The relationship of probing depth and total microscopic counts to differential subgingival plaque morphology

The goal of this study was to investigate the relationship between probing depth, total microscopic counts, and specific microbial forms in subgingival plaque from young adults. Subgingival plaque samples were collected and probing depth recorded from 415 mesial surfaces of maxillary first molars in 266 individuals aged 20–40. Using dark field microscopy in a Petroff‐Hausser chamber, total organisms in each sample were estimated, as well as proportions of spirochetes, motile and non‐motile rods, cocci, and total motile organisms. Limits for decile ranks based on total organisms were determined and the mean percentage of each microbial form within each decile rank was calculated. Correlation coefficients (r) were computed between the dependent variables of microbial forms and the independent variables of total organisms, probing depth, and the combination of total organisms and probing depth. The percentage variability of microbial forms that could be explained by the regression of each dependent variable on each independent variable or combination of independent variables was estimated by r2. The results indicate: (1) differences in bacterial composition of subgingival plaque are associated with differences in total subgingival microorganisms and in probing depths, (2) approximately 15–20% of the variability in the proportions of microbial forms in subgingival plaque can be explained by differences in total subgingival microorganisms, (3) two to four times more variability in the proportions of microbial forms in subgingival plaque can be explained by differences in total subgingival microorganisms than can be explained by differences in probing depths, and (4) considerable variability in proportions of microbial forms in subgingival plaque remains that cannot be explained by either differences in total subgingival microorganisms or by differences in probing depths.