Effects of type 2 diabetes and metformin on salivary microbiota in patients with chronic periodontitis

Diabetes is closely associated with periodontitis, however, the effects of type 2 diabetes and metformin treatment on the salivary microbiota in chronic periodontitis patients are still insufficiently studied. Saliva was collected from ten patients with moderate to severe chronic periodontitis (CP group) and 20 patients with type 2 diabetes mellitus (T2DM) and moderate to severe chronic periodontitis (ten patients were newly diagnosed with diabetes without drug treatment (DM group), and ten patients were treated with metformin (CP-DM-MET group)). Total DNA was extracted. DNA amplicons of the V3–V4 hypervariable regions of the 16S rRNA gene were generated and subjected to high-throughput sequencing. There was no significant difference in the alpha diversity of the salivary microbiota (Observed_Species, Shannon, Simpson, ACE, Chao1 index) among the three groups. The dominant phyla with relative abundances greater than 1% were Firmicutes, Proteobacteria, Bacteroidota, Actinobacteriota, Fusobacteriota, and Spirochaetota, and no significant difference was found among the three groups. Compared with the CP group, the relative abundance of twelve genera was found changed in CP-DM group, for example, Aggregatibacter, Unclassified_f_Neisseriaceae, Parvimonas, Erysipelotrichace_UCG-006, Atopobium, and Endomicrobium et al. Metformin treatment could partly restore the abundance of several genera in CP-DM, such as Acholeplasma and Comamonas. Compared with the CP group, genus Lactobacillus, Parvimonas, Norank_f_norank_o_Absconditabacteriales_SR1, and Acholeplasma changed significantly in CP-DM-MET group. Plaque index (PLI) was positively correlated with Prevotella and Lactobacillus but negatively correlated with Haemophilus, Lautropia, Unclassified_f_Pasteurellaceae, and TM7x. In conclusion, there was a significant difference in the salivary microbiota of patients with chronic periodontitis complicated by T2DM. Treatment with metformin partially alleviated the alteration in salivary microbiota caused by T2DM.