The single most important concept in probiotics is strain specificity. Two bacteria from the same species — even the same subspecies — can have entirely different health effects depending on their strain. Lactobacillus rhamnosus GG has strong evidence for preventing antibiotic-associated diarrhoea; another Lactobacillus rhamnosus strain may have none. This guide covers the most researched probiotic strains, the conditions they have been studied for, and the strength of the evidence behind them.
Understanding Nomenclature
A probiotic's full name has three parts:
- Genus — the broadest category (e.g., Lactobacillus, Bifidobacterium, Saccharomyces)
- Species — more specific (e.g., rhamnosus, longum, boulardii)
- Strain — the unique identifier (e.g., GG, 35624, CNCM I-745)
Example: Lactobacillus rhamnosus GG - Genus: Lactobacillus (recently reclassified as Lacticaseibacillus, but the old name remains widely used) - Species: rhamnosus - Strain: GG
Only the strain level tells you what the evidence supports. A product listing only genus and species provides insufficient information for evidence-based selection. For guidance on evaluating products, see our UK probiotic buying guide.
Note on reclassification: In 2020, the Lactobacillus genus was reclassified into 25 genera (Zheng et al., 2020, International Journal of Systematic and Evolutionary Microbiology). Lactobacillus rhamnosus is now officially Lacticaseibacillus rhamnosus, and Lactobacillus plantarum is Lactiplantibacillus plantarum. Most consumer products and clinical literature still use the old names, and we use both interchangeably in this guide for clarity.
Tier 1: Strongest Evidence
These strains have multiple published RCTs and/or Cochrane-level reviews supporting their use for specific conditions.
Lactobacillus rhamnosus GG (LGG)
Researched since: 1989 (isolated by Sherwood Gorbach and Barry Goldin — the "GG") Key evidence: - Antibiotic-associated diarrhoea (AAD): A Cochrane review (Goldenberg et al., 2017) concluded that LGG is among the most effective probiotics for preventing AAD, with a number needed to treat (NNT) of approximately 13 - Acute diarrhoea in children: Reduces duration by approximately 1 day (Szajewska et al., 2013, Alimentary Pharmacology & Therapeutics) - Traveller's diarrhoea prevention: Some evidence of benefit, though results are mixed - Eczema prevention in high-risk infants: When given to mothers during pregnancy and to infants — reduced eczema incidence at 2 and 4 years (Kalliomaki et al., 2001, Lancet), though subsequent studies have been inconsistent Typical dose: 10-20 billion CFU daily Available in: Culturelle (primary LGG product in the UK), some Optibac formulations
Saccharomyces boulardii CNCM I-745
What it is: A yeast, not a bacterium — which means it is not affected by antibacterial antibiotics Key evidence: - AAD: Strong evidence from multiple RCTs and meta-analyses (McFarland, 2010, World Journal of Gastroenterology) - Clostridioides difficile infection: Reduces recurrence when used alongside standard antibiotic therapy (Surawicz et al., 2000, Clinical Infectious Diseases) - Acute diarrhoea in children: Reduces duration (Cochrane Review, Szajewska & Skorka, 2009) - Traveller's diarrhoea: Modest prevention benefit (Kollaritsch et al., 1993) Typical dose: 250-500mg (approximately 5-10 billion CFU) daily Available in: Biocodex Floratil, Optibac For Those on Antibiotics, various pharmacy brands. Also available as a standalone supplement.
Bifidobacterium longum 35624 (formerly B. infantis 35624)
Developed by: APC Microbiome Ireland, University College Cork Key evidence: - IBS: Three published RCTs specifically for IBS (Whorwell et al., 2006, American Journal of Gastroenterology; O'Mahony et al., 2005, Gastroenterology). Significant reduction in overall IBS symptoms, particularly bloating, abdominal pain, and bowel dysfunction - Mechanism: Shown to normalise the ratio of anti-inflammatory (IL-10) to pro-inflammatory (IL-12) cytokines in IBS patients (O'Mahony et al., 2005) Typical dose: 1 billion CFU daily (1 capsule) Available in: Alflorex (Precision Biotics) — the sole commercial product containing this strain
VSL#3 / De Simone Formulation (8 strains)
Strains: L. plantarum, L. paracasei, L. acidophilus, L. delbrueckii subsp. bulgaricus, B. longum, B. infantis, B. breve, S. thermophilus Key evidence: - Ulcerative colitis: Multiple RCTs showing benefit for maintaining remission (Tursi et al., 2010; Sood et al., 2009) - Pouchitis: Prevention after ileal pouch-anal anastomosis surgery (Gionchetti et al., 2003, Gastroenterology) - IBS: Moderate evidence for symptom improvement Typical dose: 225-900 billion CFU daily (1-4 sachets) Note: As discussed in our UK probiotics guide, there is a legal dispute about whether current commercial VSL#3 matches the original De Simone Formulation used in the trials. Verify the source. Available in: VSL#3 sachets/capsules (pharmacy or online)
Tier 2: Good Evidence
These strains have published RCTs showing benefit, but the evidence base is smaller or the results less consistent.
