Study investigates the involvement of intestinal bacteria in rectal cancer

On comparing samples of tumors and healthy tissue, researchers found differences in the most abundant bacterial genera.

By Karina Toledo  |  Agência FAPESP – Recent studies suggest that dysbiosis, an imbalance between beneficial and pathogenic bacteria in the gut flora, may be linked to the appearance of tumors in the digestive tract. However, it is not clear whether cancer is the cause or consequence of this alteration in the intestinal microbiome.

To glean a better understanding of the condition, Brazilian researchers affiliated with A.C. Camargo Cancer Center and the University of São Paulo (USP) compared the bacteria present in samples of rectal tumors with those found in healthy tissue from this part of the intestine.

The results of the investigation, which was supported by FAPESP, were published in the journal Frontiers in Cellular and Infection Microbiology.

“What most drew our attention was the increased presence of the species Bacteroides fragilis in the tumor samples. Previous research showed that certain strains of this bacterium produce a toxin capable of inducing the formation of tumors in rats,” said Andrew Maltez Thomas, a PhD student at USP’s Chemistry Institute (IQ) and the first author of the article.

The study was supervised by João Carlos Setubal, a professor in IQ-USP’s Biochemistry Department, and Emmanuel Dias-Neto, head of the Genomics Laboratory at A.C. Camargo Cancer Center.

The group analyzed 36 samples of human rectal tissue, 18 from healthy individuals who underwent colonoscopy and 18 from cancer patients who underwent tumor resection surgery without having first been treated with radiation therapy or chemotherapy, which might have altered the bacterial flora.

Both study groups comprised the same number of men and women, with similar exposure to risk factors such as smoking and alcohol consumption. Furthermore, only samples from the same rectal region were analyzed. As the authors explain, control of these variables was important to ensure that the study focused on the microbial variation linked to cancer without the interference of other factors.

“Our work differs from other research in this field in two ways,” Thomas said. “First, we don’t treat colon and rectal cancer as one and the same disease because of the differences in survival rates and metastatic potential, as well as embryological differences in the tissue in these regions. Second, we didn’t analyze fecal samples because it has been shown that the microbiota are different in different parts of the intestinal tract, so that the bacteria found in the feces don’t necessarily reflect the composition of the entire bacterial community adhering to the rectal tissue.”

All of the DNA in the 36 samples was extracted, both human and microbial, Thomas added. Next, PCR (polymerase chain reaction) was used to amplify only the genetic information contained in the region in which a specific bacterial gene was found. 

“We sequenced the V4-V5 region of the 16S rRNA gene and thereby identified the bacteria present in the samples,” he explained.

Main findings

As the researchers report in the article, at the genus level, rectal cancer samples had higher abundances of Bacteroides, Phascolarctobacterium, Parabacteroides, Desulfovibrio and Odoribacter.

Non-cancer samples had higher abundances of Pseudomonas, Escherichia, Acinetobacter and Bacillus, as well as Lactobacillus, which is considered beneficial to human health.

“We observed that the tumors had a smaller number of biofilm-forming bacteria – species capable of sticking together and forming a cohesive unit that inhibits the survival of pathogenic agents. This shows that there is indeed dysbiosis,” Thomas said.

Two operational taxonomic units (OTUs) of B. fragilis were more abundant in tumor samples. According to Thomas, this finding reinforces evidence from previous research that indicated the involvement of this pathogen in the development of colorectal cancer. “OTU” is a term used in bacterial phylogenetic analysis to define groups with similar RNA sequences.

“We know that B. fragilis triggers a strong immune response in the colonic and rectal mucosa,” he said. “However, we don’t yet know if it causes the cancer or is more abundant as a result of the malignant lesion.” 

According to Dias-Neto, the bacterium may also play a beneficial role in the disease by attracting defense cells to the lesion and making the cancer more visible to the immune system.

“There are studies showing that if you treat an animal with antibiotics and then induce the formation of a tumor, the immune system doesn’t respond by combatting the cancer, in contrast with what happens in animals with preserved microbiota,” Dias-Neto said. “Immune activation induced by bacteria is necessary for there to be an effective response to chemotherapy.”

One of the hypotheses to be investigated in future research by the group, he added, is whether the presence of B. fragilis and other species found more abundantly in tumoral tissue influences the patient’s response to treatment.

“We’re starting to discover which bacterial groups are in which places, both in a healthy intestine and in a pathological situation,” Dias-Neto said. “The next step is to make correlations, for example, with the presence of a certain species in patients who respond very well or the absence of a bacterium in people who are developing the disease. A third step would be to start intervening in this process.”

Thomas is currently at the University of Trento in Italy where, with FAPESP’s support, he is investigating the existence of microbial markers that could help to diagnose and assess the prognosis of patients with bowel cancer.

“The idea is to determine if the presence of certain bacterial enzymes in feces, for example, can be used to distinguish whether the individual has an adenoma, an intestinal polyp or a healthy microbiota. This type of exam is feasible and will be available in the future,” Thomas said.

The article “Tissue-associated bacterial alterations in rectal carcinoma patients revealed by 16S rRNA community profiling” can be read at journal.frontiersin.org/article/10.3389/fcimb.2016.00179/full.