PHOTOINACTIVATION OF BACTERIA - USE OF A CATIONIC WATER-SOLUBLE ZINC PHTHALOCYANINE TO PHOTOINACTIVATE BOTH GRAM-NEGATIVE AND GRAM-POSITIVEBACTERIA

The photosensitization of microorganisms is potentially useful for ste rilization and for the treatment of certain bacterial diseases. Until now, any broad spectrum approach has been inhibited because, although Gram-positive bacteria can be photoinactivated by a range of photosens itizers, Gram-negative bacteria have not usually been susceptible to p hotosensitized destruction. In the present work, it has been shown tha t the Gram-negative bacteria Escherichia coli and Pseudomonas aerugino sa, as well as the Gram-positive bacterium Enterococcus seriolicida, c an be photoinactivated when illuminated in the presence of a cationic water-soluble zinc pyridinium phthalocyanine (PPC). The degree of phot oinactivation is dependent on both the concentration of PPC and the il lumination time. In contrast, the three bacteria are not photoinactiva ted by illumination in the presence of a neutral tetra-diethanolamine phthalocyanine (TDEPC) or negatively charged tetra-sulphonated phthalo cyanine (TSPC). Uptake studies have revealed that the lack of activity of TSPC is due to the fact that it has very little affinity for any o f the organisms. However, the issue appears to be more complex than si mply the gross levels of cellular uptake, since TDEPC and PPC are both taken up by the organisms but only PPC shows activity. This indicates that the localization and subcellular distribution of the phthalocyan ines may be a crucial factor in determining their cell killing potenti al. Further analysis of the uptake data has revealed a cell-bound phot osensitizer fraction, which remains tightly associated after several w ashings, and another weakly bound fraction, which is removed by succes sive washings. Analysis of the cell killing curves, carried out after successive washings of E. coli exposed to PPC, has revealed that it is the tightly associated fraction that is involved in the photosensitiz ation. Taken together with other data, these results suggest that cati onic photosensitizers may have a broader application in the photoinact ivation of bacterial cells than the anionic or neutral photosensitizer s commonly used in photodynamic therapy.