Your Tongue’s Bacteria May Reveal Whether You Have Crohn’s or Colitis

A swab of the tongue is about as low-tech as clinical samples get. But a study published in June 2025 in Microorganisms by Jiaxin Shen and colleagues suggests that the microbial film coating the dorsal tongue surface carries surprisingly specific signals about inflammatory bowel disease, and that those signals differ meaningfully between the two main IBD subtypes. That distinction matters because Crohn's disease and ulcerative colitis overlap in symptoms but diverge sharply in biology and treatment.

Tongue microbiome signatures separate UC, Crohn's, and healthy controls

The researchers enrolled 158 participants: 94 healthy controls, 45 people with Crohn's disease, and 19 with ulcerative colitis. All IBD patients had active disease. Tongue coating samples were collected with sterile swabs, snap-frozen, and analyzed by sequencing the V3-V4 region of the bacterial 16S rRNA gene on an Illumina NovaSeq 6000 platform. The approach is cost-effective and widely used, though as microbiome sequencing methods have evolved, researchers note that 16S sequencing provides limited resolution at the species and strain levels compared with full metagenomic approaches.

Despite that limitation, the compositional differences were striking. Healthy controls clustered around genera associated with a balanced oral ecosystem: Streptococcus, Haemophilus, and Rothia were the dominant markers. Ulcerative colitis patients showed a different picture entirely, with enrichment of Neisseria and Porphyromonas alongside a depletion of Firmicutes and a rise in Proteobacteria. Crohn's patients diverged from both: their tongue coatings were enriched for Prevotella, Veillonella, Actinomyces, Leptotrichia, and an unusual taxon called TM7x. Beta-diversity analyses using both Bray-Curtis and Jaccard metrics confirmed that the three groups had statistically distinct community compositions (PERMANOVA p=0.001 for both).

Ulcerative colitis shows the sharpest drop in microbial diversity

Alpha diversity, which measures how evenly a microbial community is distributed within a sample, told a particularly clear story for UC. Patients with ulcerative colitis had significantly lower Shannon and Simpson diversity indices compared with both healthy controls and Crohn's patients (p=0.010 and p=0.006, respectively), suggesting a flattened, less balanced community. Crohn's patients, interestingly, showed diversity values closer to healthy controls, pointing toward a subtler or differently structured dysbiosis rather than simple community collapse.

The co-occurrence network analysis reinforced this picture. Healthy controls had dense, interconnected microbial networks. Ulcerative colitis patients had a simplified, less connected network. Crohn's disease fell in between, with Veillonella, Streptococcus, Rothia, and Lachnoanaerobaculum emerging as major hub taxa. One finding drew particular attention: TM7x, an obligate epibiont that lives on the surface of Actinomyces cells rather than free in the environment, was consistently enriched in the Crohn's group. Its co-enrichment with Actinomyces suggests a possible symbiotic configuration specific to CD, though the authors are careful to note that network patterns reflect statistical associations, not proven ecological interactions.

Why a non-invasive oral screen for IBD subtypes is appealing

IBD diagnosis currently depends on colonoscopy, biopsy, and imaging, procedures that are invasive, expensive, and not well-suited to routine monitoring or population screening. The tongue coating has practical advantages: sampling is non-invasive, reproducible, and cheap. The dorsal tongue surface provides a stable biofilm that yields consistent microbial profiles across repeated samples, which is important for any tool intended for clinical tracking.

The idea that the mouth harbors disease-relevant microbial information is not new. Researchers have explored oral microbiome markers for IBD diagnosis in other contexts, and machine learning approaches have already shown that oral microbial profiles can separate Crohn's from colitis with meaningful accuracy. A separate Mendelian randomization study in East Asian populations found evidence for oral microbiota causally linked to IBD, adding biological plausibility to what might otherwise look like correlation. The oral-gut axis, a conceptual framework in which oral bacteria colonize the gut by being swallowed, offers one mechanism: microbes that thrive on the tongue may seed the intestine and contribute to mucosal inflammation.

