Chiang Mai Journal of Science

 Our earlier work has shown that the methanol leaf extract of Anacardium occidentale (cashew) displayed antibacterial activity. In this study, the leaf extract was assessed for quorum sensing (QS) inhibition using the disc-diffusion assay with Chromobacterium violaceum (wild-type ATCC 12472) as the test organism. The extract exhibited strong anti-QS activity based on the diameter of inhibition zone (DIZ) and minimum inhibition dose (MID) of violacein production and C. violaceum growth. From the extract, methyl gallate (methyl ester of gallic acid) was isolated by column chromatography, identified by NMR and MS analyses, and quantified using reversed-phase HPLC. Using the disc-diffusion assay and broth dilution method, methyl gallate (MG) at 500 μg/disc, inhibited violacein production with DIZ of 14.5 ± 0.1 mm, MID of 6 μg/disc, and minimum inhibition concentration of 100 μg/ml. The closely related gallic acid did not display any anti-QS activity suggesting that the methyl moiety in MG may be responsible for QS inhibition. The content of MG was quantified as 1830 ± 180 μg/g of leaves. This is the first report on the anti-QS activity of cashew leaf extract, and on the systematic isolation and quantification of MG, a potent QS inhibitor.