Amongst the millions of tiny life forms living on and inside our bodies are countless species of fungi. Our skin serves as a mosaic of these organisms, while membranes inside the nose and vagina are teeming with them. Additionally, fungi coexist alongside the bacteria in our guts, forming a complex ecosystem. While some fungi may be acquired from our mothers at birth, new species are constantly entering our bodies through our diet and environment. For instance, we ingest yeasts every time we drink beer or consume bread, and we inhale fungal spores with every breath we take. While many of these fungi are quickly eliminated by our immune systems, others can be transient passengers or lifelong inhabitants.
The notion of fungi taking complete control over our bodies might evoke images from HBO's apocalyptic series, The Last of Us. However, scientists largely agree that this idea is implausible. Instead, researchers are earnestly investigating whether certain fungi within us could contribute to brain-damaging diseases or influence our behavior and mental health. Experts emphasize that much more research is needed to explore these possibilities, as understanding our deep and complex relationships with these microbes could lead to new ways of boosting our health.
Generally, humans are quite adept at resisting fungi due to our warm body temperature, which makes it difficult for them to take hold. According to microbiologist Matthew Olm from the University of Colorado Boulder, many fungi may actually benefit us by supporting our immune systems or aiding in wound healing. "Fungi are definitely a critical part of being a healthy human," he states. However, it’s important to note that many fungi can also cause infections, ranging from athlete's foot to thrush. These infections can occur when we encounter new, harmful fungi in our environment or when resident fungi flourish under certain conditions.
While it is rare for fungi to reach the brain due to protective barriers in the lungs and intestines, fungal brain infections do occur. The incidence of such infections has increased in recent decades, primarily due to a rising number of individuals with weakened immune systems. This vulnerability is partly attributed to the global spread of the immune-crippling virus HIV, as well as the increased use of immune-suppressing medications among cancer patients and organ transplant recipients. "The more of these immune-modulating drugs we use, the more we will see fungal infections," Drummond explains.
Fungi that infect the brain often originate in the lungs, such as Aspergillus or Cryptococcus, which are inhaled as airborne spores. If left unchecked, these spores can germinate, grow, and spread. Less commonly, gut residents like Candida albicans can overgrow and invade the brain, producing nerve-killing toxins. Cryptococcus can even form tumor-like masses in the brain, leading to significant damage. The mortality rates for fungal brain infections, particularly with Aspergillus, can exceed 90%, making them challenging to treat due to the limited number of antifungal medications available.
In recent years, scientists have begun exploring the possibility that fungi may be entering the brain more frequently than previously thought and may even contribute to neurodegenerative diseases like Alzheimer's. Richard Lathe, a molecular biologist at the University of Edinburgh, points to cases where fungal and other microbial infections were found in patients initially diagnosed with Alzheimer’s. Remarkably, in some instances, the symptoms of dementia diminished after infection-fighting medications were administered, allowing patients to return to work.
Lathe posits that microbes may frequently cross the blood-brain barrier, but healthy immune systems usually suppress or eliminate them. As our immune systems weaken with age, this could allow microbial accumulation in the brain, potentially triggering inflammation that damages nerve cells. While Alzheimer's has long been associated with a build-up of certain proteins in the brain, a growing debate questions whether these proteins are a cause or merely a symptom of the disease. Lathe argues that these proteins could be a defense mechanism against microbial invasion.
Research is also examining the impact of gut-dwelling fungi on mental health. Several studies have noted differences in the makeup of gut fungi among individuals suffering from conditions such as depression or bipolar disorder. Johns Hopkins neuroscientist Emily Severance found that women with schizophrenia who showed signs of exposure to Candida albicans tended to perform worse on cognitive tests. Her research suggests that Candida overgrowth could provoke an imbalance of gut microbes, potentially increasing the likelihood of developing schizophrenia.
While the association between fungi and mental disorders is still being explored, scientists believe that understanding the role of fungi in human health is essential. Drummond emphasizes that while fungi are important, the exact nature of their influence is still under investigation. As the field of microbial research gains momentum, it may be time to shift focus from bacteria to the fungi that quietly shape our health from within.
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