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 μm) for duration times of 0.5-1.5 min at fluences ranging from 65 to 536 J/cm², 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/cm². Following irradiation with sublethal fluences of 134 and 268 J/cm², 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/cm² was from room temperature to only 35 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.