Lactobacillus reuteri DSM 17938
Key evidence: - Infant colic: Multiple RCTs in breastfed infants showing significant reduction in crying time (Savino et al., 2010, Pediatrics; Sung et al., 2014 — note: less effective in formula-fed infants) - Functional abdominal pain in children: Moderate evidence of benefit - Constipation: Some evidence for increased bowel movement frequency (Ojetti et al., 2014) Typical dose: 100 million CFU daily (drops for infants) Available in: BioGaia Protectis drops
Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14
Key evidence: - Vaginal health: Multiple RCTs showing restoration of vaginal Lactobacillus dominance and reduction in recurrent bacterial vaginosis and vulvovaginal candidiasis (Reid et al., 2003, FEMS Immunology & Medical Microbiology; Martinez et al., 2009, Letters in Applied Microbiology) - Mechanism: These strains colonise the vaginal tract (when taken orally) and produce hydrogen peroxide and biosurfactants that inhibit pathogenic bacteria Typical dose: 1-2 billion CFU daily Available in: Optibac For Women
Lactobacillus casei Shirota
Developed by: Dr Minoru Shirota (1930, Japan) Key evidence: - Constipation: Improved stool consistency and frequency (Koebnick et al., 2003) - Upper respiratory tract infections: Reduced incidence in shift workers (Gleeson et al., 2011, International Journal of Sport Nutrition and Exercise Metabolism) - Stress-related GI symptoms: Reduced cortisol and GI symptoms in stressed adults (Takada et al., 2016, Beneficial Microbes) Typical dose: 6.5 billion CFU daily (one Yakult bottle) Available in: Yakult, Yakult Light
Lactobacillus plantarum 299v
Key evidence: - IBS: An RCT by Ducrotté et al. (2012, World Journal of Gastroenterology) showed significant reduction in abdominal pain and bloating in IBS patients over 4 weeks - Iron absorption: Shown to enhance non-haem iron absorption (Hoppe et al., 2017, Nutrients) — potentially relevant for vegetarians and those with low iron Typical dose: 10 billion CFU daily Available in: Jarrow Formulas Ideal Bowel Support (online retailers)
Tier 3: Emerging Evidence
These strains show promise in preliminary research but lack the robust RCT evidence of Tiers 1 and 2.
Akkermansia muciniphila
What it is: A mucin-degrading bacterium that makes up 1-5% of the healthy gut microbiome. It strengthens the gut barrier and has anti-inflammatory properties. Key evidence: - A Belgian RCT (Depommier et al., 2019, Nature Medicine) in overweight and obese volunteers showed that pasteurised A. muciniphila improved insulin sensitivity, reduced cholesterol, and decreased markers of liver dysfunction. The pasteurised form was more effective than the live form. - Strongly associated with metabolic health in observational studies — lower in people with obesity, type 2 diabetes, and IBD Status: Next-generation probiotic. Not yet widely available as a supplement in the UK. Regulatory approval for commercial sale is pending in several jurisdictions.
Faecalibacterium prausnitzii
What it is: One of the most abundant bacteria in the healthy human gut and the largest known butyrate producer. Consistently reduced in IBD, particularly Crohn's disease. Key evidence: - Sokol et al. (2008, Proceedings of the National Academy of Sciences) demonstrated that F. prausnitzii has potent anti-inflammatory effects in animal models of colitis - Low levels predict relapse in Crohn's disease patients after surgery (Sokol et al., 2009) Status: Extremely oxygen-sensitive, making it very difficult to produce as a supplement. Currently a research target rather than a commercial probiotic.
