Repeated professional removal of plaque and tooth-stone by means of deep scaling and root planning is able to avoid a further progression of marginal periodontitis. This therapeutical measures lead to changes in the bacterial composition of subgingival plaque. Important bacteria for the etiology of periodontitis are reduced. 8-12 weeks after therapy the recolonization of the complex periodontitis flora is finished. Therefore a repeated subgingival scaling is necessary to make the treatment successful.