The oral microbiome of prediabetes: a scoping review.

OBJECTIVE: To map evidence on oral bacterial microbiome shifts in adults with prediabetes and compare them with normoglycemic and type 2 diabetes populations. DATA AND SOURCES: A scoping review in accordance with PRISMA-ScR was conducted. PubMed, Cochrane Central Register of Controlled Trials, Web of Science, Embase, and Scopus, were searched from inception to May 1, 2026, including English-language original clinical studies using high-throughput sequencing of oral samples from adults with prediabetes and normoglycemic and/or type 2 diabetes comparison groups. Findings were narratively synthesized by sample size, diversity measure, taxonomic composition and relative abundance, and predicted microbial function. STUDY SELECTION/RESULTS: 2,320 publications were screened, and nine cross-sectional studies met eligibility criteria. Samples composed of saliva, tongue coating, oral rinse, and dental plaque; all studies used 16S rRNA gene sequencing. Alpha and beta-diversity findings were heterogeneous, partly reflecting differences in sample size, glycemic definitions, study populations, and analytical methods. Prediabetes was not consistently distinguishable from normoglycemia, while diabetes more frequently differed from normoglycemia and/or prediabetes. Actinomyces and Prevotella were among the taxa more often reported as enriched at higher glycemic status, whereas Haemophilus and Neisseria were more frequently reported as depleted; however, taxonomic findings were not uniform across studies. Only one study reported computationally predicted functional profiles. CONCLUSION: Current evidence suggests that prediabetes may be associated with subtle oral microbiome alterations, but there is insufficient consistency to define a reproducible prediabetes-specific microbial signature. Standardized, longitudinal, and functionally resolved studies are needed to establish temporality, clinical utility, and potential mechanisms. CLINICAL SIGNIFICANCE: Identifying prediabetes-associated oral microbiome shifts may support earlier recognition of metabolic dysregulation and inform future preventive strategies. Longitudinal and functionally oriented studies are needed to determine whether oral microbiome changes precede, accompany, or result from progression toward diabetes.