A gut bacterium blocked a cancer-linked E. coli in mice, but you cannot buy it

Colorectal cancer rates in adults under 50 have been climbing for years, and one unsettling explanation involves a bacterium many people already carry. Certain strains of E. coli produce colibactin, a genotoxin that leaves a distinctive scar pattern in the DNA of colon cells. A new mouse study asks a practical question: can another gut bacterium stop those strains from taking over? In mice, the answer is a qualified yes, and the qualifications matter as much as the result.

Key Takeaways

• A strain of Bifidobacterium pseudolongum limited the expansion of colibactin-producing pks+ E. coli and reduced colitis severity in mice.
• It appeared to work by raising colonic acetate and activating PPAR-gamma, and its cell-free liquid inhibited the E. coli directly in a dish.
• The study never measured colibactin. Suppressing the bacterium is not the same as suppressing the genotoxin, and the authors say so explicitly.
• This is a research culture collection strain, not a product. Probiotic effects are strain-specific, and nothing on a shelf can be assumed to do this.

Bifidobacterium pseudolongum curbed pks+ E. coli expansion in colitic mice

Giving mice a specific Bifidobacterium strain limited the growth of a colibactin-producing E. coli and measurably reduced their colitis. That is the central result of a study published in Frontiers in Microbiology by Ayodeji Samuel Ajayi, Gabriela Fragoso, Manuela M. Santos and colleagues, working in a mouse model of chemically induced colitis.

The target was EcNC101, a mouse-adapted E. coli carrying the pks island (a gene cluster that lets bacteria manufacture colibactin, a DNA-damaging compound). Mice colonized with EcNC101 and then given dextran sulfate sodium, a chemical that strips the protective lining of the colon, lost more weight, scored worse on disease activity and ended up with shorter colons than uncolonized controls. Their stool carried significantly more lipocalin-2, interleukin-6 and tumor necrosis factor alpha, three standard markers of gut inflammation.

Treatment with Bifidobacterium pseudolongum subsp. pseudolongum DSM 20099 significantly reduced all three inflammatory markers, restored colon length and attenuated the microscopic damage scored in colon tissue. It also inhibited the E. coli directly: cell-free liquid from B. pseudolongum cultures markedly suppressed EcNC101 growth in a dish at every dilution tested, from quarter strength to full strength. That matters, because it means part of the effect is the bacterium releasing something, not just occupying space.

Why ulcerative colitis and early-onset colorectal cancer make this relevant

Anyone living with inflammatory bowel disease, particularly ulcerative colitis, has the most at stake here, because chronic colonic inflammation is an established risk multiplier for colorectal cancer. The link this study builds on is not speculative. In 2020, researchers exposed human intestinal organoids (miniature lab-grown guts) to pks+ E. coli for five months and found the bacterium imprints a distinctive mutational signature detectable in human colorectal tumors, identified across 5,876 cancer genomes from two independent cohorts. Colibactin leaves fingerprints.

That is why keeping pks+ E. coli from expanding is a therapeutic target rather than a microbiology curiosity. Inflammation gives these strains room to bloom, and blooming gives colibactin more opportunity to damage DNA. For readers managing ulcerative colitis or Crohn disease right now, though, this study identifies a target and a candidate, not a therapy. It changes nothing about current treatment.

From a putrescine screen to one named strain

The study's structure is what makes it more than a probiotic anecdote: the Bifidobacterium was not chosen in advance, it emerged from the data. The researchers first treated EcNC101-colonized mice with putrescine, delivered at 1% in drinking water, and found it suppressed the E. coli and reduced disease severity. Sequencing the gut communities then showed why: putrescine-treated mice had more Actinobacteriota and fewer Proteobacteria, with B. pseudolongum riding that shift.

Two correlations tied it to a mechanism. Colonic acetate, a short-chain fatty acid produced when bacteria ferment fiber, tracked closely with Bifidobacteriaceae abundance (Rho = 0.63, P < 0.0001) and with B. pseudolongum measured directly by quantitative PCR (Rho = 0.63, P < 0.001). B. pseudolongum abundance also correlated positively with colon length and negatively with colonic tumor necrosis factor alpha, meaning more of the bacterium went with less damage.

Only then did the team test the strain itself, obtained from a public culture collection and given by oral gavage during colitis induction. Female C57BL/6 mice aged 8 to 10 weeks received 2.0 to 2.5% dextran sulfate sodium for 12 days, with group sizes of 9 to 13 animals depending on the experiment. The Bifidobacterium increased both acetate and butyrate and significantly activated PPAR-gamma, a receptor in gut lining cells that dampens inflammation and constrains the oxygen supply E. coli needs to bloom. Screen first, then mechanism, then intervention: that order is what makes the causal claim credible.

