Quick answer: Your gut microbiome is the community of bacteria, archaea, viruses, and fungi living in your large intestine — about 39 trillion cells in total, encoding roughly 100 times more genes than your own body. The science is genuinely exciting and genuinely early. Strain matters more than species, and species matters more than just “probiotic” as a category — most generic “probiotic” supplement claims aren’t supported by trial evidence for that specific strain at that specific dose. Eating a varied, fibre-rich diet (the boring answer) has the most consistent evidence for a healthy microbiome. Fermented foods (kefir, kombucha, yogurt, kimchi, sauerkraut) reasonably contribute live cultures and a wider range of metabolites; specific therapeutic probiotic claims need stricter trial evidence.
What the microbiome actually is
The human microbiome is the collection of microorganisms — bacteria, archaea, viruses (mostly bacteriophages), and fungi — living on and in the human body. The largest concentration by far is in the large intestine, with smaller communities in the mouth, on the skin, in the airways, and in the urogenital tract.
The 2016 estimate from Sender, Fuchs, and Milo (PLOS Biology) puts the gut at roughly 39 trillion bacterial cells in a 70 kg adult — about a one-to-one ratio with human cells (the older “ten-to-one” figure was an overestimate). Collectively, this microbial community encodes roughly 100 times more unique genes than the human genome. These genes encode metabolic capacities the human body lacks — fermenting indigestible plant fibres into short-chain fatty acids, synthesising vitamin K and several B vitamins, modulating bile acid metabolism, and influencing the immune system.
The dominant bacterial phyla in a healthy adult Western gut are Bacillota (formerly Firmicutes) and Bacteroidota (formerly Bacteroidetes), with smaller contributions from Actinobacteriota, Pseudomonadota, and Verrucomicrobiota. Within those phyla, the species and strain composition varies enormously between individuals — a person’s microbiome is more individual than their fingerprint, and moves over months to years rather than days.
Bacterial families you’ll see on supplement labels
- Lactobacillus and related genera (the genus has been split into many — Lacticaseibacillus, Lactiplantibacillus, Lentilactobacillus, etc., as of the 2020 reclassification by Zheng et al.). Lactic-acid-producing bacteria found in fermented foods and a large fraction of probiotic supplements. Common species include L. rhamnosus, L. acidophilus, L. casei, L. plantarum, L. reuteri.
- Bifidobacterium. The dominant genus in the breastfed infant gut and an important one in adults; common probiotic species include B. longum, B. breve, B. lactis, B. infantis.
- Saccharomyces boulardii. A yeast (not a bacterium), most commonly used probiotically; some evidence in antibiotic-associated diarrhoea and in traveller’s diarrhoea.
- Akkermansia muciniphila. A mucin-degrading bacterium that lives in the gut mucus layer; correlated in observational studies with metabolic health markers; intervention trials are early. Note: regulatory status as a food supplement varies — pasteurised A. muciniphila received a favourable EFSA opinion in 2021 and was authorised as a novel food in the EU under Commission Implementing Regulation (EU) 2022/168 in February 2022.
- Faecalibacterium prausnitzii. A major short-chain-fatty-acid producer (butyrate); strongly associated in observational studies with anti-inflammatory phenotypes; not yet commercially available as a stand-alone supplement.
Two important framings:
- Strain specificity matters. “Lactobacillus rhamnosus GG” and “Lactobacillus rhamnosus HN001” are different strains of the same species; their published trial evidence is for the specific strain, not interchangeable. A label that says only “L. rhamnosus” or just “probiotic blend” without strain identifiers is a flag — most of the credible trial evidence is for named strains at named doses.
- Most credible probiotic trials use 10⁹ to 10¹¹ CFU per day (one to one hundred billion colony-forming units), often for weeks to months, for specific outcomes like antibiotic-associated diarrhoea (AAD), prevention of Clostridioides difficile-associated diarrhoea, or infant colic.
