A simple experiment to show photodynamic inactivation of bacteria on surfaces

New suitable approaches were investigated to visualize the photodynamic inactivation (PDI) of bacteria immobilized on agar surfaces. The PDI capacities of a cationic photosensitizer (5,10,15,20-tetra(4-N,N,N-trimethylammoniumphenyl)porphyrin) and an anionic photosensitizer (5,10,15,20-tetra(4-sulfonatophenyl)porphyrin) were analyzed on a typical Gram-negative bacterium Escherichia coli following two procedures. In Experiment I, the E. coli cells were grown as lawn on agar surface containing the sensitizers spread in a small area (10 nmol in similar to 0.6 cm(2)). After irradiation with visible light (10 min, 90 milliwatts/cm(2)), no cells were grown in the area containing the cationic porphyrin. In Experiment II, small colonies (similar to 2-mm diameter) of E. coli on agar were treated with a solution of sensitizer (10 nmol) and irradiated with visible light for 3 h. Overnight incubation at 37 degrees C shows a growth delay of E. coli colonies treated with the cationic photosensitizer. In contrast, the anionic porphyrin did not produce appreciable photodamage. These experiments could be either used in an undergraduate project for natural science advance students or used for a postgraduate practical training course. This methodology illustrates the application of PDI to treat bacteria growing as localized foci of infection.