Groundbreaking study finds link between common bacteria and stomach cancer

A new study has found that a type of bacteria commonly found in the body, which usually does not cause problems in healthy people, plays an important role in causing stomach cancer, the fifth most common cancer in the world.

Streptococcus anginosus bacteria are present along with other microbes in the mouth, throat, intestines and vagina. Occasionally, they can cause mild infections such as sore throats and skin infections. For patients with underlying health conditions or a weakened immune system, the bacteria can lead to more serious infections, such as those that damage the heart and brain.

However, research led by Singapore’s Nanyang Technological University (NTU Singapore) and the Chinese University of Hong Kong (CUHK) showed that S. anginosus involved in stomach infections in mice causing cellular damage and changes known to encourage stomach cancer. This includes gastritis, in which the lining of the stomach is irritated. The condition destroys the gastric lining cells and causes some of them to gradually turn into cancer cells.

Experiments on mice also revealed that the bacteria boosted the growth of stomach cancer cells, doubling the size and weight of tumors in some cases.

But the researchers also found that disrupting a protein on the surface of the bacteria, which they need to interact with cells lining the stomach, reduced S. anginosus’ ability to contribute to stomach cancer.

The findings add to the number of bacterial species known to cause stomach cancer. Another type of bacteria, Helicobacter pylori, is known to cause gastric ulcers in humans. These painful sores increase patients’ risk of developing gastric cancer. Whether other bacteria are involved remains unclear so far.

“Our latest findings in mice shed light on another pathogen that contributes to stomach cancer and how it does so,” said study leader Professor Joseph Sung, NTU Senior Vice-Chancellor (Health and Life Sciences).

“This lays an important foundation for further studies in humans that will help clinicians better treat and prevent stomach cancer caused by bacteria,” added Professor Sung, who is also dean of NTU’s Lee Kong Chian School of Medicine .

Professor Yu Jun, the study’s other co-leader who is Director of CUHK’s State Laboratory of Digestive Diseases, said the researchers will next explore the therapeutic potential of targeting S. anginosus to reduce the risk of gastric inflammation and cancer”.

The S. anginosus study, published in Cell in February 2024, contributes to one of the goals of the NTU 2025 strategic plan to address the needs and challenges of healthy living.

Abnormalities caused by bacteria

Helicobacter pylori The bacteria are classified as carcinogenic (carcinogenic) to humans. But among people infected with the bacterium, only 1 to 3 percent develop stomach cancer, suggesting that other factors are involved in its development.

Previous research has shown that 20 percent of patients with chronic gastritis—known to contribute to stomach cancer—are not infected with Helicobacter pylori. Chronic gastritis refers to long-term inflammation of the stomach.

Studies have also supported that other bacteria, incl S. anginosusit could also play a role in stomach tumor growth.

To confirm S. anginosus’ role, Professor Sung and collaborating scientists from CUHK conducted a series of experiments.

The researchers infected the stomachs of mice with S. anginosus over two weeks and mild to moderate gastric inflammation was found. This was similar to what was observed in mice infected with Helicobacter pylori in the same time frame.

when S. anginosus The infection of the mice was prolonged – up to a year – persistent and prolonged inflammation of the stomach or chronic inflammation was observed three months after the initial infection. The level of inflammation was also similar to that of rodents infected with Helicobacter pylori only.

But when the mice were infected with both S. anginosus and Helicobacter pylorithe level of chronic gastric inflammation recorded after three months was twice that of infection with either bacteria alone.

As the S. anginosus As the infection progressed, abnormalities that signal the development of cancer were also seen in the stomach. There was an increase in the number of gastric cells six months after the initial infection, stomach acidity increased after nine months, and many cells of the stomach lining turned into abnormal precancerous cells after 12 months.

Scientists noticed another way that S. anginosus The infection created an environment conducive to stomach cancer – by disrupting the population of other microorganisms in the stomach. The bacteria increased the number of gut microbes normally found in the mouth, while decreasing the number of probiotic bacteria that are important for good gut health, such as Lactobacillus.

They proved it S. anginosus was able to drive the growth of gastric tumors. When gastric cancer cells were implanted under the skin of mice or in the stomach lining of rodents, infection by S. anginosus at these points they encouraged the tumors to grow, doubling their size and weight in some cases.

The researchers also discovered that the bacteria needed a specific protein on their surface to bind to and invade the cells of the stomach lining to promote the growth of gastric cancer. When this protein was deficient in the bacteria, the ability of the microbes to attach to cells lining the stomach and encourage the growth of gastric cancer implants was impaired.

Our results suggest that in the long run S. anginosus infection causes severe chronic gastritis that is comparable to Helicobacter pylori contamination. In fact, these two pathogens may act synergistically to promote gastric inflammation and ultimately gastric cancer. This could change the way we approach disease prevention and treatment.”

Professor Joseph Sung, NTUA Senior Vice President (Health and Life Sciences)

He added that detection S. anginosus in the stool could be useful in assessing whether a patient is at risk of gastric cancer in the future.

From S. anginosus usually found in the mouth, the bacteria could be swallowed through saliva and find its way into the stomach. So one possible way to protect yourself from developing stomach cancer could be to practice good oral hygiene, Professor Sung said.