Real limitations that temper the findings

This is an emerging area of research, and the study's limitations are significant enough to warrant honest acknowledgment. The cross-sectional design means the researchers captured a single snapshot; they cannot say whether tongue dysbiosis precedes gut symptoms, follows them, or simply co-occurs. The ulcerative colitis cohort was small at just 19 patients, which reduces statistical power and increases the chance that the observed patterns are specific to this sample rather than UC broadly.

The study also lacked standardized data on diet, smoking, oral hygiene, and periodontal status, all of which influence the oral microbiome independently of IBD. Without controlling for those variables, it is hard to know how much of the difference between groups reflects disease biology versus lifestyle confounders. As oral microbiota research limitations are widely acknowledged in the field, most current studies remain correlational; direct causal evidence linking specific tongue bacteria to gut inflammation is still sparse.

The random forest and SPIEC-EASI network analyses used here were explicitly exploratory and have not been externally validated. The LEfSe analysis consistently identified the same marker taxa across methods, which is reassuring, but replication in independent cohorts is the necessary next step.

What comes next for oral microbiome-based IBD monitoring

If future studies validate these signatures in larger, prospective cohorts, the tongue swab could become a complement to existing IBD monitoring tools, useful for initial screening, tracking disease activity over time, or stratifying patients before treatment. Longer term, the microbial targets identified here, particularly the Prevotella–Veillonella-TM7x cluster in Crohn's, could inform microbiome-directed IBD therapies such as live biotherapeutic products designed to restore specific bacterial populations. That future remains speculative for now, but the groundwork, a standardized sampling framework, validated sequencing pipeline, and subtype-specific microbial signatures, is in place to build on it.

Evidence rating

EMERGING — 80/100

Caveats:

• Cross-sectional design: this study shows association, not causation — it cannot determine whether oral microbiome changes drive IBD or result from it.
• The ulcerative colitis group was small (n=19), limiting statistical power and the reliability of UC-specific findings.
• No independent validation cohort was used; random forest and network findings are exploratory only.
• 16S rRNA sequencing cannot resolve microbial differences at the species or strain level.
• Key confounders — diet, oral hygiene, smoking, and medication — were not uniformly controlled.
• Funding source was not disclosed by the authors.

Cross-sectional observational study of 158 participants (94 healthy controls, 19 ulcerative colitis, 45 Crohn's disease) using 16S rRNA V3–V4 amplicon sequencing of tongue-coating samples. The study identifies associations between IBD subtype and oral microbial composition but cannot establish causality, and the ulcerative colitis group is underpowered. Candidate discriminative taxa identified by LEfSe and random forest analyses have not been validated in an independent cohort.

References

1. Jiaxin Shen, Xing Yu, Jinni Xu, Zhihua Zheng, Weiwei Zheng (2026). Characteristics of the Tongue Coating Microbiome and Its Subtype Differences in Patients with Inflammatory Bowel Disease. Microorganisms. https://doi.org/10.3390/microorganisms14061381
2. Methods in Microbiome Research: Past, Present and Future – PMC. NIH. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6062534/
3. Sang-Bum Kang, Hyeonwoo Kim, Sangsoo Kim, et al. (2023). Potential Oral Microbial Markers for Differential Diagnosis of Crohn’s Disease and Ulcerative Colitis Using Machine Learning Models. Microorganisms. https://doi.org/10.3390/microorganisms11071665
4. Rong-Xin Xie, Xin-Yu Ci, Yu-Bao Xue, Mei Liu, Mei-Juan Zhang (2026). Association between oral microbiota and inflammatory bowel disease: A 2-sample Mendelian randomization study in East Asian populations. Medicine. https://doi.org/10.1097/md.0000000000047909
5. Xiao-Kai Chen, Tao Mao, Xiang Song, Lin-Lin Ren (2025). The Oral Microbiota and Its Implications for Inflammatory Bowel Disease: A Literature Review. International Journal of General Medicine. https://doi.org/10.2147/ijgm.s549434
6. Andrea Carolina Quiroga-Centeno, Konstantina Atanasova, Matthias Philip Ebert, Anne Kerstin Thomann, Wolfgang Reindl (2025). Emerging microbiome-directed therapies in inflammatory bowel disease: beyond diet modification and FMT. Seminars in Immunopathology. https://doi.org/10.1007/s00281-025-01066-5