Methylene blue added to suspensions of activated staphylococci in amounts sufficient to furnish 1 x 105 molecules of dye/bacterium inactivates the phage precursor content of the cells without causing cell death when the mixtures are exposed to strong light of 4000–8000 Å. There is a lag phase of approximately 15 minutes in the photodynamic inactivation of phage precursor by methylene blue. This delay seems to be due to a primary reaction between the cell and methylene blue after the completion of which exposure to light brings about the inactivation of precursor quite promptly.

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