Microbes And Mental Health: Rethinking The Antiquated Paradigm For Mental Health

There is a version of this article that opens with something like: "Mental health is a growing concern in Australia, with millions affected every year." I am not going to write that version. You already know the statistics. You may be living them.

What I want to talk about is something more specific — and, frankly, more useful. The dominant framework for understanding depression, anxiety, and related conditions has been based almost entirely on brain chemistry. Specifically, a deficit of serotonin. That model has driven antidepressant prescribing in Australia for 30 years. It is also, by the weight of current evidence, largely wrong. Not entirely. But wrong enough that it cannot be the only answer we offer people.

Here is the part that should get your attention: a large body of research now places the gut — specifically the 38 trillion microorganisms living in it — at the centre of mood regulation, neuroinflammation, and psychiatric risk. This is not fringe. It is being published in Nature Microbiology, Cell, and the Lancet Psychiatry. It is being studied at the Walter and Eliza Hall Institute here in Melbourne.

And it changes almost everything about how we should approach mental health care.

The serotonin story was wrong — or at least, drastically incomplete

The "chemical imbalance" model of depression — too little serotonin, fix it with an SSRI — dates to the 1960s. It became the dominant narrative because it gave doctors a clean story and gave patients a way to understand their suffering that removed shame from the picture. That part was genuinely useful.

The problem is the science never fully supported it. A 2022 umbrella review published in Molecular Psychiatry by Moncrieff and colleagues at University College London examined the entire body of evidence linking serotonin to depression. Their conclusion: there is no consistent evidence that depression is caused by low serotonin activity or concentrations. Multiple meta-analyses, including studies of serotonin precursor depletion, found no reliable relationship between serotonin levels and depressive episodes.

SSRIs still work for some people. That is not under dispute here. But "the drug works" does not prove the model. Aspirin reduces fever; that doesn't mean fever is caused by an aspirin deficiency.

So if the serotonin deficit story is incomplete, what is actually happening in a brain that is depressed, anxious, or chronically foggy? Increasingly, the answer points to three interconnected processes: neuroinflammation, disrupted gut-brain signalling, and a depleted or dysbiotic microbiome.

The gut-brain axis: how it actually works

The gut and the brain are in constant, two-way conversation. This happens through four overlapping pathways, and understanding them matters because each is a potential clinical target.

1. The vagus nerve

The vagus nerve runs from the brainstem to the gut and carries more traffic upward (gut to brain) than downward. Roughly 80 to 90 percent of vagal fibres are afferent — they report conditions in the gut to the central nervous system. Gut microbes directly influence vagal tone. Specific species, including Lactobacillus rhamnosus, have been shown to alter GABA receptor expression in the brain via vagal signalling in animal models. When researchers cut the vagus nerve in those same animals, the effect disappeared.

2. The HPA axis and cortisol regulation

The hypothalamic-pituitary-adrenal axis governs the stress response. Gut bacteria regulate HPA axis reactivity. Germ-free mice — raised without any gut microbiome — mount a massively exaggerated cortisol response to stress. When colonised with a healthy microbiome, the response normalises. The window for this appears to be in early life, which may partly explain why childhood gut disruptions (antibiotic use, formula feeding, caesarean delivery) correlate with later psychiatric risk.

3. Short-chain fatty acids (SCFAs)

When gut bacteria ferment dietary fibre, they produce short-chain fatty acids — primarily butyrate, propionate, and acetate. Butyrate in particular crosses the blood-brain barrier, acts as a histone deacetylase inhibitor (influencing gene expression in neurons), supports microglial function, and reduces neuroinflammation. Low-fibre diets and dysbiotic gut flora mean low SCFA production. Low SCFA production means less neurological protection and more inflammatory signalling reaching the brain.

4. The tryptophan-kynurenine pathway

This is the pathway I find most clinically significant. Tryptophan — the amino acid precursor to serotonin — is metabolised primarily by gut bacteria and immune cells. Under inflammatory conditions, tryptophan is diverted away from the serotonin pathway and into the kynurenine pathway instead. Kynurenine metabolites, particularly quinolinic acid, are neurotoxic. They activate NMDA receptors, cause oxidative damage to neurons, and have been found at elevated levels in the cerebrospinal fluid of people with major depression and suicidal ideation.

This is why gut inflammation doesn't just cause gut symptoms. It reroutes brain chemistry at the metabolic level.

What trashes the microbiome — and why Australians are particularly exposed

Australia has one of the highest per-capita rates of antibiotic prescribing in the developed world. We have high rates of caesarean delivery (around 36 percent of births, per AIHW data). We eat a diet that is, on population average, low in fermentable fibre and high in ultra-processed food. These are not incidental details. Each one directly and measurably alters gut microbial diversity.

For many patients I see in clinic, the story is not one of these factors — it is several, compounding over years. A person who had multiple courses of antibiotics in childhood, was born by caesarean, and now eats a diet low in plant diversity while managing a high-stress job is not dealing with a mild inconvenience. They are carrying years of microbiome disruption that, by the time they sit in front of me, has translated into real neurological consequences.

