Bacteriostatic Action of Irradiated Dye Media

The writer has observed that the growth of bacterial organisms is inhibited on culture plates which contain photosensitizing dyes, and which have been exposed to light of relatively low intensities. The culture media used were eosin-methylene blue, and Salle plates. the latter containing erythrosine, brom-cresol-purple, and methylene blue. When B. coli and B. aerogenes were streaked upon such irradiated plates, growth generally did not occur when the plates were subsequently incubated at 37°C. Plain agar plates containing no dyes showed no inhibition of growth when subjected to the same treatment.

In some cases the quantity of light which produced these changes in the plates was very small, e. g., an exposure 2 1/2 minutes to the diffuse light of the laboratory inhibited growth on the Salle plates, although 30 minutes was the minimum time of exposure which produced inhibition on eosin-methylene blue.

Measurements of the intensity of incident light can mean very little beyond the practical finding that ordinary diffuse laboratory light is effective in producing noticeable inhibition. This is true because the bacterial growth is limited to the surface, which is also the region of greatest absorption of light. Thus if sufficient light is absorbed at the surface layer to activate 90% of the dye molecules in this region, a great increase in intensity could not greatly increase the inhibitory effect on bacterial growth, although it might produce more change at greater depth. We have found that the inhibitory effect can be removed by washing the surface of the agar, indicating that the effects are generally limited to this region. This may also be a function of the O₂ concentration, which must be greatest at the surface, since O₂ is a necessary component of the common photochemical reaction of these dyes. 1 , 2