Something quietly changed in England’s E. coli story between 2016 and 2023, and most people didn’t notice. The number of cases from one particular type of the bacteria climbed year after year, almost without interruption, until the annual count had grown nearly tenfold in eight years. That’s not a blip. That’s a trend, and it’s still accelerating.
Then in the spring of 2024, the country got a stark reminder of what that trend looks like in real life. People who had grabbed a pre-packaged sandwich from a supermarket shelf started falling seriously ill. Some ended up in the hospital. Two died. And the culprit, investigators concluded, was almost certainly the lettuce inside.
Taken alone, either of these stories would be concerning. But there’s a third thread running alongside both of them, one coming from cancer research labs on the other side of the Atlantic, involving E. coli, colon cancer in young adults, and a toxin most people have never heard of. Put the threads together, and a much bigger picture starts to emerge.
The Quiet Rise of a Harder-to-Detect E. Coli
Not all E. coli strains are the same. The type most people are familiar with, O157, has been the subject of food safety warnings for decades. But a different category, called non-O157 Shiga toxin-producing E. coli (STEC for short), had been flying under the radar, partly because it’s harder to test for.
Cases of non-O157 STEC in England increased year after year, with the exception of 2020, from 297 in 2016 to 2,341 in 2023. That represents an almost tenfold increase in diagnoses over just eight years, according to a study published in the Journal of Medical Microbiology. The 2020 dip is attributed to pandemic-related restrictions reducing transmission and limiting testing.
Researchers behind the analysis concluded that the rise is partly explained by wider use of PCR testing technology at local hospital labs, but also “represents a true increase in the clinical and public health burden of non-O157 STEC.” In plain terms: better detection explains some of the spike, but not all of it. Cases genuinely increased.
In total, 9,378 isolates of non-O157 STEC were detected in England between January 2016 and December 2023, linked to 9,311 individuals. And the trend hasn’t stopped. The most recent annual data from the UK Health Security Agency (UKHSA) shows a further 26% rise in STEC infections in England in 2024, with 2,544 confirmed cases. Non-O157 STEC cases have increased nearly three times since 2019 alone.
What STEC Actually Does to the Body
STEC bacteria cause gastroenteritis (inflammation of the stomach and intestines), with symptoms ranging from mild to bloody diarrhea, stomach cramps, vomiting, and dehydration. In severe cases, they can cause hemolytic uremic syndrome, or HUS, a life-threatening condition that predominantly attacks the kidneys.
STEC produces the shiga toxin, which leads to a more severe form of illness than a typical stomach bug. Those with underlying health conditions are at higher risk of serious complications, including kidney failure, which can be fatal, particularly in children under five, the elderly, and the immunocompromised.
In 2024, the highest number of cases in England was among children aged one to four years old. These are not abstract statistics. They describe real families dealing with hospitalizations that began with what may have seemed, at first, like an ordinary stomach illness.
The Sandwich Outbreak That Broke Records
The sharpest demonstration of where non-O157 STEC can lead came in the summer of 2024. Contaminated lettuce in pre-packaged supermarket sandwiches was identified as the likely source of the UK’s largest recorded outbreak of STEC O145, which was declared in May 2024. Between May and November that year, 293 cases were confirmed.
Almost half of those patients were admitted to hospital, 11 developed severe complications including HUS, and two died. The outbreak surpassed a 2022 STEC O157 incident and, with its 293 confirmed cases, became the largest reported STEC outbreak in the UK since routine whole-genome sequencing began in 2015.
Investigations by the Food Standards Agency (FSA), Food Standards Scotland, local authorities, and industry identified UK-grown Apollo leaf lettuce as the likely contaminated ingredient in the pre-packed sandwiches. Food chain analysis led investigators to three sandwich manufacturers. In June 2024, Greencore, Samworth Brothers, and THIS! undertook product recalls after E. coli was detected in raw Apollo leaf and sandwich samples.