Leaky gut, leaky brain — the neuroinflammation connection

The gut lining is one cell thick. Those cells are held together by tight junction proteins — occludin, claudin, zonulin — that regulate what passes from the gut lumen into systemic circulation. A healthy microbiome maintains those tight junctions. Dysbiosis and chronic low-grade inflammation loosen them.

When the gut barrier becomes permeable, lipopolysaccharide (LPS) — a molecule from the outer wall of gram-negative bacteria — leaks into the bloodstream. LPS is a potent activator of the innate immune system. It triggers inflammatory cytokines: IL-1β, IL-6, TNF-alpha. Those cytokines cross the blood-brain barrier and activate microglia, the brain's resident immune cells.

Activated microglia produce neuroinflammatory compounds. They also — critically — upregulate the enzyme IDO (indoleamine 2,3-dioxygenase), which diverts tryptophan toward the kynurenine pathway rather than serotonin synthesis. So the sequence is: gut permeability leads to systemic LPS exposure, which leads to cytokine release, which leads to neuroinflammation, which redirects tryptophan metabolism away from serotonin and toward neurotoxic kynurenine metabolites.

This is not a hypothesis. Elevated LPS, elevated inflammatory cytokines, and elevated kynurenine metabolites have all been measured in people with major depressive disorder. A 2019 study in Translational Psychiatry found that patients with depression had significantly higher circulating LPS-binding protein than healthy controls, and that LPS-binding protein correlated directly with symptom severity.

The gut-brain axis gives us a mechanism for why depression can look like an inflammatory disease — because, for many people, it is one. The microbiome is the upstream driver.

The ROOT Method: a framework built for this

I developed the ROOT Method specifically because single-system thinking fails patients. A person comes in with depression and anxiety. They have been told their serotonin is low. They have tried one or two antidepressants with partial response and significant side effects. What the standard pathway rarely investigates is why their neurochemistry is disrupted in the first place.

What this means practically: a patient presenting with anxiety or depression at my clinic will typically be assessed for gut function, intestinal permeability markers, inflammatory load, nutrient status (B12, folate, iron, zinc, vitamin D), HPA axis function via salivary cortisol, and organic acid metabolites that reveal how tryptophan and dopamine precursors are actually being processed. This is not the same as a standard GP workup, and it is not meant to be. It is designed to find the things that standard pathology misses.

What we actually test — and what the results tell us

Functional testing for the gut-mental health connection is more accessible than most people realise. These are the panels I use regularly in clinical practice for patients with mood and cognitive presentations:

GI Microbiome Mapping

A comprehensive stool analysis that identifies species diversity, the presence of dysbiotic or pathogenic organisms, markers of intestinal inflammation (calprotectin, lactoferrin, secretory IgA), and short-chain fatty acid profiles. In patients with depression or anxiety, I commonly find reduced Bifidobacterium and Lactobacillus, elevated pathogenic species, and low butyrate-producing bacteria. This is the foundational gut test.

Organic Acids Test (OAT)

A urine test that gives a metabolic snapshot of what the body is actually doing with nutrients and neurotransmitter precursors. The OAT includes markers for tryptophan metabolism (5-hydroxyindoleacetate, kynurenate), dopamine breakdown (homovanillate), mitochondrial function, B vitamin status, and yeast/bacterial overgrowth. For patients who present with a mood disorder that has not responded predictably to standard treatment, the OAT often reveals the mechanistic explanation.

Salivary Cortisol Panel (AdrenoCortex)

The HPA axis and the gut microbiome regulate each other. Chronic HPA dysregulation — flattened cortisol curve, inverted pattern, or high nocturnal cortisol — alters gut motility, barrier integrity, and microbial composition. A four-point salivary cortisol test maps diurnal rhythm and is far more informative than a single morning serum cortisol, which misses the pattern entirely. I use the AdrenoCortex salivary panel rather than the DUTCH test, which I have found to be less reliable in my experience for this clinical application.

Comprehensive Blood Panel

Not just a standard full blood count, but targeted markers: high-sensitivity CRP (systemic inflammation), plasma zinc, serum B12 and folate, 25-OH vitamin D, ferritin, fasting glucose and insulin, and thyroid function including TSH, free T3, and free T4. Nutrient deficiencies and thyroid dysfunction are among the most commonly missed contributors to mood disorders in primary care.

Clinical targets: what the research actually supports

I want to be specific here, because this area attracts a lot of generic advice. "Take probiotics and eat more vegetables" is not a clinical strategy. Here is what the research supports, and at what level of confidence.

Psychobiotics: specific strains, specific effects

The term "psychobiotic" — coined by Ted Dinan and John Cryan at University College Cork — refers to live organisms that, when ingested in adequate amounts, produce a mental health benefit via the gut-brain axis. The research is strain-specific. Broad claims about "probiotics" improving mood are not supported. What is supported:

• L. rhamnosus (JB-1)
• L. helveticus R0052 + B. longum R0175
• B. longum 1714
• L. plantarum 299v

Lactobacillus helveticus R0052 combined with Bifidobacterium longum R0175 is probably the best-studied combination for mood and anxiety applications. A double-blind, placebo-controlled trial by Messaoudi et al. (2011) found significant reductions in anxiety, depression, and urinary cortisol in healthy volunteers over 30 days. Bifidobacterium longum 1714 has been studied at APC Microbiome Ireland and shown to reduce social anxiety and EEG-measured stress reactivity.

