Mean total DNA probe counts (×105, ± SEM) of supragingival biofilm samples taken at entry (pre-cleaning), immediately post-cleaning and after 1, 2, 4 and 7 days of biofilm development from 38 periodontally healthy subjects and 17 subjects with periodontitis. Total counts from each sample site were averaged in each subject at each time point and then averaged across subjects at each time point separately in each clinical group. Subjects did not perform oral hygiene procedures during the 7 days of the study. Significance of differences between clinical groups at each time point was determined using the Mann Whitney test. Mean total DNA probe counts differed significantly over time in each group (Friedman test, p < 0.001). The bars represent mean total DNA probe counts and the whiskers the SEM at each time point.