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 PDT_th, the threshold dose for MB photodynamic inactivation of E.coli cells, was calculated using the following equation:

where

E

is the irradiance (0.13 W/cm²),

T

is the exposure time (100 s),

ε

is the extinction coefficient of MB (4x10⁴cm^-1/M]),

C_ps

is the MB concetration inside the cell (3.8x10^-4 M),

λ

is wavelength of light (630x10^-7 cm),

h

is Planck's constant, and

c

is the speed of light in vacuo.

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