Antimicrobial activity of natural photosensitizing anthraquinones

It was demonstrated that anthraquinone-rich extracts, obtained from the phototoxic vegetal species Heterophyllaea pustulata Hook f. (Rubiaceae), exhibited bacteriostatic activity on Micrococcus luteus ATCC 9341, selectively inhibiting both oxacillin-sensitive and resistant Staphylococcus aureus, and antifungal activity against important opportunist microorganisms and against those involved in superficial mycosis, all from nosocomial origin. The acute in vitro cytotoxicity evaluation of each anthraquinone (AQ) isolated from these bioactive extracts, on a mammalian eukaryotic cell line (Vero cells), allowed us to establish the non-cytotoxic concentration range, which was used to evaluate the antimicrobial effect. Four from nine AQs tested, soranjidiol, rubiadin, damnacanthal and (S)-5,5'-bisoranjidiol, showed in vitro bacteriostatic/bactericide activity against S. aureus. The action mechanism seems to involve an increase in the levels of superoxide anion and/or singlet oxygen molecular. Moreover, the effect of actinic irradiation as a boosting agent for the production of both reactive oxygen species as well as its influence on antibacterial effect was assessed.