Sometimes ‘good’ gut microbes can turn bad NOBEASTSOFIERCE/SCIENCE PHOTO LIBRARY

The billions of bacteria that line our guts have evolved with us, and play a crucial role in our digestion, physical and mental health. But bacteria that seem beneficial for most people can cause harm in others: they can trigger an autoimmune disease in vulnerable people.

That’s what Martin Kriegel, an immunologist at Yale University, and his colleagues found when they studied people with antiphospholipid syndrome (APS), an autoimmune disorder in which a person’s own immune system attacks a protein that plays a key role in blood thinning. People with the syndrome are at risk of clots, strokes and miscarriages, and it can be fatal. “Young people can suddenly die if they have this,” says Kriegel.

He wondered if, by chance, any gut bacteria might express a protein similar to the one that can trigger APS, and so be attacked by the immune system of people with the syndrome.


When his team screened microbiome databases they found a match: Roseburia intestinalis, a species of bacteria that is thought to improve gut health.

“It’s probably random due to the sheer number of molecules in the microbiome,” says Kriegel.

The team then looked at immune system activity in the gut of people with APS and people without the condition. While R. intestinalis was present at similar levels in the guts of all people examined, the bacteria seemed to be causing inflammation in people with APS. These individuals also made antibodies to attack the bacteria – which looked very similar to the antibodies they made to attack their own proteins.

In experiments with mice, Kriegel’s team also found that, in animals genetically prone to developing APS, a dose of R. intestinalis could trigger the syndrome, with lethal outcomes. This all suggests that R. intestinalis can inadvertently trigger APS in people genetically predisposed to develop the syndrome, says Kriegel.

“It could be that a bug that is beneficial for one disease is detrimental for another,” says Kriegel. A person’s genes and lifestyle could potentially influence which way the relationship will go.

Kriegel hopes that the microbiome could offer new treatment approaches for people with autoimmune diseases like APS. Theoretically, once a person has been diagnosed with the disease, removing the microbe from their gut microbiome might help prevent future clots, he says.

Journal reference: Cell Host & Microbe , DOI: 10.1016/j.chom.2019.05.003