Sensitization of cariogenic bacteria to killing by laser light

Objectives: Aim was to investigate the effect of light on the viability of Strep. sobrinus (Ss, NCTC 10921), Strep. mutans (Sm, DSM 20523) and L. acidophilus (La, DSM 20079) cells following photosensitization with toluidine blue O (TBO), azure a (AA) or bacteriopheophorbide a (BA). Methods: The microorganisms were grown over night in trypticase-soy and de Man-Rogosa-Sharpe broth, respectively. Before irradiation, 250 µl of these cultures were incubated with TBO or AA (200 µM, 100 µM, 10 µM) or BA (10 µM, 5 µM) in 24-well microtiter plates for 30 s resp. 120 s. Light from diode lasers was emitted at 633 nm resp. 761 nm with an energy of 1.2 W. The energy density varied between 10 J/cm² and 40 J/cm². After irradiation, cell suspensions were diluted and spreaded over Columbia agar plates with 5 % sheep blood or MRS agar plates. The plates were incubated under microaerophilic conditions for 48 h. Each experiment was reproduced twice. Cell viability was measured by performing colony counts. Experimental plates were compared to controls. Results: At an energy density of 40 J/cm², the viability in a 100 µM TBO and AA solution was: Ss 5.3 ± 0.9 % and 8.1 ± 3.2 %; Sm 2.3 ± 1.8 % and 10.8 ± 3.2 %; La 19.4 ± 6.7 % and 25.0 ± 7.3 %. In a 10 µM solution of BA, the viability of Ss and Sm was 0 %, but it was 16.2 ± 5.1 % for La. Generally speaking, cell viability was reduced with increasing radiation dose and sensitizer concentration. Conclusions: The photodynamic effect varies between the cell species. BA is more effective than TBO and AA. Energy densities < 40 J/cm² result in an incomplete inactivation of bacteria. The pre-irradiation time has a minor effect on the result. Partly supported by CeramOptec GmbH, Bonn, Germany.