amounts. Hence the relative activity of mangosteen extracts
against Gram-positive bacteria was strongest in extracts with
high tartaric ester and flavonol contents. Interestingly, tartaric
acid and tartaric acid derivatives are found in significant amounts
in mangosteen pericarp.5 The antimicrobial activity of tartaric
acid and common phenolic acids with pKa values ≤ 5 is
known to increase with decreasing pH.36 The flavonol group
also includes well-known antimicrobial compounds, however,
including xanthones which have been shown to impart activity
to mangosteen pericarp extracts. Hence activity against Grampositive
bacteria reported in the present work was likely due to
the combined action of several phenolic compounds.