Thermotolerance of Acidogenic Bacteria Native to Dental Caries

Objectives: Dental caries develops, in large part, due to prolonged and localized acid production by multiple species of oral bacteria. The resulting infection causes demineralization of tooth enamel, which can lead to cavity formation. Acidogenic species implicated in caries development include Streptococcus mutans and Actinomyces odontolyticus. Previously, we have shown that microwave energy (ME) is highly effective in killing S. mutans in culture; however, effective killing (>99%) of A. odontolyticus required longer ME exposure at given intensity levels. Here, we test the hypothesis that species variation in thermal tolerance may alter bacterial response to heating effects of ME.
Methods: Initially, a pilot trial was used to determine the general thermal tolerance of S. mutans and A. odontolyticus incubated in a thermal water bath. Additional trials (n=or>4/temperature point) were repeated over 40-80^oC. Individual cultures of S. mutans and A. odontolyticus were grown in Trypticase Soy Broth and exposed to heated water baths for 30, 60, or 90 sec. The external (water bath) and internal (tube) temperatures were monitored. Following exposure, samples were serially diluted and plated for Colony Forming Units (CFUs). Experimental CFUs were counted and compared to control samples (37° C) to determine kill rate.
Results: Exposure of S. mutans to internal 65°C for 60sec showed 99.8% kill, whereas A. odontolyticus showed 40.0% kill. A. odontolyticus required an internal temperature exposure of 75°C for 60sec to achieve 99.9% kill.
Conclusions: These data indicate that A. odontolyticus is more thermally tolerant than S. mutans. They further correlate with the increased exposure time needed to effectively kill A. odontolyticus using ME. Together, we observed that heat may be an effective means of controlling two acid-producing bacterial species native to dental caries; differences in thermal tolerance may influence dose-response requirements of ME caries treatment.