Researchers may have found an unlikely contribution to the development of Alzheimer’s disease: the bacteria in your gut. Their study, published in the journal Science, suggested that certain gut bacteria increased brain damage seen in mice with an Alzheimer’s-like disease.
In Alzheimer’s disease—the most common form of dementia—proteins build up abnormally in the brain. One of these proteins is called tau, and its accumulation causes the death of memory-related brain cells, forgetfulness, and eventually permanent memory loss.
The researchers were studying a group of mice that had a genetic mutation that caused a buildup of tau protein in the brain. Mice were split into two groups: one group had a healthy gut microbiome (a community of bacteria, viruses, and fungi that live in the gut), while the other group completely lacked the gut microbiome (these mice are called “germ-free”). The germ-free mice had significantly reduced tau-induced brain damage when they reached old age.
The germ-free mice also had lower levels of tau-related inflammation in their brains. Alzheimer’s patients have high levels of inflammation in their brains that contribute to brain damage. So one of the reasons why these mice suffered less brain damage may be the lower level of brain inflammation.
One of the main causes of Alzheimer’s-related brain inflammation is the activation of a certain type of immune cell known as microglia. Microglia are known to be regulated by the gut microbiome, which may explain why germ-free mice have lower levels of active microglia.
Eliminating the gut microbiome is not the solution
If you’ve heard of the gut microbiome, you may know that it’s often thought to be important to our overall health. Therefore, the suggestion that not having a microbiota can reduce the severity of Alzheimer’s is surprising. However, if you’re wondering whether eliminating your gut microbiome will reduce your chances of getting the disease, you may be disappointed.
Everyone has a unique gut microbiome, which is home to millions of different types of bacteria, some beneficial to our health and some that can be harmful. One explanation for the study’s findings is that only a certain type of bacteria is involved in the increased brain inflammation and brain cell death caused by tau.
The researchers tested this idea by giving antibiotics to a third group of mice with a healthy gut microbiome. The antibiotics reduced the level of inflammation and microglia activation in the brain, although only in male mice. What’s interesting is that only certain types of gut bacteria were reduced by antibiotics, so it could be one of those bacteria that caused brain damage in mice.
It’s not clear why antibiotics only improve outcomes in male mice and not females. In humans, Alzheimer’s disproportionately affects women. The reason isn’t clear, but research shows that one reason women are at greater risk of Alzheimer’s may be the effect of hormone changes that occur during menopause.
It’s not surprising that the gut microbiome can influence the brain. More than a decade of research has revealed strong communication links between the two, known as the gut-brain axis. It helps regulate many bodily processes, including the gut-brain axis, the immune system, and therefore inflammation.
This is not the first study to point to the role of the gut microbiome in developing Alzheimer’s disease in mice. There is also evidence that the gut microbiome is different in people with Alzheimer’s disease than in healthy ones.
While the idea that the gut microbiome is linked to Alzheimer’s is compelling, it is important to interpret the results of the study with caution. Mice, especially those with genetic mutations, are very different from humans. Their immune systems, their genetics, and more importantly, their gut microbiome all have differences.
Germ-free mice also have an underdeveloped immune system. The gut-brain axis is also affected, and studies in germ-free mice show changes in their behavior and brain signals compared to normal mice. These differences may be one reason why the germ-free mice in the study did not develop such severe Alzheimer’s.
Based on current research, it cannot be claimed that bacteria in the gut microbiome contribute to Alzheimer’s disease in humans. Instead, the research points to the possibility of certain bacteria in the gut microbiome that may increase the risk of developing Alzheimer’s. If it can be shown to have similar bacteria with similar effects in humans, the possibility of targeting them laying a new foundation for Alzheimer’s treatment is an exciting possibility.
This article has been republished under a Creative Commons license from The Conversation. Read the original article.
Catherine Purse receives funding from the Biotechnology and Biological Sciences Research Council.