Linking bacteria to Alzheimer’s


Diego Castillo

University of Pretoria – MSc


With trillions of bacterial cells in our bodies, some say that we are more bacterial than human. Ron Milo, a renowned scientist from the Weizmann Institute of Science in Israel, recently reported that an average man with approximately 30 trillion human cells in his body could harbour more than 38 trillion bacteria. They are involved in human nutrition, digestion and growth, but also in how our brains develop and how we fight off disease.

Bacteria can also be signposts, helping us to understand diseases. In the past decade, researchers have linked specific bacteria with the onset and development of cancer, diabetes, and heart disease, among others. They have become a precious tool as markers of disease. Though bacteria are mainly found in our gut, scientists have recently discovered that there are also active bacteria living in healthy people’s blood.

Before this discovery, we thought that human blood was sterile, home only to human cells.  Previously, if bacteria or other microorganisms were present in a person’s blood, it was bad news, usually indicating blood-poisoning or septicemia. That is not to say that scientists had not detected bacteria in the blood of healthy people – there have been reports of this phenomenon since the late 1970s. But these results were written off as an anomaly, or an error due to sample contamination. However, in the past five years, more solid studies using the latest technology have shown that healthy blood does, in fact, contain bacteria.

Even though these bacteria might not cause a particular disease, hundreds of studies tell us that changes in the species of bacteria, or a disruption in their community structure in our bodies, are linked to the onset of particular diseases. For example, diabetes and asthma have been linked to specific changes in our gut bacteria. Some species of bacteria in our blood point to liver fibrosis, schizophrenia, cardiovascular disease and cirrhosis.  

Researchers at the Centre for Microbial Ecology and Genomics at the University of Pretoria focus on the impact of microbes in human health and disease. We are especially interested in Alzheimer’s disease.

Despite the condition first being reported more than 110 years ago, the scientific community is still unable to explain the exact cause and specific mechanisms that underlie the progression of Alzheimer’s disease. It affects predominantly people older than 65 years and causes their brains to undergo a slow process of decay. There are approximately 46 million people affected worldwide; they suffer from memory loss, speaking and writing difficulties, poor judgment, confusion regarding space and time, and sudden changes in mood and personality, as well as challenges in solving simple daily life problems.

But something these Alzheimer’s patients have in common is a sticky protein buildup that accumulates around their brain cells and severely damages brain function. Scientists and medical doctors are still unsure why this happens. Research suggests that microbes might be involved in this disease, because bacteria can produce a similar plaque build-up to those seen in Alzheimer’s brains.

In 1996, Brian Balin, a professor of microbiology and pathology at the Philadelphia College of Osteopathic Medicine, managed to isolate the bacterium responsible for pneumonia from postmortem Alzheimer’s disease brains. His research, published in the Journal of Medical Microbiology and Immunology, showed the presence of DNA belonging to these specific bacteria in 17 out of 19 samples he analysed. In a follow-up study, Balin and his colleagues infected healthy mice with the pneumonia bacteria isolated from Alzheimer’s brains, and found that the mice developed brain changes characteristic of Alzheimer’s disease observed in humans. In 2004, other researchers observed similar changes in brain cells grown in the lab, this time after exposing them to the bacteria that causes Lyme disease, an infection spread by ticks.

Other microbes have also been found in patients suffering from Alzheimer’s disease, such as the bacterium involved in gastritis and stomach ulcers. Some viruses and fungi are also connected with the disorder. These include herpes simplex virus, which causes cold sores, and some yeasts such as those responsible for skin infections.

Many of these microorganisms cannot be grown in petri dishes, because they cannot survive under these artificial laboratory conditions. Instead, researchers are now able to look at their DNA to figure out which microbes are present in a particular blood or tissue sample. Special footprints inherent to the DNA of microorganisms allow us to identify which species are present in a specific tissue.

Because bacteria could play a role in the origin and progression of complex disorders such as Alzheimer’s disease, we are investigating the presence of bacteria in the blood of Alzheimer’s disease patients. Using genetic tools, we first want to find out which species are present. Then we want to find out if there are differences between the species found in Alzheimer’s disease patients and the species found in healthy individuals, of similar age.

This research will help us to understand more about the presence and potential role of microbes in human health and disease, specifically Alzheimer’s disease.


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