Red light kills bacteria via photodynamic action

With the increase in the number of antibiotic resistant strains of microorg anism, the search for alternative treatments of microbial infections become s all the more important. We report a novel method for bacterial inactivati on based on the optical excitation of the naturally occurring (endogenous) photosensitzing porphyrins by red light. In particular, the pathogenic Gram -positive porphyrin producing ATCC strains Propionibacterium acnes, Actinom yces odontolyticus and Porphyromonas gingivalis were investigated. Sensitiv e autofluorescence spectroscopy revealed that these bacteria naturally synt hezise the fluorescent photosensitizer protoporphyrin IX. In addition, bact erial plaque samples of periodontitis patients were studied. Non-labeled fl uorescent bacterial colonies were exposed to red light at 632.8 nm, 100 mW/ cm(2) light intensity and 360 J/cm(2) energy density using a helium-neon la ser. The survival rate after a single phototreatment with red light was fou nd to be 0.58 +/- 0.09 in the case of Propionibacteriun acnes, 0.30 +/- 0.0 4 in Actinomyces odontolyticus and 0.59 +/- 0.10 in Porphyromonas gingivali s compared to non-exposed bacteria suspensions. No photoeffect was found fo r the bacterium Streptococcus mutans which exhibited no detectable porphyri n autofluorescence. Red-light exposed plaque samples of patients showed sig nificant reduction of colony forming units by 50% as well as a pronounced p hotoeffect on the pigmented species Prevotella intermedia. Taken together, these results suggest the treatment with red light can be potentially emplo yed as an therapeutic method to inactivate certain pathogenic strains of po rphyrin producing bacteria without the use of external photosensitizers.