Phototargeting oral black-pigmented bacteria

Abstract

We have found that broadband light (380 to 520 nm) rapidly and selectively kills oral black-pigmented bacteria (BPB) in pure cultures and in dental plaque samples obtained from human subjects with chronic periodontitis. We hypothesize that this killing effect is a result of light excitation of their endogenous porphyrins. Cultures of Prevotella intermedia and P. nigrescens were killed by 4.2 J/cm2, whereas P. melaninogenica required 21 J/cm2. Exposure to light with a fluence of 42 J/cm2 produced 99% killing of P. gingivalis. High-performance liquid chromatography demonstrated the presence of various amounts of different porphyrin molecules in BPB. The amounts of endogenous porphyrin in BPB were 267 (P. intermedia), 47 (P. nigrescens), 41 (P. melaninogenica), and 2.2 (P. gingivalis) ng/mg. Analysis of bacteria in dental plaque samples by DNA-DNA hybridization for 40 taxa before and after phototherapy showed that the growth of the four BPB was decreased by 2 and 3 times after irradiation at energy fluences of 4.2 and 21 J/cm2, respectively, whereas the growth of the remaining 36 microorganisms was decreased by 1.5 times at both energy fluences. The present study suggests that intraoral light exposure may be used to control BPB growth and possibly benefit patients with periodontal disease.

FIG. 1.

Emission spectrum of the light source provided by BriteSmile, Inc., Walnut Creek, Calif.

FIG. 2.

Survival fraction of oral species following irradiation of planktonic cell suspensions. A survival fraction of 100% represents 10⁸ microorganisms. Each plotted point is the mean of triplicate determinations.

FIG. 3.

Chromatogram of a mixture of standard porphyrins in the order of decreasing retention time and HPLC analysis of the porphyrin content of BPB.

FIG. 4.

Reduction in total CFU counts after exposure of pooled dental plaque samples to visible light at 4.2 and 21 J/cm². Each bar is the mean of values obtained from 15 patients (data from each patient were representative of three independent experiments). Error bars denote the standard error of the mean.

FIG. 5.

Inhibition of BPB growth after exposure to light. The bars represent the ratio of DNA counts before exposure to light to those after exposure to light (mean ± standard error of the mean, n = 15 subjects).

FIG. 6.

Suppression of the BPB group (four species) and that of 36 other microorganisms after exposure of dental plaque to light. The bars represent the mean growth inhibition ratios (ratios of DNA counts before exposure to light to those after exposure to light) obtained from 15 patients (data from each patient were representative of three independent experiments), and the error bars represent the standard error of the mean.