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) following photosensitization with toluidine blue O (TBO), azure A (AA), and photochlorine I, II, or III.
Methods: The microorganisms were grown over night in trypticase-soy and de Man-Rogosa-Sharpe broth, respectively. Before irradiation, 200 µl suspensions of these cultures containing 5 x 106 bacteria/ml were incubated with TBO or AA (10 µM, 100 µM, 200 µM) or photochlorine I, II, or III (5 µM, 10 µM, 100 µM) in 96-well microtiter plates for 30 s resp. 120 s. Light from diode lasers was emitted at 633 nm resp. 660 nm with an output energy of 242 mW. The energy density varied between 1 J/cm² and 40 J/cm². After irradiation, cell suspensions were serially 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 (cfu). Experimental plates were compared to controls.
Results: Mean reductions in the viable counts of bacteria were between 4.1 and 5.2 log10 in all photosensitized and irradiated suspensions. No significant reduction was seen in the controls.
Conclusions: Substantial inactivation of cariogenic bacteria in planktonic suspensions could be achieved. The photodynamic effect varies between the cell species. Ss and Sm are slightly more susceptible than La. Minor differences exist between the effects of phenothiazinium-based and photochlorine photosensitizers. The highest energy density (40 J/cm2) achieved the greatest inactivation in all microorganisms. The pre-irradiation time has a minor effect on the result.