Background: The aim of this study was to determine the distribution patterns of Porphyromonas gingivalis, Prevotella intermedia/nigrescens, and Actinobacillus actinomycetemcomitans in periodontitis patients after standard mechanical periodontal therapy, and to determine factors increasing the odds to detect these target organisms in treated sites.

Methods: Eight hundred fifty‐two (852) separate subgingival microbial samples were taken from the mesial and distal aspects of every tooth in 17 patients. Target organisms were identified culturally.

Results: The 3 microorganisms showed different persistence patterns: P. gingivalis was detected in a high percentage of subjects (59%), but in a low proportion of sites (5.4%). P. intermedia/nigrescens was detected in all subjects except one, and in 40.6% of the tested sites. Only 5 subjects were A. actinomycetemcomitans positive, but 2 of them showed a very high number of positive sites (44% and 75%, respectively). A highly significant relationship was found between a subject's tendency to bleed upon sampling and the number of P. intermedia/nigrescenspositive sites. A significant portion of the variation in frequency of persisting P. gingivalis could be explained by the frequency of persisting pockets deeper than 4 mm. No similar relationship could be established between clinical parameters and A. actinomycetemcomitans. On a site level, the odds of detecting P. gingivalis increased by a factor of 2.47 (P = 0.0001) for every millimeter of residual probing depth; the odds of detecting P. intermedia/nigrescens increased by a factor of 1.84 (P = 0.0001).

Conclusions: If, after standard mechanical periodontal therapy, a large number of sites continue to bleed, one may expect an increased number of sites positive for P. intermedia/nigrescens. If many deep pockets persist, a greater number of P. gingivalis‐positive sites can be expected. J Periodontol 2000; 71:14‐21.