What’s Fueling Colorectal Cancer in Young Adults

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This story originally appeared in the winter 2025 issue of UC San Diego Magazine as “What’s Fueling Colorectal Cancer in Young Adults.”

In the past decade, as the rate of colorectal cancer has dramatically increased among people under 50 years of age, scientists have been trying to understand why. 

UC San Diego bioengineers have identified a potential culprit: a toxin called colibactin produced by certain strains of E. coli bacteria living in the gut. Their research shows that exposure to colibactin during early childhood can leave distinct mutation patterns in the DNA of colon cells that increase the risk of developing colorectal cancer — which occurs in the colon or rectum — before age 50.

“Producing this toxin is a way for the bacteria to defend themselves,” says Ludmil Alexandrov, a professor in both the Shu Chien-Gene Lay Department of Bioengineering and the Department of Cellular and Molecular Medicine. Colibactin helps bacteria fend off competitors in the gut, but in doing so, it can mutate the DNA of human cells lining the colon. And over time, these mutations may lead to cancer.

When the team analyzed samples from 981 patients across 11 countries, they found that these colibactin-related mutations were 3.3 times more prevalent in adults under 40 than in those over 70. The researchers went a step further, tracing the timing of these mutations, and found that they likely originated during childhood, most likely in the first decade of life.

“This suggests that early-life exposure to colibactin could put people on track to develop colorectal cancer as young adults,” says Alexandrov.

Certain strains of E. coli that produce colibactin may live in our gut without our knowledge. Unlike the strains of E. coli that cause food poisoning and lead to symptoms such as diarrhea — often acquired from undercooked meat or unwashed produce — these colibactin-producing strains can reside in our systems without causing noticeable illness. Because these bacteria often do not produce symptoms, it’s tough to know if someone has been exposed to them. And while not everyone who harbors these bacteria will develop cancer, the DNA mutations caused by colibactin can increase the risk over time. 

Alexandrov and his team do not yet know exactly why colibactin exposure may be affecting a greater number of young people today, but they have several hypotheses. One possibility is that the rise in cesarean births is altering the way babies’ gut bacteria develop, which could potentially allow more colibactin-producing strains to take hold early in life. Decreased breastfeeding, increased use of antibiotics in early childhood and diets higher in ultra-processed foods could also be affecting gut health in ways that allow harmful bacterial strains to thrive. 

“All of these factors may be contributing to the rise in early-onset colorectal cancer, but we are just beginning to understand the connections,” says Alexandrov.

Currently, there is no test to detect colibactin-producing bacteria in the gut; however, Alexandrov and his team are working on developing one. A stool test that could identify DNA mutations associated with colibactin exposure could help doctors monitor people at higher risk.

“Early detection is key,” Alexandrov says. “If you catch it early enough, you have a better chance of beating it. That’s why it’s important to pay attention to your body — even when you’re young — and not ignore signs that something might be wrong.”

Researchers in Alexandrov’s lab are also investigating the use of probiotics for children that could help eliminate harmful gut bacteria. But these solutions will still take several years to develop.

In the meantime, people should watch for any symptoms of colorectal cancer, regardless of age. These include bowel habit change, blood in the stool, unexplained weight loss and persistent abdominal pain. Those with a family history of colorectal cancer or concerns about symptoms should speak with their doctor about the possibility of early screening. A diet rich in fiber, fruits and vegetables may also help support a healthy gut microbiome.