1. Primary objective To measure the gingival crevicular levels of A.actinomycetemcomitans and P.gingivalis after intra-pocket application of diode laser in severe periodontitis (stage 3 grade C) patients.
2. Secondary objective To compare the levels of the same microorganisms as in cases of diode laser therapy to those after conventional treatment with systemic antibiotic administration in severe periodontitis (stage 3 grade C) patients.
3. to measure clinical parameters probing depth,attachment loss and mobility grade

Inclusion Criteria:

1. Patients of both sexes having severe periodontitis (stage 3 grade C) with interdental clinical attachment loss (CAL) ≥ 5.
2. Patients' age between 15 and 35 years old with rapid bone loss showed by radiograph.
3. Systemically healthy patients.
4. Radiographic bone loss extending to middle or apical third of root.
5. Probing depth (PD) ≥ 6 mm.

Exclusion Criteria:

1. Use of antibiotics during the last 3 months.
2. Pregnancy.
3. Any systemic condition that might affect the study.
4. Use of mouthwash containing antimicrobials during the previous 3 weeks.
5. Smoking.