Comparative photodynamic inactivation of antibiotic resistant bacteria by first and second generation cationic photosensitizers

Photodynamic inactivation (PDI) is an efficient approach against a wide range of microorganisms and can be viewed as an alternative for the treatment of microbial infections. In this work we synthesized first and second generation photosensitizers (PSs), the tetra-cationic porphyrin 2b and the new penta-cationic chlorin 3b, respectively, and evaluated their efficiency against two antibiotic resistant bacterial strains, Staphylococcus aureus and Pseudomonas aeruginosa. The PS 3b was obtained in very good yield by an easy synthesis method. The PDI studies were performed in parallel with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin tetra-iodide (1), a widely studied PS in PDI, and the obtained results were compared. Two different light ranges were used: white light (400-800 nm) and red light (530-800 nm) delivered at a fluence rate of 150 mW cm(-2). The results show that both strains, even though antibiotic resistant, were efficiently inactivated by the three PSs, chlorin 3b being the most effective. For the Gram positive bacterium S. aureus a 7.0 log reduction was observed after 5-10 min of irradiation, at a concentration of 0.5 mu M, whereas for the Gram negative P. aeruginosa, similar photoinactivation occurred at a higher PS concentration (10 mu M) and after a longer irradiation period (30 min). The synthetic chlorin 3b can be regarded as promising for the treatment of bacterial infections under red light, which penetrates deeper in living tissues. The results of this study open the possibility to prepare a new series of chlorin-type derivatives to efficiently photoinactivate Gram (+) and (-) antibiotic resistant bacteria. The efficient PDI with the chlorin 3b indicates high potential for the use of a TPPF20 scaffold in the preparation of new generation PSs based on cationic chlorin derivatives.