Differences in the On- and Off-Tumor Microbiota between Right- and Left-Sided Colorectal CancerOriginal paper
Karen Pendergrass is a microbiome researcher specializing in microbiome-targeted interventions (MBTIs). She systematically analyzes scientific literature to identify microbial patterns, develop hypotheses, and validate interventions. As the founder of the Microbiome Signatures Database, she bridges microbiome research with clinical practice. In 2012, based on her own investigative research, she became the first documented case of FMT for Celiac Disease, four years before the first published case study.
Read MoreWhat was studied?
This pilot study asked whether gut bacteria differ between right- and left-sided colorectal cancer, on the tumor and in adjacent tissue. All patients had iron deficiency anemia and received intravenous iron to remove that confounder. Paired tumor and tumor-adjacent mucosal biopsies were taken after surgery. Bacterial DNA underwent 16S ribosomal RNA V4 sequencing. Diversity and taxon enrichment were compared by colon side and by tissue type using LEfSe.
Who was studied?
The cohort was 24 anemic patients with non-metastatic colorectal adenocarcinoma from the IVICA trial, 17 right-sided and 7 left-sided. All received intravenous ferric carboxymaltose before surgery. Anemia was defined below sex-specific World Health Organization thresholds. Patients on chemotherapy, with pre-diagnosis anemia, or with blood disorders were excluded. This was a human tissue-biopsy study. Parenteral rather than oral iron was used to limit gut luminal iron effects on bacteria.
What were the most important findings?
Off-tumor (tumor-adjacent) microbiota showed significantly greater diversity, richness, and abundance in the right colon than the left (p < 0.05), and formed distinct beta-diversity clusters (p = 0.003). On-tumor microbiota showed no such side differences. Sequencing yielded 4.4 million reads, averaging 109,122 per sample. Firmicutes, Bacteroides, Proteobacteria, and Fusobacteria made up over 95 percent of phyla. Right off-tumor tissue was enriched for Lachnoclostridium, Selenomonas, and Ruminococcus, while left off-tumor tissue was enriched for Epsilonbacteraeota, Campylobacteria, and related taxa. Left tumors differed from adjacent tissue at 24 taxa versus only 3 on the right.
What are the greatest implications of this study?
The right and left colon carry distinctive bacterial communities in tumor-adjacent tissue, but the on-tumor microbiota is more consistent across sites. This suggests a cancer-defined microbiota less governed by colon location. Left-sided tumors showed a large shift away from their surrounding flora toward a right-colon-like profile. That may relate to why right-sided tumors present later and more advanced. The sample is small (24 patients, uneven sides) and observational, so it identifies patterns rather than causal roles for bacteria in tumor progression.