Scientists have linked some forms of intestinal bacteria to an increased risk of colorectal cancer, two new studies suggest.
Researchers have found higher levels of almost 30 different types of bacteria in patients with the disease compared to healthy individuals.
Some bacteria were commonly found in the mouth, while others turned the nutrients found in red meat and poultry into carcinogens.
The team, led by the University of São Paulo (USP) in Brazil, says it hopes the findings will lead to new ways to track, diagnose and treat cancer.
A new study led by the University of São Paulo found higher levels of almost 30 different types of bacteria in patients with the disease compared to healthy individuals. One was Fusobacterium nucleatum (photo), an oral bacterium that plays a role in periodontal disease
Colorectal cancer is a cancer that starts in the colon or rectum, located in the lower end of the digestive tract.
Cancer usually begins with growths called polyps. They are located in the inner lining of the colon or rectum and become cancerous for many years.
A study published in 2017 found that colorectal cancers diagnosed in adults under 55 years of age have doubled from 1990 to 2013, although no one knows for certain why.
In response, the American Cancer Society has updated its screening guidelines for colorectal cancer by lowering the age at which average at-risk people begin regular 50- to 45-year exams.
According to the American Cancer Society, colorectal cancer is the third most commonly diagnosed cancer in the US, both among men and women.
It is also the third leading cause of cancer deaths in American men and women and is estimated to cause more than 51,000 deaths by 2019.
Between the two studies, both published in Nature Medicine, the team found 29 species of bacteria in cancer patients in the US, Europe and Asia.
For the first study, the researchers analyzed more than 600 faecal samples from cancer patients and controls from five previous studies.
They found that the high levels of bacteria normally found in saliva were in stool samples from cancer patients.
This included Fusobacterium nucleatum, an oral bacterium that plays a role in periodontal disease and has been associated with colorectal cancer in the past.
A greater number of oral bacteria tends to travel to the intestine in patients with colorectal cancer. This migration may cause inflammation in the intestine, giving rise to the tumor, "said the first author, Dr. Andrew Maltez Thomas, a researcher at USP.
"However, we do not know the real reason for the migration, only that there is a link between the presence of these bacteria in the intestine and colorectal cancer and that the link deserves to be further investigated."
The second study, conducted by the University of Trento in Italy, found that choline, an essential nutrient found in various foods such as meat, poultry and fish, is broken down by intestinal bacteria and made into a metabolite necessary for metabolism.
This metabolite has been associated in previous studies with an increased risk of cardiovascular disease, and the team now says it also raises the risk of colorectal cancer.
The co-author of the study, Nicola Segata, associate professor of computational metagenomics at the University of Trento, said that some of the species of bacteria found were only recently discovered.
"The gene that degrades choline in a potentially dangerous metabolite is a good example," she said.
"This gene is often carried in the intestinal microbiome by a bacterial species that has no name and that we discovered in other works earlier this year."
Researchers say that future studies will determine whether the bacterium increases the risk of colorectal cancer or whether the cancer allows certain species of bacteria to flourish.
"We detected an association, but this does not necessarily imply a causal link," said co-author Dr. João Carlos Setubal, professor of bioinformatics at USP.
The question is whether specific bacteria can cause cancer or cancer, creating a different environment in the colorectal duct and thus favor certain bacteria over others.
"We do not yet have an answer, which would be central to the research results described in the article to help develop therapies for the treatment of colorectal cancer."