Investigators hypothesize that good oral hygiene may inhibit P. gingivalis, a keystone periodontal pathogen, entering mature dental plaque, which leads to a higher ratio of S. cristatus and P. gingivalis. Investigators will educate and train periodontitis patients following periodontal therapy to maintain periodontal health. Initial and intense education of periodontal health maintenance will be provided and the ratio of S. cristatus and P. gingivalis in dental plaques will be compared and analyzed for correlation between patient's execution of periodontal health maintenance and education tools or the ratio of S. cristatus and P. gingivalis.

The periodontal health maintenance will be evaluated by plaque index, periodontal pocket, and gingival inflammation status. Participants will be assigned into two groups after the second dentist visit based on their execution of periodontal health maintenance. Participants whose number of P. gingivalis is unchanged or higher compared to that found at baseline will be considered poor maintenance. Those not having maintained a good dental hygiene will further be randomly assigned to two groups. One group will be given more intense oral health education, while the other one will receive the same education as post therapy. The result of intensive education and a shift in ratio of S. cristatus and P. gingivalis will then be measured for both groups at the third dentist visit (9 months after initial periodontal treatment) and the effectiveness of the intervention will be tested at this stage.

Inclusion Criteria:

• Subjects should have > 16 teeth, 2 or more interproximal sites with CAL >4 mm and 2 or more with pocket depth >5 mm.

Exclusion Criteria:

• No scaling and root planing within the previous year or periodontal surgeries in the previous five years.

De-identified individual participants data for all primary and secondary outcome measures will be made available.