What’s in fermented foods, and how they differ from probiotic supplements
Kefir
Traditional milk kefir is a fermented milk drink originating from the Caucasus region. It is made by inoculating milk with kefir grains — a symbiotic culture of bacteria and yeasts (a SCOBY) embedded in a kefiran matrix. Common organisms include Lactobacillus kefiranofaciens, L. kefiri, Lactococcus lactis, Acetobacter species, and yeasts including Kluyveromyces marxianus and Saccharomyces unisporus. Live cultures count varies by batch but typically reach 10⁸ to 10⁹ CFU per ml in artisanal preparations. Commercial supermarket kefirs are pasteurised in some markets and live in others — read the label.
Water kefir is a sugared-water fermented variant, with a different microbial composition (lactobacilli plus yeasts, lower in dairy organisms). Commercial water kefir is widely available in UK shops.
The randomised-trial evidence for kefir is limited but generally positive for short-term gut symptoms in lactose-intolerant adults (the fermentation reduces lactose content and the bacteria assist digestion). For broader health outcomes, observational evidence is suggestive but causal trials are small.
Kombucha
Kombucha is fermented sweetened tea, traditionally made by inoculating sweetened black or green tea with a SCOBY. The dominant organisms are acetic-acid bacteria (notably Komagataeibacter species, formerly Gluconacetobacter) and yeasts (Brettanomyces, Zygosaccharomyces, Saccharomyces). The fermentation produces acetic acid, gluconic acid, ethanol (typically 0.5 to 3 percent in commercial product, with strict UK regulations on labelling above 0.5%), small amounts of B vitamins, and various polyphenols from the tea base.
Health-claim caveats: there is currently no GB-authorised health claim for kombucha. The randomised-trial evidence is limited; most published claims about gut, liver, or immune effects in humans are from very small or uncontrolled studies. Kombucha can be enjoyed as a fermented food without expecting it to deliver specific therapeutic outcomes.
Yogurt and yogurt-style products
Live yogurt is fermented milk, traditionally made with Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. The 2010 EFSA opinion (also retained in the GB NHC framework) authorises a specific health claim for live yogurt cultures and lactose digestion: “Live cultures in yoghurt or fermented milk improve lactose digestion of the product in individuals who have difficulty digesting lactose”, conditional on a minimum of 10⁸ CFU per gram of the named species.
Yogurts described as “probiotic” or “bio” usually contain additional strains (commonly Bifidobacterium species) on top of the standard yogurt cultures.
Kimchi, sauerkraut, miso, tempeh, natto
These traditionally-fermented vegetable, soy, and bean products contain a wide range of lactic-acid bacteria during fermentation. Pasteurised commercial versions (some supermarket sauerkraut) are dead-cell preparations and don’t deliver live cultures. Unpasteurised, refrigerated, “live” or “raw” labelled versions do.
The 2021 American Gut Study work and subsequent randomised crossover trial by Wastyk et al. (Cell, 2021) showed that increasing daily fermented-food intake over 10 weeks measurably increased gut microbial diversity and reduced inflammatory markers — better than a high-fibre intervention alone. The result has not yet been replicated at scale but is the strongest evidence to date that fermented foods, as a category, do something measurable in the gut.
Where probiotic supplements have the strongest evidence
- Antibiotic-associated diarrhoea prevention. The 2019 Cochrane review of probiotics for paediatric AAD prevention (Guo et al., CD004827.pub5) found that probiotics reduce AAD from around 19% to 8% (number-needed-to-treat ~9), with the strongest effect at ≥5 × 10⁹ CFU/day of Lactobacillus rhamnosus GG or Saccharomyces boulardii. For Clostridioides difficile-associated diarrhoea prevention, the 2025 Cochrane update (Esmaeilinezhad et al., CD006095.pub5) softened earlier conclusions to “may be effective” — the most recent large trials did not show clear benefit, downgrading the certainty.
- Infant colic. Lactobacillus reuteri DSM 17938 has multiple supportive trials in breastfed infants with colic; the evidence is less consistent in formula-fed infants.
- Lactose digestion. Live yogurt cultures (per the EFSA-authorised claim above).
- Acute infectious diarrhoea in children. Mixed evidence; recent large trials have weakened earlier positive findings; clinical guidelines are evolving.
