Specimen catalogue · Interactive explainer
Your gut is a garden.
Four types of biotics. Each works differently. All connected — including to your brain. Here's the science, simply explained.
WHO/FAO definition · 2001
"Live microorganisms that, when administered in adequate amounts, confer a health benefit on the host."
Think of it like...
Planting new seeds in your garden. Living organisms that, if the conditions are right, take up residence and contribute to the ecosystem. The variety of seed — and the quantity — matters as much as the act of planting.
How they work
Three main mechanisms
1. Compete with harmful bacteria for space and nutrients along the gut lining.
2. Produce antimicrobial compounds (bacteriocins, organic acids) that suppress pathogens.
3. Interact with immune cells via the gut epithelium to modulate immune responses.
Areas studied in published literature
Antibiotic-associated diarrhoea
Prevention & recovery
Strong
Infectious diarrhoea
Duration reduction
Strong
Infant colic
Symptom reduction
Strong
Irritable bowel syndrome
Bloating, pain, transit
Emerging
Immune modulation
Infection frequency
Emerging
Vaginal microbiome
Balance restoration
Emerging
Metabolic health
Weight, glucose
Early
Skin health
Eczema, atopic dermatitis
Early
Important distinction
Not all live cultures are probiotics
A microorganism only qualifies as a probiotic when studied at a specific dose and demonstrated a health benefit. Fermented foods contain live cultures — but unless clinically studied at adequate amounts, they do not meet the scientific definition.
🧠 Gut-brain axis connection
Certain probiotic strains have been studied for their influence on the gut-brain axis — the bidirectional communication network between the gut microbiome and the central nervous system. Proposed mechanisms include modulation of neurotransmitter precursor production (serotonin, GABA) and regulation of the HPA stress response axis. Human trial evidence in anxiety and mood is emerging but remains small-scale and heterogeneous.
ISAPP definition · 2017
"A substrate that is selectively utilised by host microorganisms conferring a health benefit."
Think of it like...
Fertiliser for your garden. Not planting new seeds — nourishing the beneficial organisms already present. Selectivity is the key word: not all fibre qualifies, and the molecular structure determines which bacteria benefit.
How they work
Three main mechanisms
1. Resist digestion in the upper GI tract and arrive intact in the colon.
2. Are selectively fermented by beneficial bacteria — particularly Bifidobacterium and Lactobacillus.
3. Fermentation produces short-chain fatty acids (SCFAs) — butyrate, propionate, acetate — which benefit the gut lining and systemic health.
Common prebiotic types studied in literature
Galacto-oligosaccharides (GOS)
Bifidobacteria growth, infant microbiome
Strong
Fructo-oligosaccharides (FOS)
Gut motility, microbiome diversity
Strong
Human milk oligosaccharides (HMOs)
Infant immunity, gut colonisation
Strong
Inulin
Bifidobacteria, calcium absorption
Strong
Beta-glucan
Cholesterol, glycaemic response
Strong
Resistant starch
Butyrate production, satiety
Emerging
Pectin
Gut barrier, immune signalling
Emerging
Arabinoxylan
Microbiome diversity
Emerging
Important distinction
Not all dietary fibre is prebiotic
Fibre must be selectively fermented by beneficial microorganisms to qualify. The ratio and molecular structure matter — GOS and FOS were specifically studied to replicate the bifidogenic activity of human milk oligosaccharides in infant nutrition.
🧠 Gut-brain axis connection
Prebiotics influence the gut-brain axis indirectly — primarily through SCFA production. Butyrate and propionate produced during prebiotic fermentation cross the gut barrier and influence systemic inflammation, increasingly linked to neuroinflammation and mood. Some prebiotic fibres have been studied in association with reduced cortisol awakening response and self-reported anxiety.
ISAPP definition · 2021
"A preparation of inanimate microorganisms and/or their components that confers a health benefit on the host."
Think of it like...
The fruit your garden produces. When bacteria ferment fibres, they release bioactive compounds your body directly uses. No living organisms required — giving postbiotics stability and formulation advantages that probiotics don't have.
How they work
Three main mechanisms
1. Short-chain fatty acids (SCFAs) like butyrate directly nourish colonocytes and reinforce the gut epithelial barrier.
2. Inanimate microorganism components interact with immune receptors independently of live bacteria.
3. Bioactive metabolites produced during fermentation modulate inflammatory pathways systemically.
Postbiotic types studied in literature
Butyrate (SCFA)
Gut barrier, IBD, colonocyte fuel
Strong
Acetate & propionate
Immune regulation, appetite
Emerging
Fermentation metabolites
Gut comfort, tolerance
Emerging
Heat-inactivated bacteria
Immune signalling
Emerging
Urolithin A
Mitochondrial health, muscle
Emerging
Bacteriocins
Pathogen suppression
Early
Indole derivatives
Gut-brain signalling
Early
Exopolysaccharides
Immune modulation
Early
Important distinction
Postbiotics are not simply "dead probiotics"
While some postbiotics are inanimate microorganisms, the category covers any bioactive compound produced by or derived from microorganisms. The specific metabolite and its concentration determines the effect.
🧠 Gut-brain axis connection
Postbiotics may be the most direct biotic link to the gut-brain axis. Indole derivatives — produced when gut bacteria metabolise tryptophan — are precursors to serotonin synthesis. SCFAs like butyrate reinforce gut barrier integrity, reducing systemic LPS translocation linked to neuroinflammation. Mechanistically well-characterised in preclinical models; human evidence in mood outcomes remains early.
ISAPP definition · 2020
"A mixture comprising live microorganisms and substrate(s) selectively utilised by host microorganisms that confers a health benefit on the host."
Think of it like...
Planting seeds with their own matched fertiliser. The prebiotic is specifically selected to benefit the probiotic strain it accompanies — improving survival through the GI tract and enhancing colonisation. The pairing is the science.
How they work
Two synbiotic approaches
1. Complementary synbiotics — probiotic and prebiotic each independently confer benefits.
2. Synergistic synbiotics — the prebiotic is specifically designed to be metabolised by the accompanying probiotic strain, enhancing its effect beyond what either achieves alone.
Areas studied in published literature
Infant microbiome restoration
C-section born babies
Strong
Allergy prevention
Atopic dermatitis, asthma
Emerging
Inflammatory bowel disease
UC, pouchitis
Emerging
Gut barrier integrity
Permeability reduction
Emerging
Immune development
Neonatal immunity
Emerging
Metabolic health
Glucose, lipid metabolism
Early
Important distinction
Combining any probiotic and prebiotic does not make a synbiotic
A true synbiotic requires that the prebiotic substrate is selectively utilised by the specific probiotic strain it accompanies. Random combination without strain-substrate matching is not synbiotic by scientific definition.
🧠 Gut-brain axis connection
The synbiotic approach may amplify gut-brain axis effects by combining the direct microbial signalling of probiotics with the SCFA-mediated pathways of prebiotic fermentation. Research at this specific intersection is early — but mechanistically the combination creates conditions each independently associated with reduced neuroinflammation and improved gut-brain communication.