The Gut-Brain Axis and the Microbiome

Introduction to the Microbiome:
- The microbiome consists of trillions of microbes that live on and in every human being.
- Science is establishing links between these microbes and physical health, but a new, controversial concept suggests they act as an "invisible hand" altering the human brain.
- Conditions currently being linked to the microbiome include:
  - Depression.
  - Autism.
  - Neurodegenerative diseases.
The Shift in Perspective:
- Historically, it was understood that emotional states affect the gut (e.g., the "butterflies" felt before an exam or job interview).
- Current research suggests this is a two-way street, where the gut significantly influences mental health and brain function.
- Researchers are exploring "mood microbes" or psychobiotics as a potential revolution for mental health treatment.

The Kyushu University Study (Japan):
- This study is credited with igniting the concept of microbial influence on the brain.
- Methodology: Researchers compared "germ-free" mice (those never in contact with microbes) against normal mice.
- Findings: Germ-free mice produced $2 \times$ the amount of stress hormone compared to normal mice when distressed.
- Conclusion: Because the animals were identical except for their microbes, the difference in stress response was attributed to their micro-organisms.
Expert Insight - Dr. Jane Foster:
- Dr. Jane Foster, a neuropsychiatrist at McMaster University in Canada, identifies the Kyushu paper as the starting point for neuroscientists considering microbes.
- She describes it as a "powerful" discovery for those studying depression and anxiety, representing the first hint of microbial medicine in mental health.

The Brain-Gut Connection Paths:
- The Vagus Nerve: Described as an "information superhighway" connecting the brain and the gut directly.
- Chemical Signaling: Bacteria break down dietary fiber into chemicals known as short-chain fatty acids, which have effects throughout the human body.
- Immune System: The microbiome influences the immune system, which is increasingly implicated in brain disorders.
- Genetic Alteration: Emerging evidence suggests gut bugs use microRNAs (tiny strips of genetic code) to alter the operation of DNA within nerve cells.

Diversity as a Health Metric:
- A healthy microbiome is characterized by high diversity, meaning a wide variety of different species living throughout the body.
- Prof. Ted Dinan (Cork University Hospital):
  - In clinically depressed patients, there is a measurable narrowing in the diversity of the microbiota compared to healthy individuals.
  - While not suggesting it is the sole cause, Dinan believes the microbiome plays a role in the "genesis of depression."
  - Lifestyles that weaken gut bacteria, such as a diet low in fiber, increase vulnerability to these conditions.

Cellular and Genetic Data:
- Cellular Ratio: Humans are more microbe than human. By cell count, only $430\%$ of the body is comprised of human cells.
- Microbiome Components: Includes bacteria, viruses, fungi, and single-celled archaea.
- The "Second Genome":
  - The human genome contains approximately $20,000$ genes (genetic instructions).
  - The microbial genome (the sum of all microbial genes in the body) ranges between $2 \times 10^6$ and $2 \times 10^7$ genes ( $2$ million to $20$ million).
- Linked Diseases: This second genome is linked to allergy, obesity, inflammatory bowel disease, Parkinson's, and the efficacy of cancer drugs, as well as depression and autism.

Behavioral Transmission via Microbiota:
- Scientists at the APC Microbiome Centre (University College Cork) performed transplants of microbiome samples from depressed humans into rats—a procedure colloquially known as a "trans-poo-sion."
- Prof. John Cryan's Findings:
  - The researchers were surprised to find they could reproduce many features of a depressed individual in a rat simply through microbiome samples.
  - Anhedonia: A symptom of depression where individuals lose interest in pleasurable activities. In the study, rats normally crave sugary water, but once they received the microbiome from a depressed human, "they no longer cared" for the treat.

Gut-Brain Link in Neurodegeneration:
- Parkinson's is classically defined as a brain disorder involving the death of brain cells, resulting in muscle control loss and tremors.
- Prof. Sarkis Mazmanian (Caltech):
  - Argues that gut bacteria are causal or at least driving the motor symptoms of Parkinson's.
  - Studies on mice genetically predisposed to Parkinson's showed that gut bacteria were necessary for the disease to manifest.
  - Experiment: When stool from Parkinson's patients was transplanted into these mice, their motor symptoms became "much worse" compared to those receiving stool from healthy individuals.

Therapeutic Potential:
- There is potential for psychiatrists to prescribe "probiotic cocktails" of healthy bacteria to treat mental health and neurodegenerative conditions.
- Dr. Kirsten Tillisch (UCLA):
  - Highlights the need for much larger studies to identify specific species and sub-species of bacteria that affect the brain and their chemical products in the gut.
  - Views this as a way to potentially prevent disease in addition to helping those already suffering.
The Malleability of the Second Genome:
- Unlike human DNA, the microbiome is highly changeable. It is altered by:
  - The food we eat.
  - The pets we live with.
  - Medicinal drugs.
  - The way we are born.
- Predictions for the Future (Prof. John Cryan):
  - Within the next $5$ years, microbiome assessments will become standard medical practice, similar to cholesterol testing.
  - The microbiome is considered the fundamental future of personalized medicine.