This study was undertaken to evaluate the effect of Alternathera brasiliana (Amaranthaceae) extracts as photosensitizing
agents in photodynamic antimicrobial therapies (PACT) against Staphylococcus aureus, Staphylococcus epidermidis
and Candida dubliniensis. The crude hexane and ethanol extracts were obtained from A. brasiliana whole plant
and showed absortion from 650 to 700nm. Also, singlet molecular oxygen (1O2) production (type II photosensitization reaction)
was examined, and the results show that 1,3-diphenylisobenzofuran photodegradation was greatly enhanced in the
presence of the A. brasiliana extracts. One plate in each assay was irradiated while the other was not irradiated, the number
of colony-forming units per milliliter (CFU/mL) was obtained, and data analyzed by the Tukey test. The chemical
composition of the extracts was determined by chromatographic and spectrometric techniques; steroids, triterpenes, and
flavonoids were identified. Laser irradiation alone at 685nm using diode laser, output power of 35mW, and energy of 28
J/cm2, or non-irradiated crude extracts in sub-inhibitory concentration did not reduce the number of CFU/mL significantly,
whereas irradiated hexane and ethanol extracts, in sub-inhibitory concentrations, inhibited the growth of these microorganisms.
The photoactivation of hexane and ethanol extracts of A. brasiliana, in sub-inhibitory concentrations, using
red laser radiation at 685nm had an antimicrobial effect.