*2.2. Subgingival Biofilm Sample Collection*

Subgingival samples were collected from four sites, selected by clinical criteria, namely the deepest PD with BOP per quadrant [30]. Sampling was performed after the isolation of the area by cotton rolls and after removing supragingival biofilm and/or calculus. Then, two medium-sized sterile paper tips (Maillefer, Ballaigues, Switzerland) were consecutively inserted in each site for 10 s, as subgingival as possible [30]. All eight paper points were pooled in a vial containing 1.5 mm of reduced transport fluid (RTF) [31] and sent to the Laboratory of Microbiology (Faculty of Dentistry, University Complutense of Madrid, Spain), to be processed within 24 h.
