Nearly 18,000 cases are diagnosed among this age group every year in the US compared to 12,000 a year pre-2000.
An even bigger rise has been seen among all cancers affecting the digestive tract, which has also been linked to poor diets.
In the study, scientists tested stool samples from 94 colon cancer patients. Twenty-four of these patients’ tumors had a KRAS mutation — which makes the cancer more aggressive.
Of the more than two dozen bacterial species they tested for, the team found three were more common among those with the more aggressive form of the disease.
Fusobacteriums can cause inflammation in the gut, the researchers warned, while Colstridium — such as C. difficile — can release toxins.
These raise the risk of damage to DNA in cells, which in turn increases the chance that they could turn cancerous.
For Shewanella, this bacteria proliferates on the surface of tumors, the team wrote — helping to support their growth.
Colon cancer is surging among under-50s, with cases of the disease rising 50 percent over two decades to nearly 18,000 per year.
It has also become the number one cause of cancer deaths among men in the age group and the number two cause among women.
Early-onset cancers of all forms are also on the rise, increasing by 79 percent from 1990 to 2019 globally, according to an analysis published in BMJ Oncology last year.
Scientists are stumped as to the cause of the uptick, blaming everything from antibiotics to obesity and even a type of fungus.
A $25million global probe has also been launched to attempt to track down the causes.
Young patients diagnosed with colon cancer include Evan White, 24, from Dallas whose tumor was not detected until it had progressed to stage three — when the cancer is harder to treat.
He had been on track to marry his girlfriend and move to California, but he died after a four-year battle with the disease.
Others include Marisa Maddoz, a paralegal from Delaware, who was diagnosed with the disease at the age of 29 years.
She told DailyMail.com that treatment for the cancer has left her unable to have another child.
Kate Middleton, 42, bravely revealed last month that she had also been diagnosed with cancer following abdominal surgery.
She has not revealed the type of cancer that doctors detected, but her comments prompted an outpouring of support — and inspired some cancer survivors to share their stories.
Evan White, from Dallas, was diagnosed with stage three colon cancer at the age of 24 after going into hospital to get an abscess removed from his tonsils. On the right is Marisa Maddox who was diagnosed with the cancer at the age of 29 years, She now can’t have anymore children