How ‘Bad’ Gut Bacteria Can Change Their Evil Ways

Could the idea that there are “good” and “bad” bacteria be a false dichotomy? A study appearing today (July 21) in the journal Science Immunology suggests so.

In a study on mice, scientists found that a group of bacteria called Helicobacter, long associated with ulcers, stomach cancer and intestinal distress, turned “bad” only when placed in a bad gut environment.

These bacteria triggered two very different kinds of immune-system responses, depending on the health of the mice. In healthy mice raised in a nearly germ-free, controlled environment, the Helicobacter induced an immune response associated with tolerance, as if the body were saying it accepted the new bacteria along with its existing gut bacteria, collectively known as the gut microbiome.

 

However, in mice bred to have colitis, a condition that involves inflammation of the bowel, the Helicobacter made the inflammation worse. The immune systems treated the bacteria as foreign invaders.

The study suggests that Helicobacter and similar bacteria labeled as “bad” may, in fact, be neutral or even beneficial, depending on the health of the individual. A person’s level of stress, poor diet or genetics all may influence the good or bad nature of gut bacteria, the scientists said.

“An interesting issue about Helicobacter species is that they’re thought of as pathobionts, which means they don’t necessarily have a well-described function in terms of promoting host health,” said Dr. Chyi-Song Hsieh, an assistant professor of medicine and of pathology and immunology at the Washington University School of Medicine in St. Louis, who led the study. “But in the wrong context, in the wrong person, with the wrong genetics, it can cause inflammation in various parts of the gastrointestinal tract.”

Hsieh said the discovery could lead to a better understanding of the causes of inflammatory bowel disease, as well as treatments for the condition, which affects upward of 3 million Americans, according to the Centers for Disease Control and Prevention. [5 Ways Gut Bacteria Affect Your Health]

The human gut contains trillions of bacteria and other microorganisms that mostly contribute to good health by aiding in digestion and regulating the immune system. Many of these bacteria elicit responses from the immune system cells, called T cells. These responses improve the body’s tolerance to beneficial molecules and keep the immune system in check, so it doesn’t run rampant and attack the body’s own tissues.

“Gut bacteria [are] constantly interacting with immune cells of the host and can promote barrier function [or protection] in the intestinal tract,” said Jiani Chai, a graduate student in Hsieh’s lab who was the first author on the paper.

Some bacteria, however, such as the Helicobacter species, cause the T cells to increase inflammation and attack cells within the body that they recognize as foreign.

The study on mice doesn’t imply that all gut bacteria are inherently neutral, waiting for the gut to determine their fate as good or bad, Hsieh told Live Science. After all, one type of Helicobacter, called H. pylori, clearly can cause dangerous ulcers and stomach cancer. But it is interesting to observe that Helicobacter, thought to be solely bad, can trigger an immune response that is good for the body, he added.

It remains unclear exactly why Helicobacter elicits certain responses from T cells, but this could be key to maintaining tolerance to bacteria. And figuring this out could potentially lead to the development of new drug targets for treating IBD, he said.

Hsieh said his group’s future studies may investigate the possibility of using bacteria as sort of a medical delivery system, like a vaccine, to directly access the immune system to help regulate autoimmune diseases.

 

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