This Gut Bacterium Has a Built-In Weapon for Causing Colon Cancer

Doctors have known for decades that patients diagnosed with a gut bacterium called Streptococcus gallolyticus subspecies gallolyticus, or Sgg, are unusually likely to also have colorectal cancer. The connection is so reliable that a bloodstream infection with Sgg routinely triggers a colonoscopy referral. What has been missing is a mechanism: does the bacterium simply flock to tumors that are already growing, or does it help cause them? A study published in PLOS Pathogens supplies a concrete answer, and it comes down to a single piece of bacterial machinery.

Bacterial Secretion System Drives Colon Tumor Growth in Mice

Researchers identified a type VII secretion system, a molecular syringe-like apparatus bacteria use to inject proteins into neighboring cells, in an Sgg strain called TX20005. They named the locus SggT7SST05. Using Western blotting, the team confirmed the system is active: it secretes a protein called SggEsxA into the bacterial culture fluid, the calling card of a working T7SS. To test what this system actually does, the researchers built a mutant strain, TX20005Δesx, with the secretion system genetically deleted, then compared it head to head with the normal, "wild-type" bacteria.

The difference showed up immediately in cell culture. Wild-type Sgg adhered readily to HT29 human colon cancer cells and drove those cells to proliferate faster. The T7SS-deficient mutant lost much of its ability to stick to the cells and, notably, lost its ability to trigger that proliferation entirely. In mice, the pattern held at the level of a whole tumor. Using an azoxymethane (AOM)-induced mouse model of colorectal cancer, a standard method for studying how chemical carcinogens progress to tumors, mice colonized with wild-type Sgg developed significantly higher tumor burden than mice given the T7SS-deficient mutant or saline alone. The wild-type-colonized mice also showed higher average dysplasia grade (a measure of how abnormal the tissue looks under a microscope) and increased activation of beta-catenin, a signaling protein at the center of the Wnt pathway that governs how fast intestinal cells divide.

Why a Bacterial Weapon System Matters for a Cancer Linked to Nearly Two Million Diagnoses a Year

Colorectal cancer is diagnosed in roughly 1.9 million people worldwide each year, according to the World Health Organization's cancer agency, and researchers have spent the last two decades trying to sort out which of the trillions of microbes in the human gut are bystanders and which are active contributors. Sgg has stood out because its clinical link to colon cancer is unusually strong. Case reviews summarized in a mini review in the journal Gut Microbiome note that a large share of patients diagnosed with Sgg bloodstream infections or heart-valve infections turn out to have an undiagnosed colon adenoma or carcinoma once they are scoped. A case report in a gastrointestinal medicine journal documented one such patient whose Sgg heart infection was the first clue that led doctors to a colon tumor. This new study gives that long-observed clinical pattern a mechanical explanation: it is not simply that tumors create a friendly environment for Sgg to move in. The bacterium appears to bring its own tool for helping tumors grow once it arrives, by adhering to gut epithelial cells and flipping on a proliferation switch through the Wnt/beta-catenin pathway, a signaling route already well known for its role in colorectal cancer.

A Genetic Knockout Made the Case for Causation, Not Just Correlation

The strength of this study is in its design. Rather than simply comparing Sgg-positive and Sgg-negative tumors, which only shows association, the researchers deleted a single gene locus and compared otherwise identical bacteria side by side. That head-to-head comparison, wild-type versus T7SS-deficient mutant, in both a cell culture assay and a live animal model, is what lets the authors argue the secretion system itself, not just the presence of the bacterium, is driving tumor promotion. Isolating one bacterial system this cleanly and tracing its effect all the way to tumor burden and dysplasia grade in a live animal is a demanding standard, and it is the kind of dissection that is hard to do with correlational human data alone.

The Mouse Model Is a Real Limit on What This Study Can Claim

The tumor data here comes entirely from the AOM-induced mouse model of colorectal cancer, not from human patients. Mice given a chemical carcinogen and then colonized with bacteria are a well-established way to study cancer-microbe interactions, but they are still a simplified stand-in for human disease, and mouse and human gut environments differ in ways that can matter. The Sgg-CRC association itself is well documented in human patients, as the case reports and reviews above show, but the specific causal chain identified in mice (T7SS drives adhesion, which drives beta-catenin activation, which drives tumor growth) has not yet been confirmed to work the same way in the human gut. The authors also worked with one Sgg strain and one T7SS locus; whether this mechanism generalizes across the diversity of clinical Sgg isolates remains open.

What Comes Next for Sgg and Colon Cancer Research

The obvious next step is figuring out what SggEsxA and any other T7SS-secreted proteins are actually doing once they reach a human colon cell, at the molecular level, and whether blocking the secretion system could blunt the bacterium's tumor-promoting effect without requiring broader antibiotic treatment. There is also a diagnostic angle: if T7SS activity turns out to be a reliable marker of which Sgg strains are the risky ones, it could eventually help refine which patients found to carry the bacterium need the most urgent colon cancer workup. None of that is established yet. What this study does establish is that a specific, nameable piece of bacterial hardware, not just a bacterium's general presence, has a demonstrated role in pushing tumors to grow, at least in the mouse gut.

References

1. John Culver Taylor, Xinsheng Gao, Juan Xu, et al. (2021). A type VII secretion system of Streptococcus gallolyticus subsp. gallolyticus contributes to gut colonization and the development of colon tumors. PLOS Pathogens. https://doi.org/10.1371/journal.ppat.1009182
2. Chukwunonso Chime, Harish Patel, Kishore Kumar, Ahmed Elwan, Manoj Bhandari, Ariyo Ihimoyan (2019). Colon Cancer with Streptococcus gallolyticus Aortic Valve Endocarditis: A Missing Link?. Case Reports in Gastrointestinal Medicine. https://doi.org/10.1155/2019/4205603