The invention relates to the use of 20-(4-carboxyphenyl)-2,13-dimethyl-3,12-diethyl-[22] pentaphyrine as antibacterial agent in photodynamic treatment. This expanded porphyrin derivatized in position 4 with a carboxyphenyl group proved very active after photo-oxidation both against S. aureus and E. hirae. Its high antibacterial activity and its low toxicity make this pentaphyrine particularly useful as antimicrobial agent both for photodynamic therapy against bacterial infections and in the disinfection of microbiologically contaminated liquids.
