Changes in Gene Expression of Streptococcus mutans in Planktonic Environment Following CO2 Laser Irradiation

Objective: The aim of this work was to study the effect of CO2 laser light on gene expression of Streptococcus mutans in a planktonic environment irradiated by sublethal energies. Background data: The use of the CO2 laser for oral clinical applications has great benefits over the traditional procedures, including a reduced bacterial environment. Materials and methods: In order to determine the sublethal irradiation dose, S. mutans in suspension was irradiated with a continuous wave CO2 laser (wavelength 10.6 mu m) for duration times of 0.5-1.5 min at fluences ranging from 65 to 536 J/cm(2), following bacterial growth and viability measurements. The effect of CO2 laser irradiation at sublethal dose on gene expression was evaluated by DNA microarray. Temperature rise in the suspension caused by laser irradiation was measured using thermocouple electrodes. Results: Bacterial growth and viability began to decrease after exposure to laser irradiation using 284 J/cm(2). Following irradiation with sublethal fluences of 134 and 268 J/cm(2), changes in gene expression were found with seven downregulated genes being common to both fluences. The temperature rise in the bacterial sample after irradiation with 268 J/cm(2) was from room temperature to only 35 degrees C. Conclusions: CO2 laser irradiation inhibited viability and growth of S. mutans in suspension in a dose-dependent manner. Irradiation of planktonic bacterial suspensions with sublethal fluences resulted in changes mostly of downregulation in gene expression in a fashion similar to S. mutans in biofilm, except that different genes were involved.