Despite that extensive investigation, the root cause of the outbreak remains unknown, and authorities have warned that a re-emergence is possible. Ready-to-eat salad vegetables can be contaminated at the pre-harvest stage through flooding, rainwater run-off, or irrigation water containing animal feces. Once bacteria are embedded in lettuce leaves, washing doesn’t reliably remove them. Most raw produce like lettuce will have already been washed by the manufacturer, but this is only partially effective at removing bacteria like E. coli. Heavily contaminated produce can remain infectious even after washing.
The practical implication is uncomfortable: if the contamination occurs before the food reaches the factory, even robust hygiene processes at the manufacturing stage may not be enough.
Why Lettuce Carries Particular Risk
You might wonder why a leafy green keeps appearing at the center of E. coli outbreaks. Part of the answer lies in how the plant grows. As Aidan Taylor, a microbiology lecturer at the University of Reading, explained, E. coli that ends up on crops starts in the gut of animals. Contaminated water can also reach crops during heavy rainfall as run-off from adjacent fields or through large-scale flooding.
The other part of the answer is simpler: we don’t cook lettuce. Heat kills E. coli. A sandwich that is not heated and suffers temperature abuse, left in a warm car between a store and consumption for example, could reach an optimal state for E. coli to grow. That said, the FSA confirmed that temperature abuse was not considered a likely primary factor in the 2024 outbreak specifically.
Outbreaks of STEC infection have previously been associated with pre-packed sandwiches and salad vegetables, mainly lettuce, in the UK and elsewhere. This was not a freak occurrence. It fits a pattern that food safety agencies have been tracking for years.
For people who buy pre-packaged sandwiches regularly, which in the UK and increasingly in the US is a large portion of the working population, this matters. With ready-to-eat foods like sandwiches and salads, the consumer is reliant on the manufacturer to ensure safe preparation and testing. That’s a significant amount of trust to place in supply chains that investigators are still trying to fully understand.
A Separate E. Coli Concern: Young Adults and Colon Cancer
While the outbreak story was unfolding, researchers in a different field were piecing together a separate but related puzzle. Colon cancer rates in young adults have been rising sharply for years, and no one had been able to fully explain why. Colorectal cancer is now on the rise among young people in at least 27 countries. Its incidence in adults under 50 has roughly doubled every decade for the past 20 years, and if current trends continue, it is projected to become the leading cause of cancer-related death among young adults by 2030.
According to the American Cancer Society’s 2026 Colorectal Cancer Statistics report, CRC incidence is rising by 3% per year in adults aged 20 to 49, while decreasing among those 65 and older by 2.5% per year. These numbers don’t fit the old model of colon cancer as a disease of aging. Something else is going on.
A major study published in April 2025 in Nature offered a striking potential piece of that puzzle. Researchers analyzed 981 colorectal cancer genomes from patients with both early- and late-onset disease across 11 countries with varying cancer risk levels. What they found in the tumors of younger patients was unexpected: the fingerprint of a bacterial toxin called colibactin.
If you want to know more about the warning signs that may signal a problem, read about colon cancer red flags that doctors say shouldn’t be dismissed.
What Colibactin Is and Why It Matters
Colibactin is a toxin produced by specific strains of E. coli that live in the colon. Produced by certain strains of E. coli residing in the colon and rectum, colibactin is capable of altering DNA. It isn’t produced by the same STEC strains responsible for food poisoning outbreaks. These are different bacterial variants, part of the normal gut microbiome in a significant portion of the population.
About 20% to 30% of people carry strains of E. coli that can synthesize this toxin. Most of the time, this doesn’t appear to cause any observable problem. But the new research suggests that in some people, the toxin’s DNA damage adds up over years and decades.
The findings show that colibactin leaves behind specific patterns of DNA mutations that were 3.3 times more common in early-onset cases, specifically in adults under 40, than in those diagnosed after the age of 70. By molecularly timing each mutational signature, the researchers demonstrated that colibactin-associated mutations arise early in tumor development, consistent with prior studies showing that such mutations occur within the first 10 years of life.
