Type 1 diabetes: associations with the oral microbiota, but no validated biomarker

A review that distinguishes targeted detection from overall microbiota profiling

The authors included 42 human studies on type 1 diabetes following a search of PubMed, Embase and Scopus covering publications through June 2025. The protocol had been prospectively registered in PROSPERO. Two reviewers independently performed study selection, data extraction and risk-of-bias assessment.

Most studies were cross-sectional: they observed glycemic control and microbial characteristics without tracking changes over time. Glycemic control was generally assessed using glycated hemoglobin, or HbA1c, with thresholds varying across studies. Saliva and subgingival plaque were the most frequently sampled sites.

The methods were too heterogeneous to allow a combined quantitative synthesis. The authors therefore compared the findings qualitatively, distinguishing studies of specific microorganisms from more comprehensive analyses of microbial communities.

Clearer associations for certain microorganisms

Targeted studies frequently reported an association between poorer glycemic control and certain microorganisms involved in periodontal disease, or the presence of Candida in the mouth. Findings for microorganisms associated with tooth decay, however, were inconsistent.

Only four studies used culture-independent profiling of overall microbial communities. They described differences in composition or diversity, but their small number and differing methods prevent any conclusion about a consistent increase or decrease in diversity.

The distinction matters: more frequent detection of a targeted microorganism is not enough to demonstrate an imbalance in the oral microbiota as a whole.

What these findings change, and what they do not establish

These associations cannot determine whether glycemic control influences oral microorganisms, whether oral changes contribute to glycemic control, or whether other factors explain their coexistence. Longitudinal studies using standardized methods are needed.

The authors consider that these observations support attention to established preventive dental care and periodontal assessment as part of diabetes follow-up. However, they do not justify using oral microbiota analysis to estimate glycemic control or predict the risk of oral disease.

What we can take away

The signal mainly concerns certain periodontal microorganisms and Candida. It demonstrates neither consistent oral dysbiosis nor the clinical usefulness of a microbiota test in people with type 1 diabetes.