Bifidobacterium lactis HN019
Key evidence: - Improved immune function markers in elderly subjects (Gill et al., 2001, American Journal of Clinical Nutrition) - Some evidence for improved gut transit time and reduced constipation (Waller et al., 2011) Typical dose: 1-10 billion CFU daily Available in: Some multi-strain products (check labels for HN019 specifically)
Strains to Be Aware Of (Context-Dependent)
Some strains that are generally safe may not be suitable for everyone:
- Lactobacillus casei — some strains produce histamine, making them potentially problematic for people with histamine intolerance
- Lactobacillus bulgaricus — commonly found in yoghurt but also a histamine producer
- Lactobacillus reuteri — while L. reuteri DSM 17938 is well-studied, some L. reuteri strains produce histamine
- Enterococcus faecium — found in some multi-strain products (including Symprove). Generally considered safe but is intrinsically resistant to vancomycin, raising theoretical concerns about antibiotic resistance transfer
If you have histamine intolerance, choose strains known not to produce histamine — Bifidobacterium species and L. rhamnosus GG are generally safe choices.
How to Use This Guide
- Identify your condition — what are you trying to address?
- Find the strain with the best evidence for that condition in the tiers above
- Find a product containing that strain at the dose used in the trials
- Trial it for at least 4 weeks (consistent with NICE guidance for IBS)
- Assess your response — if no improvement after 4-8 weeks, the strain may not be right for you
Remember that probiotics are supplements, not treatments. They should complement, not replace, medical care for diagnosed conditions. Always discuss with your GP before using probiotics if you are immunocompromised, critically ill, or have a central venous catheter — rare but serious infections have been reported in these populations.
Key Takeaways
- Probiotic benefits are strain-specific — the strain identifier (e.g., GG, 35624, CNCM I-745) determines the evidence, not the species name
- Saccharomyces boulardii and Lactobacillus rhamnosus GG have the broadest and strongest evidence bases
- For IBS, Bifidobacterium longum 35624 (Alflorex) has the most dedicated trial evidence
- For vaginal health, L. rhamnosus GR-1 and L. reuteri RC-14 (Optibac For Women) are the best-studied strains
- Next-generation probiotics (Akkermansia muciniphila, F. prausnitzii) show enormous promise but are not yet commercially available
- Some commonly used strains produce histamine — relevant for people with histamine intolerance
- Always match the strain to the condition and the dose to the trial evidence
- Probiotics supplement, not replace, dietary and medical strategies for gut health
Medical disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making changes to your diet, starting supplements, or making decisions about your health.
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Key Terms
Glossary terms mentioned in this article
IBS (Irritable Bowel Syndrome)
A chronic functional gastrointestinal disorder characterised by abdominal pain, bloating, and altered bowel habits without identifiable structural cause. The microbiome is increasingly recognised as a contributing factor, with IBS patients typically showing reduced microbial diversity and altered bacterial composition compared to healthy controls.
Faecalibacterium prausnitzii
One of the most abundant and important bacteria in the healthy human gut. A major butyrate producer with strong anti-inflammatory properties. Reduced abundance is consistently associated with Crohn's disease and other inflammatory conditions.
Clostridioides difficile
A bacterium that can cause severe diarrhoea and colitis, typically after antibiotic use disrupts the normal gut flora. C. difficile infection is one of the clearest examples of how microbiome disruption leads to disease.
Lactobacillus rhamnosus
One of the most extensively studied probiotic species. The strain L. rhamnosus GG has strong evidence for preventing antibiotic-associated diarrhoea and acute gastroenteritis in children. The strain L. rhamnosus JB-1 has shown anxiolytic effects in animal models via the vagus nerve, though human trials have been less conclusive.
Bifidobacterium longum
A beneficial bacterial species naturally abundant in the human gut from infancy. The strain B. longum 1714 has been studied for its effects on stress and cognitive function, showing modest benefits in human trials. B. longum subsp. infantis is particularly important for infant gut health and breast milk digestion.
Bifidobacterium
A genus of beneficial bacteria that dominates the infant gut and remains important throughout life. Various species (B. longum, B. infantis, B. breve) are among the most studied probiotics, with evidence for IBS, immunity, and infant health.
Lactobacillus
A genus of lactic acid-producing bacteria found naturally in the gut, mouth, and vaginal tract. Certain strains (L. rhamnosus GG, L. plantarum 299v) have strong clinical evidence for specific health benefits. Now reclassified into multiple genera including Lacticaseibacillus and Limosilactobacillus.
Akkermansia
Akkermansia muciniphila is a mucin-degrading bacterium that lives in the intestinal mucus layer. Higher abundance is associated with improved metabolic health, lower inflammation, and better gut barrier function. It thrives on polyphenol-rich foods.