Scanning electron microscopy of dentin caries. Experimental in vitro studies with Streptococcus mutans

This study was performed to gain better insight into the mechanisms involved in carious destruction of human dentin by Streptococcus mutans. In particular, bacterial colonization of dentin surfaces and bacterial invasion in dentin were studied. Streptococcus mutans (S. mutans), strain NCTC 10449, was grown on sterile dentin blocks in a 10% CO2 atmosphere at 37 degrees C. After 72, 120, 144 and 288 h of incubation the specimens were processed for scanning electron microscopic examination. The colonization of the dentinal surface progressed slowly and was nearly complete after 288 h. Invasion of S. mutans into the dentinal tubules was found occasionally and was limited to the initial 5 micron of the tubular lumen. The acid metabolites produced by S. mutans, caused lesions of the dentinal structures in the close proximity of the bacteria. From the results of this in vitro study it appears that carious destruction of exposed dentinal surfaces by S. mutans mainly occurs at the exposed dentin after it has been colonized by bacteria. Destruction of the deeper dentinal layers by bacteria invading the dentinal tubules may play a less important role. However, in the few cases where S. mutans invaded the dentinal tubules, rapid destruction of the peritubular dentin sheath occurred. In addition, the possibility remains that acid metabolites produced by S. mutans diffuse into the dentinal tubules and cause tissue damage in the deeper parts of the dentin.