5. Results: PDT threshold dose

PDT threshold dose for inactivation:
The threshold dose for inactivation was expressed as the number of photons absorbed by MB within E. coli per 1 cm3 volume of E. coli cells that prevented colony formation. With the following equation, the photons absorbed by MB per unit time was calculated based on the energy threshold denoted as the product of light irradiance and irradiation time, which were specified from the previous survival curve experiment. The value of PDTth, the threshold dose for MB photodynamic inactivation of E.coli cells, was calculated using the following equation:


is the irradiance (0.13 W/cm2),
is the exposure time (100 s),
is the extinction coefficient of MB (4x104cm-1/M]),
is the MB concetration inside the cell (3.8x10-4 M),
is wavelength of light (630x10-7 cm),
is Planck's constant, and
is the speed of light in vacuo.

(Note: E T = 16 J/cm2 and λ/(hc) = 3.2x1018 photons/J.) The value of PDTth calculated above is

~1.5x1021 photons absorbed by photosensitizer per cm3 of cells,

which is comparable to literature reports of thresholds for photodynamic therapy of cells in tissues, in the range of 1018-1020. The value is rather high, however, consistent with the observation that E. coli gram (-) bacteria are difficult to kill with PDI.

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