Photodynamic inactivation of Escherichia coli immobilized on agar surfaces by a tricationic porphyrin

The photodynamic activity of 5,10,15-tris[4-(3-N,N,N-trimethylammoniumpropoxy)phenyl]-20-(4-trifluoromethylphenyl)porphyrin iodide (A(3)B(3+)) has been studied in vitro on a typical Gram-negative bacterium Escherichia coli immobilized on agar surfaces. The results obtained for the tricationic A(3)B(3+) porphyrin were compared with those of 5,10,15,20-tetra(4-N,N,N-trimethylammoniumphenyl)porphyrin p-tosylate (TTAP 41), which is a standard active sensitizer established to eradicate E coli in cellular suspension. The photobleaching of these porphyrins in solution was evaluated by decay in absorbance and in fluorescence. In both cases, a higher photostability was found for A(3)B(3+) than for TTAp(4+). Photodynamic inactivation capacities of these sensitizers were analyzed in E coli cells immobilized on agar surfaces. Small colonies were treated with different amount of sensitizer (0-14 nmol) and irradiated with visible light for 3 h. The light source used was either a projector or midday sun. The A(3)B(3+) porphyrin produced a growth delay of E coli colonies on a at surfaces. Similar result was obtained irradiating only one isolated colony through an optical fiber. Under these conditions, A(3)B(3+) porphyrin shows a high activity to inactivate localized bacterial cells. The higher photodynamic activity of A(3)B(3+) was confirmed by mechanical spreading of the colonies before treatment. This procedure produces complete inactivation of E coli cells on the agar surface. Therefore, tricationic A(3)B(3+) porphyrin is an interesting sensitizer with potential applications in photodynamic inactivation of bacteria growing as localized foci of infection. (c) 2006 Elsevier Ltd. All rights reserved.