This is the part that changes the calculus. If the damage is being done in childhood, then by the time a person reaches their 30s or early 40s, they may already have decades’ worth of accumulated genetic changes in their colon cells, a biological head start toward cancer that typical risk assessments don’t account for.
The study also found that colibactin-related mutations account for approximately 15% of what are known as APC driver mutations, some of the earliest genetic changes that directly promote cancer development, in colorectal cancer. As lead researcher Ludmil Alexandrov at the University of California, San Diego, put it in a press release from UC San Diego: “If someone acquires one of these driver mutations by the time they’re 10 years old, they could be decades ahead of schedule for developing colorectal cancer, getting it at age 40 instead of 60.”
Important Caveats the Research Makes Clear
This finding is significant, but researchers are careful about how far to take it. The study found an association, a pattern, not a proven causal chain. While the study shows a strong association, the data can’t prove that colibactin caused these patients to develop cancer at a younger age.
Researchers in the field don’t expect E. coli, or any single microbe, to be the skeleton key for the surge in colorectal cancer. As Christian Jobin, a Distinguished Professor and microbiome researcher at the University of Florida, put it: colibactin is likely one “hit” among many to the microbiome, which together may place some people on a faster path toward cancer.
Cancer Research UK, which co-funded the research, has also been clear on this point. There is no evidence linking the E. coli strains that cause food poisoning, such as from contaminated lettuce, to cancer. The colibactin research involves different strains of E. coli. These are two separate stories about the same genus of bacteria, not the same story.
What the research does open up are new questions worth pursuing. Researchers are exploring whether probiotics could safely eliminate the harmful bacterial strains, and they are developing early detection tests that analyze stool samples for colibactin-related mutations. Both approaches are still in early stages, but the fact that researchers have a specific molecular target to work with is a meaningful step forward.
Read More: Doctor Warns Gen Z: ‘Silent’ Signs of Bowel Cancer May Go Unnoticed as Cases Rise
What to Do Now
Two threads run through all of this, one about food safety and one about cancer biology, and both have practical implications for the same person reading this.
On the E. coli and food safety side, the advice is grounded. People can reduce their risk of food poisoning by following good hygiene practices and the four Cs of food hygiene: chilling, cleaning, cooking, and avoiding cross-contamination. When product recalls are issued, follow them. And if you experience symptoms like bloody diarrhea, severe stomach cramps, or vomiting that doesn’t resolve quickly, don’t wait it out. See a doctor. STEC can cause hemolytic uremic syndrome, which is a medical emergency, especially in young children, the elderly, and anyone with a compromised immune system.
On the colon cancer side, the most actionable step right now is awareness and screening. Contributors to higher death rates among younger adults include a lower suspicion for colorectal cancer in younger age groups, symptom misattribution, and related delays in diagnosis. Research shows that young adults with rectal cancer wait an average of seven months from first symptoms to treatment, compared to about one month in patients 50 and older. That delay can mean the difference between an early-stage and a late-stage diagnosis.
The colibactin research doesn’t yet offer a preventive action to take today. That science is still developing. But it does reinforce something that matters to everyone in their 30s and 40s: colon cancer is no longer a disease that can be set aside until you’re older. Among adults under 50, colorectal cancer is now the number one cause of cancer death in men and the number two cause in women, according to the American Cancer Society. Current US guidelines recommend starting screening at age 45 for average-risk individuals, and earlier if you have a family history. If you haven’t had that conversation with your doctor, now is the time to start.
The bacteria living in and around us have always been part of our biology. What research is beginning to show is how much earlier in life that relationship starts to matter, and how much earlier we should be paying attention to it.
Disclaimer: The author is not a licensed medical professional. The information provided is for general informational and educational purposes only and is based on research from publicly available, reputable sources. It is not intended to constitute, and should not be relied upon as, medical advice, diagnosis, or treatment. Always consult a licensed physician or other qualified healthcare provider regarding any medical condition, symptoms, or medications. Do not disregard, avoid, or delay seeking professional medical advice or treatment because of information contained herein.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.
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