The role of bacteria in periodontal disease will be briefly reviewed. The lecturer will present his work on the morphological features of dental plaque formation on epoxy resin crowns and describe the differences in dental plaque morphology between healthy and diseased surfaces. The importance of bacterial successions will be presented to explain shifts in the composition of the microbiota. The main bacterial species associated with various periodontal conditions will be reviewed. The

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... and disadvantages of monitoring the periodontal microbiota with dark field microscopy will be discussed. This will be followed by a presentation of the main data from several clinical studies. In these studies, differential dark field microscopy of the subgingival mcrobiota was related to the clinical status of the patients studied. The main result were as follows: 1. At healthy sites, the proportion of coccoid cells is elevated and the proportions of motile rods and spirochetes depressed as compared to diseased sites in the same mouth. 2. Mechanical and/or chemical treatment of dise-ased sites shifts the bacterial proportions of indicator microganisms from those see at untreated diseased sites toward those see at healthy sites. 3. A single episode of mechanical debridement causes a significant shift in the composition of the local microbiota for a mean duration of 42 days. 4. Patients previously treated for chronic periodontitis who have eievat proportions of spiochetes and/or motile bacteria are more likely to develop recurrences of the disease, as measured by localized increases in probing depth. 5. Results from ongoing longitudinal studies indicate that differential dark field microscopy of the subgingival microbiota can be used to customize recall intervals for patients with mild to moderate periodontal disease. The results indicate that some patients remain in good periodontal health despite the absence of professionally administered prophylaxes, while others require frequent visits. Microbial monitoring of the periodontal microbiota is helpful in identifying appropriate recall intervals for periodontally treated subjects.