Inactivation of Staphylococcus aureus by photodynamic action of hypocrellin B

Staphylococcus aureus is a common opportunistic pathogen causing human infections. In the present study, we investigated photodynamic inactivation on S. aureus using hypocrellin B from a traditional Chinese herb.

After incubation of S. aureus cells with different concentrations of hypocrellin B for 60 min, subsequent light irradiation from a novel LED light source with wavelength of 470 nm was combined with the energy density of 0.3 J/cm(2). Phototoxicity of hypocrellin B on S. aureus was investigated by colony forming unit assay. Membrane permeability was measured using confocal laser scanning microscopy (CLSM) with propidium iodide (PI) staining. Intracellular reactive oxygen species (ROS) level was measured using flow cytometry (FCM) with DCFH-DA staining. Morphology and structure of treated and controlled bacteria were observed using transmission electron microscopy (TEM).

Activity of S. aureus was substantially inhibited by photodynamic action in hypocrellin B dose-dependent manner. Notable damage to S. aureus were found in TEM after photodynamic treatment of hypocellin B. Red fluorescence of PI dye was observed more frequently in bacterial cells treated by photodynamic action of hypocrellin B than those of bacterial cells treated by the controls including sham control, hypocrellin B treatment alone and light irradiation alone. Intracellular ROS increase was also found in S. aureus treated by photodynamic action of hypocrellin B.

Photodynamic action of hypocrellin B markedly increased intracellular ROS level and caused damage to membrane permeability, resulting in cell death of S. aureus.