Thermal tolerance of E. faecalis to pulsed heating in the millisecond range.

Our aim was to evaluate thermal damage to endodontic pathogen Enterococcus faecalis (E. faecalis) caused by sub-second laser-generated heat pulses by determining the parameters for the thermal damage survival curve (TDSC). A novel experimental method for thermal pulsing of bacteria in the millisecond range was developed. After cultivation, E. faecalis was inoculated on anodized aluminum substrate and heated with a pulsed Nd:YAG laser. Viability was assessed with both plate count and flow cytometry methods. An E. faecalis TDSC for single-pulse millisecond range heating times was derived from the Arrhenius equation. Results gained from single-pulse heating viability measurements were used to predict the bactericidal effect of multiple sequential pulses (pulse train), and compared to experimental measurements. The thermal damage model was then applied to determine the relationship between laser fluence, pulse width, and the viability decrease of E. faecalis in a simulated root canal disinfection procedure. The application of the model to calculate the required lethal laser fluence levels on dentin during endodontic laser procedures seems to indicate that for endodontic procedures, the sub-millisecond pulsed Nd:YAG lasers are more effective in comparison with continuous-mode diode lasers and will cause less undesirable bulk heating of the tooth and surrounding tissues. The results of the study can be applied to create a model for predicting the impact of sub-second temperature increase on viability of bacteria on various surfaces and calculate required fluences and pulse widths to achieve the aforementioned effects with laser pulses.