Action of light on Micrococcus roseus

Micrococcus roseus is killed by exposure to a continuous spectrum of white light (3000 ft-c (32 292 lm/m2)) in the presence of air and the dye, toluidine blue. Comparison of rates of photodynamic killing for log and stationary phase cells and for two pigmentation mutants indicated that photosensitivity could not be correlated with amount of carotenoid or with carotenoid-chromophore length. Moreover, cells grown in medium containing diphenylamine (DPA) did not contain significant quantities of colored carotenoids; they were not as sensitive to photodynamic killing as fully pigmented cells. Photodynamic killing was accompanied by selective release of magnesium and calcium and by release of UV-absorbing material. In the absence of added photosensitizer, M. roseus, the two mutants, or DPA-inhibited cells were not killed when exposed to white light at intensities as high as 22 000 ft-c (236 806 lm/m2) for as long as 9 h, and cells did not release UV-absorbing material. Under these conditions M. roseus and the yellow mutant did not release as much magnesium or calcium as when subjected to photodynamic killing, but DPA-grown cells leaked sodium, magnesium, and calcium. These observations are consistent with several suggestions: (i) carotenoids do not serve as photochemical buffers in M. roseus; (ii) growth in medium containing DPA or mutations leading to altered pigmentation change membrane organization leading to altered photosensitivity; (iii) M. roseus has an efficient repair mechanism which renders it resistant to photodynamic killing unless an exogenous photosensitizer is present.