- Irritable bowel syndrome (IBS). Variable. Some specific strains (Bifidobacterium infantis 35624 is the most-studied) have modest symptom-improvement evidence; the broader category claim is less supported.
Where probiotic claims are weak
- “Boost your immune system” / “improve mood” / “treat depression” / “weight loss” — currently no GB-authorised health claim that any probiotic strain is permitted to claim a benefit for these. Some early-stage research (the gut-brain axis) is interesting; commercial claims significantly outrun the trial evidence.
- “Restore your gut after antibiotics” — observational studies suggest probiotics may slow the natural recovery of microbiome diversity after antibiotics, not speed it up. The evidence is contested but not nothing.
- “Survives stomach acid” / “billions of strains” claims without strain-and-dose specificity should be treated as marketing.
The gut-brain axis — exciting but early
Animal studies (germ-free mice, microbiota-transfer experiments) show clearly that the gut microbiome can influence the brain via the vagus nerve, immune signalling, short-chain fatty acid metabolism, and microbial-derived neurotransmitter precursors. Translation to humans is much less clear. The “psychobiotic” probiotics marketed for anxiety or depression rest on small early-phase trials. The honest framing: it’s a rich research area, the consumer products are getting ahead of the evidence, and there is no GB-authorised claim that any probiotic strain is permitted to claim a benefit for anxiety or depression.
What actually works for a healthy microbiome
The boring, well-evidenced answer:
- Eat a wide variety of plant fibres. The American Gut Project found that people eating 30+ different plant species per week had measurably more diverse gut microbiomes than people eating fewer than 10. Variety beats any single “superfood”.
- Eat fermented foods regularly. Per the Wastyk et al. 2021 trial, this is the intervention with the most direct measurable effect on diversity and inflammatory markers in healthy adults.
- Include prebiotic-rich foods — onions, garlic, leeks, asparagus, oats, pulses, slightly under-ripe bananas. These fermentable fibres feed your existing microbiome.
- Limit ultra-processed foods. The PURE study and others link high ultra-processed-food intake to reduced microbial diversity, though the causal direction is debated.
- Don’t take antibiotics you don’t need. Each course measurably perturbs the microbiome for weeks to months.
- Sleep, exercise, and stress. Each has been shown in human studies to influence the microbiome. The mechanisms are less direct than diet but the effects are real.
Notice that none of these requires a probiotic supplement. The most reliable interventions are dietary patterns and lifestyle, not pills.
The bottom line
Your gut microbiome is real, individual, dynamic, and responsive to diet. The science is genuinely exciting and genuinely early. For specific clinical indications (antibiotic-associated diarrhoea, infant colic, lactose intolerance), specific probiotic strains at specific doses have credible trial evidence. For the broader claims that fill the supplement aisle (immune, mood, weight loss, anti-aging), the trial evidence does not yet support what the marketing implies. The interventions with the most consistent evidence for a healthy microbiome are dietary variety, fermented foods, and avoiding unnecessary antibiotics. Camden Medicals does not currently stock probiotic supplements; we will revisit this when there is a specific commercial-grade strain we can stand behind in evidence terms.
Related Camden Medicals reading
Sources and further reading
- Sender, Fuchs & Milo (2016) — Revised estimates for the number of human and bacteria cells in the body, PLOS Biology
- Wastyk et al. (2021) — Gut-microbiota-targeted diets modulate human immune status, Cell
- Cochrane CD004827 (Guo et al. 2019) — Probiotics for the prevention of paediatric antibiotic-associated diarrhoea
- Cochrane CD006095 (Esmaeilinezhad et al. 2025) — Probiotics for the prevention of Clostridium difficile-associated diarrhoea (updated, softer conclusions)
- gov.uk — GB Nutrition and Health Claims Register (yogurt cultures + lactose digestion claim)
- NHS — Probiotics
Camden Medicals editorial. Information, not medical advice — talk to your GP, dietitian, or pharmacist before starting any probiotic supplement, especially if you are immunocompromised, critically ill, or in hospital, where some probiotic preparations carry specific risks.