Antimicrobial activity of 6-oxophenolic triterpenoids.: Mode of action against Bacillus subtilis

Zeylasteral and demethylzeylasteral are 6-oxophenolic triterpenoids isolated from the root of Maytenus blepharodes, which have antimicrobial activity against Gram-positive bacteria and the yeast Candida albicans. The time-kill curves for zeylasteral and demethylzeylasteral at concentrations twice their MICs, against Bacillus subtilis showed that the colony forming units were reduced in 3-log(10) and > 4-log(10) respectively. This reduction was dependent on inoculum size and the growth phase of cells, and was greater when the compounds were incorporated in the exponential phase, indicating a bacteriolytic effect. Treatment with both agents, particularly with zeylasteral (20 mu g/mL) caused a reduction of optical density at 550 nm. With regard to the synthesis of DNA, RNA, protein and cell wall, the compounds exhibited the fastest inhibition against cell wall synthesis. Thus, the predisposition to lysis, the morphological changes seen by microscopy, and the complete inhibition in the incorporation the N-acetyl-D-[1-(14)C]glucosamine, suggest that the phenolic compounds compromise the cell wall synthesis and/or cytoplasmic membrane.