Your Relatives’ IBD Could Already Be Written in Your Gut Microbiome

Before any symptoms appear and long before a diagnosis, the guts of people with a family history of Crohn's disease may already be drifting toward a recognizable microbial pattern. A new study published in Biomarker Research by Serrano-Gómez G and colleagues analyzed fecal samples from hundreds of participants using shotgun metagenomics, finding that healthy first-degree relatives of Crohn's disease patients show specific taxonomic and virulence-gene signatures that partially mirror those seen in patients themselves. The findings are preliminary, but they point toward a potential microbiome-based early warning system for one of medicine's more difficult-to-predict diseases.

F. prausnitzii Depletion and Virulence Genes Mark Crohn's Relatives

The core signal came from a group the researchers called CD-HFDRs: healthy first-degree relatives of Crohn's disease patients. Despite having no diagnosis of inflammatory bowel disease, these individuals showed depletion of Faecalibacterium prausnitzii, a bacterium widely recognized as a marker of gut health whose loss is a consistent feature of active Crohn's disease. That alone would have been noteworthy. What made the finding more striking was a simultaneous enrichment of virulence factor genes (VFGs), genetic sequences carried by bacteria that help them adhere to and invade host tissue. Of 18 VFGs elevated in CD-HFDRs, 16 were also elevated in confirmed Crohn's patients, a substantial overlap suggesting these relatives are not simply a random slice of the healthy population.

One gene stood out in particular: fdeC, an adhesin gene carried by E. coli that helps the bacterium stick to intestinal cells. It ranked as a top predictive feature in machine learning models trained to distinguish high-familial-risk individuals from unrelated healthy controls. Together, these findings suggest that gut microbiota and genetic factors interact in IBD in ways that manifest in the microbiome before disease is clinically apparent.

Why Catching IBD Risk Early Matters for Millions of Families

IBD affects tens of millions of people worldwide, and its global prevalence is rising, compounding the burden on healthcare systems and patients alike. Having a first-degree relative with Crohn's disease or ulcerative colitis is one of the strongest known risk factors for developing the disease yourself. Yet there is currently no reliable, non-invasive way to identify who among those at-risk relatives will actually go on to develop IBD, and who will remain healthy.

A validated microbiome-based risk test could change that calculus significantly. Clinicians could theoretically monitor at-risk individuals more closely or consider microbiome-directed interventions before irreversible intestinal damage occurs. The study's machine learning classifiers performed impressively on this question: random forest models using species-level data and VFG profiles achieved an AUC of 0.966 for distinguishing CD relatives from healthy controls, and 0.946 for UC relatives. Those numbers suggest strong signal, though they will need to hold up in independent cohorts before anyone draws clinical conclusions.

How Shotgun Metagenomics Made the Study Work

The methodology here matters. The team used shotgun metagenomics on fecal samples from 68 Crohn's patients, 77 ulcerative colitis patients, 37 CD-HFDRs, 30 UC-HFDRs, and 497 unrelated healthy controls. Unlike 16S rRNA gene sequencing, which catalogs only bacterial identity at a coarse level, shotgun metagenomics sequences the entire genetic content of a fecal sample. That allowed the researchers to profile not just which species were present, but what functional pathways those microbes were running and, critically, which virulence genes they were carrying.

This kind of multi-omics profiling approach integrates species-level taxonomy, MetaCyc functional pathways, and VFG databases into a single analytical pipeline. The extra resolution was essential: functional pathway profiles in CD-HFDRs actually resembled healthy controls more than they resembled Crohn's patients, even while the taxonomic and virulence-gene layers showed clear disease-associated signals. This taxonomic-functional dissociation is one of the study's more intriguing puzzles and suggests that microbial community structure can shift before metabolic output changes meaningfully.

Caveats: Sample Size, Cross-Sectional Design, and UC's Subtler Signal

The study has genuine limitations that temper enthusiasm. The HFDR groups were relatively small, 37 for CD and 30 for UC, which limits statistical power and increases the risk that findings reflect cohort-specific noise rather than a generalizable biological pattern. The design is cross-sectional, meaning participants were measured once; there is no longitudinal data showing that individuals with these microbiome signatures actually go on to develop IBD at higher rates. That causal link remains unproven.

The signal was also notably weaker in UC relatives than in CD relatives. UC-HFDRs showed subtler microbiome alterations overall, even though classifier performance remained comparable. The clinical and molecular heterogeneity of IBD makes it difficult to determine whether this difference reflects genuine biology or the smaller sample sizes and shorter disease windows typical of UC studies. Additionally, the healthy control group of 497 was drawn from a different population context than the HFDR groups, a potential confound the researchers adjusted for statistically but could not fully eliminate.

Longitudinal Studies and Microbiome-Directed Therapies as Next Steps

The most important next step is a prospective longitudinal cohort: recruit healthy first-degree relatives now, characterize their microbiomes using this panel of species and VFGs, and follow them for years to see whether the flagged individuals actually develop IBD at higher rates. That kind of study would turn a cross-sectional association into predictive evidence.

If the virulence-gene signatures hold up as genuine risk markers, they could also become therapeutic targets. Emerging microbiome-directed IBD therapies, including rationally designed bacterial consortia and engineered bacteria, are already being developed for active disease; applying them earlier, in at-risk relatives before inflammation takes hold, is a plausible next horizon. The identification of specific pathobionts carrying adherence and invasion genes gives researchers a concrete molecular handle to work with, something more actionable than broad dysbiosis scores. Whether that handle translates into a clinical tool remains to be seen, but this study has made the case that looking for it is worth the effort.

Evidence rating

EMERGING — 80/100

Caveats:

• This is a cross-sectional observational study — it shows association, not causation; it cannot confirm that these microbiome changes precede or drive disease development.
• HFDR sample sizes are modest (n=37 for CD, n=30 for UC), which may limit the generalizability of machine learning model performance.
• Funding source was not disclosed, so independent replication cannot be confirmed at this stage.
• No longitudinal follow-up was performed, so it is unknown whether relatives with these microbiome signatures actually develop IBD.

Cross-sectional observational study using shotgun metagenomics in a multi-group design (n=709 total, including patients, healthy first-degree relatives, and unrelated controls). Results show associations between familial IBD risk and specific microbial and virulence gene signatures, but cross-sectional design precludes causal or temporal conclusions. Findings are internally consistent with prior IBD microbiome literature but have not yet been independently replicated in a separate cohort.

References

1. Gerard Serrano-Gómez, Soler Zaida, Marc Pons-Tarín, et al. (2026). Microbial, functional, and virulence biomarkers associated with familial risk of Crohn’s disease and ulcerative colitis. Biomarker Research. https://doi.org/10.1186/s40364-026-00950-y
2. The Gut Microbiota in Inflammatory Bowel Disease – Frontiers. frontiersin.org. https://www.frontiersin.org/articles/10.3389/fmicb.2021.718828/full
3. Christopher A. Lamb, Aamir Saifuddin, Nick Powell, Florian Rieder (2022). The Future of Precision Medicine to Predict Outcomes and Control Tissue Remodeling in Inflammatory Bowel Disease. Gastroenterology. https://doi.org/10.1053/j.gastro.2021.09.077
4. Precision Medicine: How multi-'omics will shape the future of IBD? – PMC – NIH. NIH. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8900010/
5. Stefan Schreiber, Konrad Aden, Florian Tran, Philip Rosenstiel (2025). Rise of precision medicine: can it deliver on its promise in IBD?. Gut. https://doi.org/10.1136/gutjnl-2023-330000
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