Colorectal cancer has traditionally been associated with older age. But in recent years, researchers have become increasingly concerned about rising diagnoses among younger adults in several countries.
The trend has prompted scientists to investigate possible explanations, including changes in diet, lifestyle, environmental exposures and the gut microbiome.
A significant clue emerged in a study published in Nature in April 2025. Researchers analysed the DNA of 981 colorectal cancers from 11 countries and discovered that a distinctive mutation pattern associated with a toxin produced by certain gut bacteria was more common in people diagnosed at younger ages. The findings offer a promising new direction for research, although they do not establish a single cause of early-onset cancer.
The Bacterial Toxin Drawing Scientific Attention
Our intestines contain a vast community of microorganisms, collectively known as the gut microbiome. Many help maintain normal digestive function, and most strains of Escherichia coli (E. coli) do not cause disease.
However, some strains carry genetic machinery that allows them to produce colibactin, a substance capable of damaging the DNA of human cells.
DNA contains the instructions that regulate how cells grow, repair themselves and function. When DNA is damaged, cells can sometimes repair it correctly. But if mutations remain, they may affect genes involved in controlling cell growth.
One important gene is APC, which helps regulate cell division in the lining of the colon. Changes in this gene can contribute to the early stages of colorectal cancer.
Researchers have found that colibactin can leave a recognisable pattern of mutations in DNA. Scientists can identify this pattern in tumour genomes, much like investigators recognising a distinctive signature left behind by a particular process.
What Did the Study Discover?
The international research team examined 981 colorectal cancer genomes, looking for patterns that could reveal how DNA damage had occurred.
Two mutation signatures, known as SBS88 and ID18, are associated with exposure to colibactin.
The researchers found that these signatures were 3.3 times more common in colorectal cancers diagnosed before age 40 than in cancers diagnosed after age 70. They also linked colibactin-associated mutations to changes in the APC gene.
These results matter because mutations caused by colibactin appear to occur relatively early in the development of some colorectal cancers.
The findings raise the possibility that exposure to colibactin-producing bacteria earlier in life may contribute to cancer risk decades later.
However, the researchers studied tumour DNA after cancer had developed. They did not follow each patient from childhood to diagnosis, so the study cannot establish exactly when exposure occurred or prove that the bacterial toxin caused an individual person’s cancer.
Does Carrying These Bacteria Mean You Will Develop Cancer?
No. This distinction is essential.
The presence of bacteria capable of producing colibactin does not mean someone will develop colorectal cancer. Cancer usually develops through a combination of genetic changes and other biological influences over time.
Researchers are still investigating why some people are exposed to these bacteria, how the microbes interact with the intestinal lining, and what other factors influence whether DNA damage progresses towards disease.
The new findings also do not explain every case of early-onset colorectal cancer. Other biological, inherited, environmental and lifestyle factors may contribute, and their relative importance remains under investigation.
There is currently no established routine screening test for colibactin exposure that can reliably predict who will develop colorectal cancer. Antibiotics or probiotics should not be used specifically to eliminate these bacteria or prevent cancer without evidence-based medical advice.
What Can You Do to Protect Your Bowel Health?
Although scientists have not yet developed a proven intervention targeting colibactin, there are practical ways to support digestive health and reduce recognised colorectal cancer risks.
- Eat a varied, fibre-rich diet. Include vegetables, fruits, beans, lentils and whole grains. These foods support normal bowel function and contribute to a balanced diet.
- Stay physically active. Regular movement is associated with better overall health and a lower risk of several chronic diseases, including colorectal cancer.
- Limit processed meat and avoid smoking. These are established cancer-prevention considerations. Limiting alcohol can also reduce cancer risk.
- Know your family history. A close relative with colorectal cancer or certain precancerous polyps may mean you need earlier or more frequent screening.
- Do not ignore persistent symptoms. Rectal bleeding, ongoing changes in bowel habits, unexplained abdominal discomfort, unexplained weight loss or iron-deficiency anaemia deserve medical assessment.
A healthy diet is beneficial, but no particular food, supplement or probiotic has been proven to prevent colibactin-related cancer.
Why Age Should Not Be the Only Consideration
In the United States, average-risk adults are generally advised to begin colorectal cancer screening at age 45. People with certain inherited conditions, a significant family history or other risk factors may need earlier screening. Recommendations differ between countries, so individuals should follow local guidance and their clinician’s advice.
Screening is intended for people without symptoms. If concerning symptoms develop at a younger age, a medical assessment should not be delayed simply because someone is below the routine screening age.
This distinction is increasingly important as researchers work to understand early-onset colorectal cancer.
A New Clue, Not the Final Answer
The discovery of colibactin-associated DNA mutations gives scientists a valuable way to investigate how some colorectal cancers begin. It also opens potential research avenues for understanding early-life exposures and developing future prevention strategies.
But important questions remain unanswered. Scientists still need to establish how much this bacterial toxin contributes to the overall rise in younger patients and whether targeted interventions can safely reduce the risk.
For now, the most useful response is neither fear nor false reassurance. It is continued research, awareness of symptoms, attention to established prevention measures and appropriate screening.
The takeaway: Gut bacteria may play a role in some cases of early-onset colorectal cancer, but they are only one part of a much larger scientific puzzle. Understanding that role could eventually help researchers improve prevention and detection—without blaming individuals for a disease whose causes are complex.
Photo by julien Tromeur on Unsplash