Inhibition of Gram-positive bacteria by mangosteen bark and
fruit pericarp extracts
The antimicrobial activity of pericarp and bark extracts prepared in
water (60 ◦C, 1:60 ratio) and methanol against the Gram-positive
bacteria Listeria spp. and S. aureus was examined in greater detail
at pH 4, 5, 6 and 7. Results presented in Table 4 show that MIC
and MBC measurements varied according to source and solvent
used for extraction. Overall, pericarp extract prepared in methanol
had the highest activity, followed by methanolic bark extract. MIC
and MBC measurements were significantly affected by medium
pH (P ≥ 0.05) and the lowest measurements were achieved at
pH 4. Aqueous pericarp and bark extracts were comparatively
less effective although here the influence of medium pH was
more pronounced. Aqueous bark extract, for example, had little
activity against most Listeria species at medium pH > 5. The
effect of pH on the activity of plant extracts and individual
phenolic compounds has been reported previously. Wen et al.36
showed that the antilisterial activity of caffeic acid increases with
decreasing pH. This effect was ascribed to the influence ofmedium
pH on the state of dissociation of the molecule, which has a