Once the bacteria get into the olfactory nerve, which is directly exposed to the air (it’s how we smell odours), it’s like a direct highway between the outside world and your brain. It’s a short distance and the route also bypasses the blood-brain barrier, which is one of the brain’s best protections against infection by bacteria and viruses.
We’ve also shown that bacteria and viruses can go up through the trigeminal nerve in a similar way. This is a nerve that provides sensory and motor innervation of the face and then goes into the brainstem. That means that two regions of the brain where some of the first symptoms of Alzheimer’s and dementia show up in humans, the olfactory region and the brainstem, are the locations where these two nerves go into the brain.
We don’t think it’s a one-off bacterial infection that causes these pathologies. It can happen slowly over your life. These bacteria are living inside the cells of the nose and olfactory nerve. And we think they might stay there for years, living inside our brains and doing small amounts of damage over time, and that might be what contributes to Alzheimer’s disease.
The damage happens in small increments, so we can think now about how we can reduce infections or how to treat them early on and reduce the long-term risk of degenerative disease. It’s our great hope that we can now reduce the progression or symptoms of Alzheimer’s disease using this knowledge.
One of the symptoms of Alzheimer’s disease is the presence of beta amyloid peptides or amyloid peptides. People with Alzheimer’s disease often have more accumulation of these amyloid peptides in plaques. This is a protein. This peptide is a protein that’s normally at low levels in the brain but when we infect it with bacteria, we see a large accumulation of this amyloid peptide protein around where the bacteria are.
And so we’re thinking now that these bacteria can set off this secretion of these proteins that lead to these plaques in humans. And now another aspect of it is that if we all have bacteria in our noses, in our brains, and I’m sure we all do, why do some people get it and other people don’t? it might be just because of your genetics or it might be something else, some other environmental factor at the same time.
There are now a lot of people who are looking at how pathogens, viruses and bacteria can cause many diseases in our brain and a lot of fingers are being pointed at bacteria contributing to Alzheimer’s disease. So it’s certainly a great area for developing drug treatments, and our team at the Clem Jones Centre for Neurobiology and Stem Cell Research is already looking at new ways to test for, prevent and treat for Alzheimer’s and dementia.
For example, because the one of the first symptoms of Alzheimer’s is losing your sense of smell, we could screen people using their sense of smell. So perhaps we should be thinking about getting people to do a simple smell test when they get to the age of 60 and use that knowledge to create therapies and potentially treat people earlier.