Methanolic mangosteen pericarp and bark extracts evidently
contain potent inhibitors of Gram-positive bacteria. Vaquero
et al.39 studied the antimicrobial properties of polyphenols
extracted from wine including the flavonoids rutin, catechin and
quercetin, and the non-flavonoid gallic, vanillic, protocatechuic
and caffeic acids against L. monocytogenes. Caffeic acid exhibited
the strongest activity and 0.5mgmL−1 was reported to completely
inhibit growth, although the effect of pH was not considered. In
the present work MIC measurements well below this value were
obtained with pericarp extract prepared in methanol, particularly
at low pH. Aqueous extracts were comparatively less effective
and both MIC and MBC values were significantly higher under
all experimental conditions (P ≥ 0.05). Weaker antimicrobial
activity in plant extracts obtained with water compared with
other solvents including alcohols or non-polar solvents has been
reported previously. For example, Bassam et al.40 showed that
methanol or ethanol extracts of several aromatic herbs were more
effective against a range of microorganisms than those prepared
in hot water. Baram et al.41 studied the antimicrobial activity of
ethanol, methanol, water and acetone extracts of Tulipa sintenisii
(Baker). All exhibited antimicrobial effects against S. aureus, E. coli
and Pseudomonas syringae but acetone extracts had the highest
activity.