Photoinactivation of Deinococcus radiodurans: An unusual gram-positive microorganism

Deinococcus radiodurans is an extremely resistant bacteria to both the mutagenic and lethal effects of ionizing radiation. In the present study, we examined photoinactivation of D. radiodurans by two photosensitizers, the hydrophilic cationic 5,10,15,20-tetra(4-N-methylpyridyl)porphine (TMPyP) and deuteroporphyrin (Dp), which is more hydrophobic. The effects of illumination with various lights at different wavelengths were also examined. Efficient photoinactivation was obtained when D. radiodurans cells were irradiated by blue light (400-450 nm) and treated with TMPyP at a concentration of 0.73 μM. Under these conditions the viability of the culture was found to be decreased by almost seven orders of magnitude after 2 min of treatment at an intensity of only 2 J/cm². Treatment of the bacterial cells by TMPyP or Dp resulted in an efflux of potassium and magnesium from the cells as well as phosphate loss as a function of light dose. These results can be regarded as a consequence of membrane damage, because transmission electron microscopy revealed cell membrane damage shortly after exposure of the treated bacteria to light. In addition, asymmetric septation and filamentous chromosomal DNA were seen. It seems that the cytoplasmic membrane damage in this bacterial species plays a more important role than in other gram-positive or even gram-negative bacteria, rendering this radiation-resistant bacterium susceptible to photosensitization.