Strain selection matters. Dose matters. Duration matters. This is why buying a generic $15 probiotic from the supermarket and expecting neurological benefit is, at best, optimistic.

Dietary interventions with robust evidence

The SMILES trial (2017), conducted partly through Deakin University here in Victoria, is the landmark study. It was a randomised controlled trial comparing a Mediterranean-style dietary intervention against social support for people with moderate-to-severe depression. The dietary intervention produced significantly greater reductions in depressive symptoms — a mean reduction of 11 points on the MADRS scale, compared to 4 points in the social support group. Roughly one-third of the dietary group achieved full remission versus 8 percent of the control group.

The mechanisms are multiple: increased fibre drives SCFA production, polyphenols from vegetables and olive oil modulate microbial composition, omega-3 fatty acids reduce neuroinflammation via prostaglandin pathways, and plant diversity directly correlates with microbial diversity.

Butyrate support

Where microbiome damage is significant, direct butyrate support via tributyrin or sodium butyrate supplements can provide a bridge while dietary changes take effect and the microbiome rebuilds. Butyrate reduces intestinal permeability, downregulates inflammatory signalling, and crosses the blood-brain barrier to directly influence neuroinflammation and microglial activation.

Tryptophan pathway support

Where organic acids testing reveals excessive kynurenine pathway activity, targeted interventions include: anti-inflammatory dietary modification, specific B6 and B2 support (cofactors for the enzymes that return tryptophan toward serotonin), and, where indicated, NAD+ precursor support to address downstream kynurenine accumulation. This is precision work — it requires the test data to guide it safely.

Addressing intestinal permeability

Reducing LPS translocation requires repairing the gut barrier. Evidence-supported interventions include L-glutamine (epithelial cell fuel, supports tight junction protein expression), zinc carnosine, colostrum, and the removal of dietary contributors to permeability — primarily gluten in susceptible individuals, alcohol, and non-steroidal anti-inflammatory drugs.

Where to start if this resonates

This is where I get practical, because a 3,000-word article is only useful if it leads somewhere actionable.

If you have been dealing with depression, anxiety, brain fog, chronic fatigue, or mood instability that has not responded fully to standard treatment — or that you have been managing without finding the root cause — there are a few reasonable starting points.

The first is honest dietary assessment. Not guilt, not overhaul — just honest assessment. How many different plant foods do you eat in a week? Research by Tim Spector's group at King's College London found that people who eat 30 or more different plant foods per week have significantly more diverse gut microbiomes than those eating fewer than 10. Most Australians eating a typical Western diet are well below that threshold. That alone is a meaningful target.

The second is considering whether the conventional workup you have had actually tested the right things. A normal TSH, a normal full blood count, and a normal serum B12 do not rule out functional nutrient insufficiency, HPA dysregulation, gut dysbiosis, or disordered tryptophan metabolism. Standard pathology is designed to catch pathology. Functional medicine testing is designed to catch dysfunction before it becomes pathology — which is where most people with mood and energy complaints actually sit.

The third is getting a proper clinical history taken by someone who understands the gut-brain axis as a clinical system, not as a metaphor. Because the questions that matter — about childhood antibiotic use, delivery mode, diet history, stress exposure, sleep architecture, bowel habits — are often not the questions that get asked in a 15-minute GP appointment. They take time. They require a clinical framework built to hold that complexity.

The clinical picture we are building toward

The old model — brain chemistry as the problem, medication as the solution — was never entirely wrong. It was just narrow. Antidepressants help some people meaningfully, and for those people they should be available and properly prescribed.

But a large proportion of people with mood disorders do not respond adequately to medication alone. Antidepressant response rates in randomised controlled trials are, at best, around 50 percent over placebo. The STAR*D trial, the largest antidepressant study ever conducted, found that after four medication trials only about a third of participants achieved remission and stayed in it. The other two thirds remained symptomatic despite multiple medication changes.

Those are the people who deserve a different question: not "which medication have we not tried yet" but "what upstream process have we not yet investigated?"

Gut dysbiosis, intestinal permeability, neuroinflammation, HPA axis dysregulation, tryptophan pathway diversion — these are not alternative medicine concepts. They are published, peer-reviewed mechanisms with growing clinical evidence behind them. They are testable. And in a meaningful proportion of patients with treatment-resistant mood disorders, they are treatable.

That is the paradigm shift worth having.

Related reading: Chronic fatigue: naturopathic treatment  |  The ROOT Method for anxiety relief  |  Book a discovery call

Disclaimer: This article is for educational purposes and does not constitute medical advice. If you are experiencing mental health concerns, please speak with a qualified health professional. If you are in crisis, contact Lifeline on 13 11 14 or Beyond Blue on 1300 22 4636. This content is prepared by Domenic Pisanelli, naturopath and functional medicine practitioner (ATMS registered), at Vital Health and Natural Medicine, Kealba VIC.