Why no probiotic on a shelf is this bacterium

Nobody should read this and buy a Bifidobacterium supplement, because the strain tested here is not sold as one. DSM 20099 is a deposit in a research culture collection, used by laboratories. The distinction is not pedantry. The National Center for Complementary and Integrative Health is blunt about strain specificity, noting that if a specific kind of Lactobacillus helps prevent an illness, "that doesn't necessarily mean that another kind of Lactobacillus or any of the Bifidobacterium probiotics would do the same thing," and that in most instances "we still don't know which probiotics are helpful and which are not."

Two products can both say Bifidobacterium on the label and behave entirely differently, because what mattered here, the compound secreted into the culture liquid, the acetate output, the PPAR-gamma activation, is a property of one strain rather than of the genus. Most probiotics are sold as dietary supplements, which do not require FDA approval before marketing. So the practical instruction is genuinely nothing. Buying an unrelated Bifidobacterium on the strength of this paper would be acting on a name, not on evidence.

Colibactin itself was never measured

The most important limitation is one the authors state outright: they did not measure colibactin. No pks gene expression, no colibactin quantification, no assessment of the DNA damage marker gamma-H2AX. As they write, "the observed suppression of EcNC101 expansion therefore does not establish direct inhibition of colibactin-mediated genotoxicity." Fewer genotoxic bacteria is a reasonable proxy for less genotoxicity, but it is a proxy.

The colitis model has its own boundary. Dextran sulfate sodium reproduces chemical damage to the gut lining and the immediate immune reaction, but the authors note it "does not fully replicate the chronic adaptive immune dysregulation characteristic of human ulcerative colitis." No tumors were induced or counted, so this is a colitis study that gestures at cancer rather than a cancer study.

Only female mice were used, which the authors flag given sex differences in microbiota, short-chain fatty acid production and susceptibility to this model. They also concede that whether supplementation achieves durable colonization in an adult human gut "has not been demonstrated," and that the acetate concentrations needed to kill E. coli directly in a living gut may exceed what this bacterium produces. Group sizes of 9 to 13 mice are modest, and many outcomes are reported graphically rather than as numbers in the text.

What has to happen before this reaches people

The next step is measuring the thing that actually matters: colibactin-driven DNA damage. That means repeating the intervention while quantifying pks expression and genotoxicity markers, and moving to a model where colon tumors are the endpoint rather than inflammation. Testing in male animals and in a chronic colitis model would close two acknowledged gaps.

Beyond that sits the colonization question, since a strain that works when gavaged into a mouse during active colitis may not establish itself in an adult human gut that is already full. The work was funded by the Canadian Institutes of Health Research under grant PJT-196003, with support from the Fonds de recherche du Québec-Santé and the Institut du cancer de Montréal, and the authors declared no commercial or financial conflicts, worth noting for a probiotic-adjacent finding.

Common Questions

Should I take a Bifidobacterium supplement to lower colorectal cancer risk? No. This study used a research culture strain not sold to consumers, tested it in mice, and never measured cancer or DNA damage. Probiotic effects are strain-specific, so a different Bifidobacterium product cannot be assumed to behave the same way.

Does everyone with pks+ E. coli get colorectal cancer? No. Colibactin-producing E. coli are carried by a substantial share of healthy people, and the established finding is that these strains leave a detectable mutational signature in a subset of colorectal tumors, not that carriage causes cancer in any individual.

Did the Bifidobacterium kill the E. coli or just crowd it out? Both appeared. Cell-free liquid from B. pseudolongum cultures suppressed EcNC101 growth without the bacterium present, indicating something secreted, while in mice the treatment also raised acetate and activated PPAR-gamma, making the gut less hospitable to E. coli.

Does this apply to people with ulcerative colitis? Only as a research direction. The dextran sulfate sodium model captures chemical injury to the gut lining but not the chronic immune dysregulation of human ulcerative colitis, and no human has been treated with this strain.

References

Ayodeji Samuel Ajayi, Gabriela Fragoso, Annie Calvé, et al. (2026). Expansion resistance against genotoxic E. coli: protective effects of Bifidobacterium pseudolongum in DSS-induced colitis in mice. Frontiers in Microbiology. https://doi.org/10.3389/fmicb.2026.1877186