Periodontitits is a bacteria induced inflammatory disease that destroys the supporting tissues of the tooth and leads to tooth loss. Treatment consists mainly of the mechanical cleaning of the tooth surfaces in order to remove the bacterial deposits (plaque and calculus). This procedure can reduce the level of pathogenic bacteria but it can not eradicate them. In severe cases, for the complete resolution of the disease, the elimantion of certain species of bacteria is needed. In order to achieve that, the combination of different regimens of antibiotics adjunctive to the mechanical treatment has been proposed. However, dosage and duration of antimicrobial therapy should be optimal and not excessive as issues may arise related to increased antimicrobial resistance in the population and the individual due to habitual prescription of wide-spectrum antibiotic regimens, horizontal gene transfer and genetic mutation. In the present study, in an effort to optimize the dosage and duration of the antimicrobial regimen, we will determine the pharmacokinetics (PK) and pharmacodynamic (PD) properties of the MET-AMO combination and of AZI in Gingival Crevicular Fluid (GCF), saliva and serum in severe periodontitis patients during and after either a 3-day or a 7-day course of treatment.