__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 cm^{3} 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

where

>- E
- is the irradiance (0.13 W/cm
^{2}), - T
- is the exposure time (100 s),
- ε
- is the extinction coefficient of MB (4x10
^{4}cm^{-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/cm^{2} and λ/(hc) = 3.2x10^{18} photons/J.)
The value of PDT_{th} calculated above is

~1.5x10^{21} photons absorbed by photosensitizer per cm^{3} of cells,

which is comparable to literature reports of thresholds for photodynamic therapy of cells in tissues, in the range of 10^{18}-10^{20}. The value is rather high, however, consistent with the observation that *E. coli* gram (-) bacteria are difficult to kill with PDI.