Protochlorophyllide: a new photosensitizer for the photodynamic inactivation of Gram-positive and Gram-negative bacteria

Abstract The growing resistance against antibiotics demands the search for alternative treatment strategies. Photodynamic therapy is a promising candidate. The natural intermediate of chlorophyll biosynthesis, protochlorophyllide, was produced, purified and tested as a novel photosensitizer for the inactivation of five model organisms including Staphylococcus aureus, Listeria monocytogenes and Yersinia pseudotuberculosis, all responsible for serious clinical infections. When microorganisms were exposed to white light from a tungsten filament lamp (0.1 mW cm−2), Gram-positive S. aureus, L. monocytogenes and Bacillus subtilis were photochemically inactivated at concentrations of 0.5 mg L−1 protochlorophyllide. Transmission electron microscopy revealed a disordered septum formation during cell division and the partial loss of the cytoplasmic cell contents. Gram-negative Y. pseudotuberculosis and Escherichia coli were found to be insensitive to protochlorophyllide treatment due to the permeability barrier of the outer membrane. However, the two bacteria were rendered susceptible to eradication by protochlorophyllide (10 mg L−1) upon addition of polymyxin B nonapeptide at 50 and 20 mg L−1, respectively. The release of DNA and a detrimental rearrangement of the cytoplasm were observed.