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A world-first study has demonstrated that bacteria can travel through the olfactory nerve in the nose and into the brain in mice, where they create markers that are tell-tale signs of Alzheimer's disease.
It's thought human brains are not unlike mouse brains, with researchers at Griffith University in Queensland finding that picking your nose may increase risk for Alzheimer's or other brain disorders.
The study is published in the journal Scientific Reports.
Professor James St John, head of the Clem Jones Center for Neurobiology and Stem Cell Research at Griffith University, joined Sunday Morning to discuss the findings.
The study looked at the bacteria Chlamydia pneumoniae which causes respiratory tract infections.
"These bacteria are quite common. We have them in our bodies all the time," St John said.
"We might be more familiar with them causing pneumonia in the lungs but they're present in the nose too and it's also been associated with Alzheimer's disease in humans.
"But we just don't know they get there and we don't know what they do on their journey to the brain."
Photo: Armin Kübelbeck, CC BY-SA 3.0
Droplets of the bacteria were put into the nostrils of mice during the experiment.
"Chlamydia pneumoniae is particularly interesting because of its association with Alzheimer's disease," St John said.
"It's been extracted from plaques inside the brains of people who have died from Alzheimer's.
"Scientists have said what's it doing there, is it causing their pathologies?"
Of course, bacteria is a fact of life, but it's how certain bacteria react with humans that's the worry.
"We breathe in millions of bacteria every day, the vast majority of them do nothing," St John said.
Infections can take years to show any effect, he said.
“We think these bacteria are going in at very low levels and then doing incremental damage over time.
“It’s that long-term sort of slow progression that likely contributes to Alzheimer’s disease.
“It’s not a matter of people getting infections when they’re 60. It’s infections when they are 30, or 40, or 50, that might be causing the problem.”
As a rule of thumb for good hygiene and disease prevention, St John said he generally warns people not to forcefully pick their noses or pluck their nose hairs.
"We're warning people of all ages because we said before, we might be getting these bacterial infections coming in at any time of our lives.
"Any sort of damage you might do the lining of your nose is probably not wise. You can think about a cut in your skin anywhere, on your arm or your leg, it can get infected by bacteria.
"If you do it inside your nose, even though you can't see it, you'll still be exposing that to bacteria being able to penetrate."
"If you've got problems with hair in your nose, just trim it with scissors, that sort of thing."
Professor James St John. Photo: Supplied / Griffith University
Not to get too graphic, but if you do pick at your nose, you want to be gentle, St John said.
"If you're careful you should be OK, but then again you have to think about why you want to be picking your nose."
St John said we have millions of bacteria in our nose, but "it's when it gets out of balance it's the problem".
More investigation needs to be made on the leap from mice to humans and what that might mean for Alzheimer's research.
"That's what we're going to be doing next, we need to look at some people who've got Alzheimer's disease or early stages of it, find out what bacteria are present in their nose and see if we can detect any gene or protein changes that might be similar to what we see in mice.
"And then after that, we can work out how to treat it, that's the most important thing.
"We need to find treatments so we can reduce the onset and the progression for this contribution to Alzheimer's disease."
Bacterial and viral infections that reach the brain can cause a variety of problems, as the Covid-19 pandemic has shown us.
"In the old days people used to think the brain was sterile, there's no bacteria or viruses in the brain, but we know that's not true.
"Covid is a classic example of how suddenly we are much more aware of what these micro-organisms can do.
"People with Covid often report that they lose their sense of smell, and that's because the virus kills some of the cells in the lining of the nose, which then exposes the nerves to infection.
“And we do know that Covid can get into the brains of people and cause long-term systems such as long Covid.
"We all have bacteria in our brains but not all of us get Alzheimer's disease. So why do some people get it? It might be a combination of different bacteria and viruses and it might be genetics as well."
"Some bacteria are very sneaky," he said, and noted that low levels of bacteria can cause serious damage over time.
Our nostrils take in air millions of times in our lifetime, and we're obviously not all getting sick every single day. What keeps the bacteria at bay?
"I think it's more common than we think" that 'bad' bacteria gets into the noses, St John said.
"It's just that our bodies are very good defence mechanisms."
