Morphologic changes correlating to different sensitivities of Escherichia coli and Enterococcus faecalis to Nd : YAG laser irradiation through dentin

Background and Objective: Previous studies demonstrated the disinfecting po tential of Nd:YAG laser irradiation on the root canal system from an overal l quantitative viewpoint. The aim of this study was to evaluate the specifi c effect of irradiation through dentin on gram-negative and gram-positive b acteria with regard to their cell structure. Study Design/Materials and Methods: Sterile dentin samples of standardized size were divided into two sets of four groups with eight samples each. The first set was inoculated with Escherichia coli as the gram-negative test s train, the second set was inoculated with Enterococcus faecalis, which serv ed as the gram-positive test organism. The samples were then irradiated on the bacteria-free side in contact mode under constant scanning movement at an angle of 10 degrees by use of the fiber optic of the Nd:YAG laser. Upon laser treatment they were critical point dried and subjected to SEM investi gation. Another two sets of samples were prepared and irradiated in the sam e manner and evaluated by standard microbiological procedures to verify whe ther the observed morphologic alterations correlated to cell death. Results: SEM investigations revealed damage pattens that increased with the amount of energy applied. Whereas the gram-negative test organism showed i mmediate structural injury, the gram-positive test organism required repeat ed application of irradiation. The microbiological examination showed reduc tion of both bacterial strains, yet to different extents. Conclusion: Our study demonstrates the different morphologic impact of Nd:Y AG laser irradiation through dentin on representatives of the two main grou ps of bacteria. It shows that the construction of the cell wall is crucial for their individual sensitivity to laser treatment. Lasers Surg. Med. 26:2 50-261, 2000. (C) 2000 Wiley-Liss, Inc.