mature leaves and bark—to test the hypothesis of greater antimicrobial activity in the traditionally used part. Cytotoxicity of the extractions was determined by trypan blue assay on human lymphocytes. The microbial inhibition assays yielded six species that inhibited the growth of S. aureus, E. coli and/or C. albicans at rates of 40% or above compared to the controls. The only species that inhibited all three microbes at above 40% was S. corynocarpum. Subsequent testing on S. corynocarpum revealed that both the methanol and hexane extractions of young leaves inhibited S. aureus (MIC 125 and 500 μg/ml respectively) and MRSA (MIC 250 and 500 μg/ml respectively), while only the methanol extract of the bark showed inhibition against both microbes (MIC = 250 μg/ml), and the extractions from the mature leaves showed no activity at the concentrations tested. An additional assay with extractions of S. corynocarpum on human lymphocytes suggests no cytotoxicity compared to control cells. These results support the traditional use of several of the tested plants. In particular S. corynocarpum, which has not previously been studied for antimicrobial activity, showed greater activity by the traditionally used part, emphasizing the importance of documenting specific plant parts used during ethnobotanical interviews.

Key words: Ethnobotany, antimicrobial, Tonga, Syzygium corynocarpum, minimum inhibitory concentration (MIC).