Photosensitizing anthraquinones isolated from Heterophyllaea pustulata Hook f. (Rubiaceae), namely soranjidiol, rubiadin, damnacanthal and 5,5'-bisoranjidiol, showed antibacterial activity (bacteriostatic/bactericide) on Staphylococcus aureus. The mechanism of action seems to involve an increase in the levels of superoxide anion (O(2)(·-)) and/or singlet molecular oxygen ((1)O(2)). Moreover, the effect of actinic irradiation as a boosting agent for the production of both reactive species of oxygen as well as its influence on antibacterial activity was assessed. The routine susceptibility assay (minimum inhibitory concentration determination) was carried out by means of the broth macrodilution method. Bactericide activity was determined counting the colony-forming units per milliliter (cfu/mL) in plate. The O(2)(·-) production was determined by means of an indirect photobiological assay (Nitroblue Tetrazolium test), and the production of (1)O(2) was followed using an indirect steady-state method, with methionine as the (1)O(2) chemical quencher.
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