Harm caused by gut bacteria might contribute to the increasing rates of colon cancer among young adults.

The reason colon cancer rates have doubled in people under 55 over the past 20 years remains unclear — a troubling trend that has caught the attention of doctors and cancer researchers.

One possible contributor could be colibactin, a DNA-damaging toxin produced by certain strains of E. coli and other gut bacteria. A new study published this week identifies a strong link between this toxin and colon cancer in younger adults.

Researchers from the University of California, San Diego, analyzed tumor samples from nearly 1,000 colorectal cancer patients across four continents. Most of the cancers showed genetic mutations consistent with previous exposure to colibactin.

“You can think of it as a bacterial weapon used to fight off other microbes,” says lead researcher Ludmil Alexandrov.

Interestingly, patients under 40 were three to five times more likely to carry these mutations compared to those in their 70s or older. This suggests the toxin may be playing a role in early-onset cases of the disease.

Scientists believe that in some individuals, this microbial “weapon” — technically a genotoxin — may inadvertently damage human gut cells, triggering mutations that increase cancer risk. According to the study, these damaging interactions likely happen in childhood.

“We estimate it occurs within the first 10 years of life,” Alexandrov says. “If a child gets the mutation at age 5, they might develop colorectal cancer 20 to 30 years earlier than expected.”

Still, the study doesn’t prove colibactin causes cancer — it only shows a strong association. Experts also doubt that a single bacteria like E. coli is solely responsible for the surge in cases.

“That would be too simple,” says microbiome expert Christian Jobin from the University of Florida. More likely, colibactin is just one of several factors impacting the microbiome that together accelerate cancer risk.

While E. coli is a common early resident of the human gut, only about 20% to 30% of people carry strains capable of making colibactin. And even when present, it doesn’t always lead to harm. What activates this bacterial weapon remains unclear — diet, inflammation, and medication may all play a role.

Importantly, not all colorectal cancers show signs of colibactin-related mutations, meaning other causes are also in play.

The study provides another clue in the broader mystery of why colorectal cancer is rising in younger adults. Experts suspect a mix of environmental factors — including modern diets, sedentary habits, medication use, and possibly the increased consumption of ultra-processed foods — may be influencing the gut microbiome in harmful ways.

Researchers also found these colibactin-linked mutations were more common in patients from Western countries than in rural areas of Africa and Asia, adding weight to the idea that lifestyle and environment are key.

Some early-life factors the team is investigating include birth delivery method (C-section vs. vaginal), breastfeeding, antibiotic use, and childhood diet. Each of these can significantly shape the microbiome, and may influence the presence or activity of colibactin-producing bacteria.

Antibiotics, in particular, stand out. Previous studies have linked early antibiotic exposure to a higher risk of colorectal cancer later in life.

Looking ahead, scientists hope to identify specific gut microbes that act as early warning signs for cancer risk. With more research, this could eventually lead to treatments that target harmful bacterial toxins like colibactin — though experts caution that any intervention must be carefully balanced to avoid disrupting the microbiome in unintended ways.