Bugs in the System

New research has revealed how the microbes in our gut influence our central nervous system, and healing these microorganisms may, in turn, be the answer to everything from spinal cord injury and stroke damage to mental illness. Celeste McGovern investigates…

Imagine you’ve been in a car accident and your spinal cord is severed. You are paralyzed from the neck down with major nerve damage. The neurologist assessing you begins with a treatment of probiotics, vitamins and nutrients to feed certain bacteria and stimulate your immune system. Over the course of the coming weeks, your stool is continually tested, and your urine and blood sampled, to gauge markers of the types of bacteria thriving in your gut. You receive a transplant of bacteria into your colon from a healthy ‘faecal donor’, and your diet and probiotics are monitored to see how your immune system and microbes are responding, hopefully encouraging them to kick-start your damaged nervous system into self-repair mode.

Sound like quackery? It’s more like a distant, but not impossible, dream of a rapidly growing field of science studying how the bacteria inhabiting our gut are intimately connected to our brain and central nervous
system (CNS).

Take, for example, a recent study by scientists at the Center for Brain and Spinal Cord Repair at Ohio State University. They demonstrated for the first time that an injury to the spinal cord profoundly alters the kinds of microbes living in the intestines. What’s more, mice fed antibiotics prior to a spinal cord injury have intestines that are overrun with antibiotic-resistant superbugs, and suffer even worse nerve damage and impaired healing. More astonishingly, mice with spinal cord injuries fed probiotics—living ‘friendly bacteria’—suffered less nerve damage and enjoyed greater motor recovery than the injured mice that didn’t get probiotics.

Two-way brain chatter

Your gut is around 6–10 metres long from entry to exit, and every square centimetre of it is densely colonized by hundreds of species of bacteria, viruses, fungi and archaea—around 100 trillion of them. While you sleep, these organisms are busy replicating, competing for food, digesting your food, disintegrating your toxins and churning out a host of byproducts—vitamins, antioxidants and anti-inflammatory agents—that ‘talk’ through your immune system and CNS which, in turn, influences the rest of you—your brain, your hormones, your heart and more.

The microbiome—the microscopic cosmos of thousands of species of bacteria, viruses and other microbes that reside in our gut, mouth, skin, nose, among others—is one of the most explosive and revolutionary fields of research.

Scientists have discovered that these bugs, once viewed as agents of disease to be attacked with sterilizers and antibiotics, in fact perform endless vital functions for us. They digest our food, produce our vitamins, compete with pathogens to defend us, churn out metabolites that regulate our immune system and, as the latest research is showing, even command our CNS
and brain.

For decades, alternative practitioners have been pointing to yeast overgrowths—Candida infections and bacterial imbalances—and the ‘leaky gut’ that results from the damage these overgrowths do to the intestinal lining as a major underlying culprit in a wide array of chronic disorders, including mental illness.

And now, thanks to new technology, the emergence of microbiome science is validating their theories. What began a decade or so ago as research confirming a role for microbes in gastrointestinal disease has rapidly expanded to include research into the impact of bugs on the immune system, metabolism and neurological diseases.

The ability of bugs to control behaviour was recently shown experimentally with the jaw-dropping example of a parasite (Toxoplasma gondii) that ‘hijacked’ the brains of rats, manipulating those animals’ sexual and defensive behaviours to favour its own reproduction. Indeed, it turns out that our bugs can manipulate our mood and mental health too.

While researchers have long described a ‘gut–brain axis’, there is now growing evidence for a gut–microbiota–immune system–brain pathway that bears constant four-way traffic.

Gut microbiota ‘talk’ to the CNS directly by interacting with immune cells and nerve fibres, and indirectly by secreting metabolites that bypass the blood–brain barrier, say the Ohio State University researchers in their report.1 As 70 to 80 per cent of immune system cells are in the gut—specifically, in gut-associated lymph tissue (GALT)—alongside the microbes, there is an ongoing back-and-forth chatter between the two.

Other, similar studies are just beginning to fill in the details of how this chatter works and to determine the roles that microbes play, beyond what was previously suspected in neurological diseases and even in traumatic injuries. An entirely new field of ‘immunopsychiatry’, focused on this gut–immune system–brain trafficking is now emerging.

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