A Common Gut Pathogen Is Quietly Rewiring Tumors in Early Colorectal Cancer

Clostridioides difficile is not the bacterium most oncologists think about when they consider colorectal cancer. Yet a new preprint from Nicholas O. Markham and colleagues, posted to bioRxiv in June 2026, suggests it deserves serious attention: even at low abundance inside tumors, C. difficile appears to disproportionately alter the gene expression landscape of early-stage colorectal cancer. The findings are preliminary and have not been peer-reviewed, but the methodology is unusually rigorous for this kind of microbial mapping work.

C. difficile toxin B activates WNT signaling in colorectal tumors

The central finding is striking in its specificity. When the team examined the relationship between C. difficile and tumor tissue, they traced the effect to a single effector molecule: toxin B, also called TcdB. In experiments using human colonic organoids, TcdB triggered the nuclear localization of ELF3, an E-twenty-six family transcription factor. At the same time, TcdB caused cytosolic beta-catenin protein to accumulate and drove upregulation of WNT signaling, a pathway whose hyperactivation is a well-established driver of colorectal cancer progression. What makes this notable is the disconnect between the bacterium's prevalence and its apparent impact: C. difficile was present at relatively low numbers, yet its transcriptomic footprint was outsized compared to more abundant species. Research on C. difficile's role in colorectal carcinogenesis has been building for several years, with prior work pointing to its toxins as agents of epithelial barrier disruption and DNA damage, but direct spatial evidence inside human tumors has been lacking until now.

Why early-stage CRC and the absence of antibiotics matter

Colorectal cancer is the second most common cause of cancer-related death in the United States, and incidence rates are climbing in adults under 50, making early-stage biology an urgent research priority. This study was deliberately restricted to early-stage tumors, and the cohort was specifically chosen to exclude patients who had received preoperative oral antibiotics. That exclusion matters: antibiotics profoundly reshape the gut and tumor microbiome, and including antibiotic-exposed patients would have made it nearly impossible to read the native microbial signals. By focusing on antibiotic-naive tissue, the team captured what the tumor microbiome looks like before clinical intervention scrambles it. The eight distinct intratumoral bacterial niches they identified, each associated with unique tumor features and gene expression patterns, reflect the kind of ecological complexity that only becomes visible when you look carefully at untreated tissue.

How a spatial multi-omics atlas mapped 756 tumor regions

The methodological core of the study is a molecular atlas built from surgically resected tissue from 20 early-stage colorectal cancer patients. The team combined histopathological analysis, fluorescence in situ hybridization (FISH), whole exome sequencing, 16S rRNA amplicon bacterial DNA sequencing, and CODEX multiplex immunofluorescence. The element that ties it together is spatial transcriptomics, a technology that maps gene expression patterns to precise physical locations within a tissue section rather than averaging across a bulk sample. Applied to 756 distinct regions across the 20 specimens, spatial transcriptomics let the researchers ask not just which bacteria are present, but where they sit within the tumor architecture and what the surrounding human cells are doing at exactly that location. To make bacteria detectable within the spatial framework, the team designed and validated custom bacterial probes using 16S rRNA sequencing data, a technically demanding step that allowed them to co-register microbial and host transcriptomic data in the same tissue sections. The result is one of the more comprehensive spatial catalogues of the colorectal tumor microbiome produced to date, covering not only well-studied species like Escherichia coli and Fusobacterium nucleatum but also less-characterized organisms like Enterocloster aldenensis alongside C. difficile. Spatial transcriptomics technologies have been rapidly adopted in cancer research precisely because they resolve the spatial heterogeneity that bulk sequencing erases.

Open questions about C. difficile abundance, causality, and patient scope

Several important caveats apply. Most fundamentally, this is a preprint: the data and methods have not yet undergone formal peer review, and the conclusions should be treated as early-stage evidence. The cohort of 20 patients, while carefully chosen, is small, and it is not yet clear how the microbial niches identified here map onto broader CRC populations, different disease stages, or patients from diverse geographic and demographic backgrounds. The organoid experiments establish a plausible mechanistic chain from TcdB to ELF3 nuclear localization to WNT pathway activation, but organoids are simplified systems and the chain has not been validated in a larger clinical dataset. The low abundance of C. difficile in the tumor tissue also raises a question the study acknowledges: if the bacterium is present in such small numbers, what determines whether and when it produces enough TcdB to have a meaningful transcriptomic effect? The threshold and the conditions that set it remain undefined.

Bacterial niches as potential diagnostic and therapeutic targets in CRC

If the findings hold up under peer review and in larger cohorts, the implications extend in two directions. First, C. difficile and its toxin may represent an under-recognized contributor to early colorectal tumorigenesis, potentially relevant to explaining some of the rising incidence in younger adults who lack traditional risk factors. Second, the spatial niche framework itself could become a tool: if specific bacterial niches reliably predict distinct tumor gene expression states, they might eventually inform patient stratification or point toward microbiome-targeted interventions. The authors note that further studies are needed to define diagnostic and therapeutic targets for bacteria-associated CRC. Understanding the microbiome as an active participant in cancer progression rather than a passive bystander is a rapidly maturing area of oncology research, and a spatially resolved atlas of this kind adds a layer of precision that earlier sequencing studies could not provide. For now, the most honest read is that C. difficile has earned a place on the shortlist of microbial suspects in colorectal cancer biology, and the mechanism the team has sketched out is worth pursuing with urgency.

Evidence rating

INVESTIGATIONAL — 78/100

Caveats:

• This is a preprint and has not yet undergone peer review — findings should be considered preliminary.
• Study examined only 20 patient tissue specimens; findings require validation in larger, independent cohorts.
• The causal mechanism linking TcdB to WNT activation was validated in organoids, not in living humans, so clinical translation remains unconfirmed.
• Funding source was not disclosed by the authors.

Multi-omics observational study of 20 surgically resected early-stage colorectal cancer specimens, incorporating spatial transcriptomics across 756 tissue regions and organoid validation experiments. The study identifies associations between intratumoral bacterial niches and tumor gene expression patterns and proposes a mechanistic pathway linking C. difficile toxin B to WNT signaling activation; however, the small sample limits statistical power and the design cannot establish causation in human disease. This work has not yet been peer-reviewed, and independent replication in larger cohorts is needed.

References

1. Nicholas O Markham, Julia L Drewes, Emily H Green, et al. (2026). Spatial atlas highlights contribution of C. difficile in early-stage colorectal cancer. https://doi.org/10.64898/2026.06.18.733268
2. Ju Zou, Biyue Tian, Yuanyuan Xiao, Anhua Wu, Chunhui Li (2025). Clostridioides difficile: A suspected pro-carcinogenic bacterium for gastrointestinal tumors. Chinese Medical Journal. https://doi.org/10.1097/cm9.0000000000003834
3. Colorectal cancer statistics, 2026. NIH. https://pubmed.ncbi.nlm.nih.gov/38487779/
4. Spatial Transcriptomics – NCI – National Cancer Institute. cancer.gov. https://www.cancer.gov/research/technology/data-science/spatial-transcriptomics
5. Qichao Yu, Miaomiao Jiang, Liang Wu (2022). Spatial transcriptomics technology in cancer research. Frontiers in Oncology. https://doi.org/10.3389/fonc.2022.1019111
6. Can the Microbiome Decide the Fate of Cancer? A New Frontier in Oncology. NIH. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10300958/