Altered Oral Microbiota Composition and Upregulation of Gingival ACE2 and TMPRSS2 Expression in Patients with Periodontitis and Type 2 Diabetes Mellitus

Periodontitis (P) and Type 2 Diabetes Mellitus (T2DM) are chronic inflammatory diseases that share pathophysiological pathways involving immune dysregulation and oxidative stress. Both conditions have been associated with increased susceptibility to viral infections, including SARS-CoV-2. In this regard, molecules associated with viral infection include angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2). This study aimed to evaluate the clinical periodontal status, oral microbiota composition, and the expression of ACE2 and TMPRSS2 in the oral epithelium and gingival tissue of patients with and without T2DM and P. Methods: This cross-sectional study enrolled 120 participants allocated into four groups based on periodontal and glycemic status: periodontally healthy non-diabetic individuals (PH non-T2DM), periodontitis without diabetes (P non-T2DM), periodontally healthy individuals with type 2 diabetes mellitus (PH T2DM), and periodontitis with T2DM (P T2DM), with 30 participants per group. Full-mouth clinical periodontal parameters were recorded by a calibrated examiner. Oral microbiota was assessed from unstimulated whole saliva, labial swab samples, and subgingival biofilm by selective culture and checkerboard DNA-DNA hybridization. Gingival exfoliative cytology and full-thickness gingival biopsies were obtained for immunohistochemical evaluation of ACE2 and TMPRSS2 expression. Cytomorphometric analysis and polymorphonuclear cell counts were performed on epithelial smears. Additionally, primary human gingival fibroblasts (HGFs) isolated from each group were stimulated with bacterial ligands (LPS, LTA, and PGN) to assess ACE2 and TMPRSS2 modulation by Western blot. Intergroup comparisons were performed using one-way ANOVA with Bonferroni post hoc correction and the Mann–Whitney U test, with statistical significance set at p < 0.05. Results: Diabetic patients exhibited higher plaque accumulation, clinical attachment loss, and bleeding on probing compared with non-diabetic individuals (p < 0.05). The diabetic groups showed significantly higher levels of Actinomyces, Fusobacterium, and Streptococcus spp., with decreased Staphylococcus counts. ACE2 and TMPRSS2 expression were markedly elevated in gingival epithelial cells of P T2DM patients, predominantly in basal and suprabasal layers. The nuclear-to-cytoplasmic ratio and polymorphonuclear cell counts were also increased in diabetic subjects. Conclusions: T2DM and P synergistically upregulate ACE2 and TMPRSS2 expression and alter the oral microbiota.