Gram-negative Escherichia coli B. and Gram-positive Streptococcus faecalis were incubated with a water-soluble, second generation photosensitizer - Zn(II) - phthalocyanine - tetrasulphonate (ZnPcTS). The dark toxicity of the drug was checked at its different concentrations. Growth curves of treated and untreated cultures were assessed by turbidimetric measurement and compared by two-samples t-test. The quantity of incorporated phthalocyanine was determined by its fluorescence intensity and then correlated to the protein content of bacteria. ZnPcTS showed significantly greater uptake by Gram-positive S. faecalis than by Gram-negative E.coli B. The dye appears to interact with the two types of bacteria according to different pathways as suggested by the differences in temperature-dependence of the binding process. The amount of photosensitizer associated with the cells was also determined for different numbers of washing steps. The weakly bound fraction can be removed by two washing steps for both bacteria.

Keywords: Zinc-phthalocyanine-tetrasulphonate, cellular uptake, dark toxicity, photosensitization, Gram-Positive Bacteria, Gram-Negative Bacteria