Human Lingual Biofilm Signatures in Gastrointestinal Disease: A Scoping Review

ABSTRACT Introduction and aims The tongue dorsum represents a structurally complex oral biofilm niche that has traditionally been regarded as indicative of systemic health. Recent advances in oral microbiome research and multi-omics technologies facilitate the systematic evaluation of the association between tongue coating biofilm signals and gastrointestinal disease states. However, it remains unclear whether these tongue-derived signals indicate systemic gastrointestinal pathology or merely reflect localised oral ecological disturbances. This review synthesises current evidence on tongue-derived microbial and multi-omics signatures across inflammatory, precancerous, and malignant gastrointestinal conditions, and evaluates their ecological, biological, and clinical significance. Methods A scoping review was conducted in accordance with PRISMA-ScR guidelines. Five electronic databases were searched (2010-2025) for human studies analysing tongue-coating samples using microbiome or multi-omics approaches. Results A total of twenty-five cross-sectional studies involving more than 4500 participants, primarily from East Asian populations, were included. Three recurrent patterns were identified: (1) stage-associated microbial restructuring, which involved mild non-specific alterations in inflammatory states, structured dysbiosis in precancerous conditions, and more consistent ecological configurations in malignancy; (2) convergence of functional multi-omics signals on lipid metabolism pathways across independent cohorts; and (3) significant modification of microbial and functional profiles by tongue coating phenotype, including colour, thickness and classification system. Conclusions Tongue-derived microbial and multi-omics signatures demonstrate reproducible cross-sectional associations with gastrointestinal diseases, exhibiting functional convergence across multiple omics layers. However, the reliance on cross-sectional study designs, absence of external validation and insufficient adjustment for confounding variables currently limit their clinical application as diagnostic biomarkers. Clinical relevance Tongue examination is clinically useful for assessing oral biofilm burden, mucosal pathology and oral hygiene in dental practice. It should not be used to diagnose gastrointestinal disease until validated by longitudinal, confounder-controlled studies.