Methods: S. mutans (UA159) were grown overnight and transferred into 96 well microplate (NUNC). Samples were irradiated with CO2 laser (wavelength 10.6μm) at different power and irradiation time (energy densities ranging from 12 J/cm2 to 378 J/cm2). Viability of S. mutans was measured by CFU/ml and the sub-lethal energies were determined. Microarray analysis was conducted to test sub-lethal effect on bacterial DNA. S. mutans cDNA was prepared from isolated RNA of irradiated samples following overnight bacterial growth. In addition immediately after irradiation, Real-time PCR experiment was conducted for selected genes associated with stress response.
Results: CFU results showed that the lethal effect of CO2 laser irradiation on S. mutans begun at 378 J/cm2 (~1/2 log of bacterial reduction) under this experimental condition. Microarray analysis of overnight irradiated bacteria (86 J/cm2) showed that most genes were not affected significantly compared to control samples, whereas, Real Time RT-PCR results of genes associated with stress response showed a high up-regulation of groEL (24 fold change), dnaK (5.6 fold change) and recA (2 fold change).
Conclusions: Sub-lethal energies of CO2 laser had no significant continuous effect on S. mutans genome, whereas, a significant effect shown on mRNA levels immediately after irradiation, suggesting a stress response of S. mutans due to sub-lethal laser irradiation.
* This study was supported by the United States Israel Binational Science Foundation (BSF), No. 2005094.