How gut bacteria ensure a healthy brain – and could play a role in treating depression

One of medicine’s greatest innovations in the 20th century was the development of antibiotics. It transformed our ability to combat disease. But medicine in the 21st century is rethinking its relationship with bacteria and concluding that, far from being uniformly bad for us, many of these organisms are actually essential for our health.

Nowhere is this more apparent than in the human gut, where the microbiome – the collection of bacteria living in the gastrointestinal tract – plays a complex and critical role in the health of its host. The microbiome interacts with and influences organ systems throughout the body, including, as research is revealing, the brain. This discovery has led to a surge of interest in potential gut-based treatments for neuropsychiatric disorders and a new class of studies investigating how the gut and its microbiome affect both healthy and diseased brains.

The microbiome consists of a startlingly massive number of organisms. Nobody knows exactly how many or what type of microbes there might be in and on our bodies, but estimates suggest there may be anywhere from three to 100 times more bacteria in the gut than cells in the human body. The Human Microbiome Project, co-ordinated by the US National Institutes of Health (NIH), seeks to create a comprehensive database of the bacteria residing throughout the gastrointestinal tract and to catalogue their properties.

The lives of the bacteria in our gut are intimately entwined with our immune, endocrine and nervous systems. The relationship goes both ways: the microbiome influences the function of these systems, which in turn alter the activity and composition of the bacterial community. We are starting to unravel this complexity and gain insight into how gut bacteria interface with the rest of the body and, in particular, how they affect the brain.

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Disputed theory on Parkinson’s origin strengthened

Parkinson’s disease is strongly linked to the degeneration of the brain’s movement center. In the last decade, the question of where the disease begins has led researchers to a different part of the human anatomy. In 2003, the German neuropathologist Heiko Braak presented a theory suggesting that the disease begins in the gut and spreads to the brain. The idea has since, despite vocal critics, gained a lot of ground. Researchers at Lund University in Sweden now present the first direct evidence that the disease can actually migrate from the gut to the brain.

The so-called Braak’s hypothesis proposes that the disease process begins in the digestive tract and in the brain’s center of smell. The theory is supported by the fact that symptoms associated with digestion and smell occur very early on in the disease.

Researchers at Lund University have previously mapped the spread of Parkinson’s in the brain. The disease progression is believed to be driven by a misfolded protein that clumps together and “infects” neighboring cells. Professor Jia-Yi Li’s research team has now been able to track this process further, from the gut to the brain in rat models. The experiment shows how the toxic protein, alpha-synuclein, is transported from one cell to another before ultimately reaching the brain’s movement center, giving rise to the characteristic movement disorders in Parkinson’s disease.

“We have now been able to prove that the disease process actually can travel from the peripheral nervous system to the central nervous system, in this case from the wall of the gut to the brain. In the longer term, this may give us new therapeutic targets to try to slow or stop the disease at an earlier stage”, says Professor Jia-Yi Li, research group leader for Neural Plasticity and Repair at Lund University.

The research team will now carry out further studies in which the mechanisms behind the transport of the harmful protein will be examined in detail. The current study suggests that the protein is transferred during nerve cell communication. It is at this point of interaction that the researchers want to intervene in order to put a stop to the further spread of the disease.