6. Conclusion

1. PDT using methylene blue (MB) can inactivate E. coli cells, requiring about 16 J/cm² of 630 nm wavelength light to achieve 99% killing.
2. The amount of MB that accumulates in E. coli cells when the cells are suspended in MB at 0.001 mg/ml solution for 30 min was 0.122 [mg MB/cm ³ cells], which corresponds to a molar concentration of 381 [μM]. Hence, the E. coli cells concentrate the MB 122-fold.
3. The PDT threshold for inactivation of E. coli is defined as the number of photons absorbed by MB within E. coli per cm³ of cells, and equals 1.5x10²¹ [photons/cm³]. This value is a bit higher than the range of PDT thresholds reported elsewhere for various photosensitizers and target cells, which extends from about 10¹⁸-10²⁰. This observation is consistent with the known difficulty in killing gram negative bacteria using photodynamic inactivation.

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