Probiotics: Which Strains Work, for What, and When They're Useless

First: Understanding the Microbiome

Your gut contains approximately 38 trillion bacteria — roughly equal to the number of human cells in your body. These microbes perform essential functions: they ferment dietary fibre into short-chain fatty acids (SCFAs) that feed gut epithelial cells, produce certain vitamins (K2, some B vitamins), regulate immune responses, modulate neurotransmitter precursors, and keep pathogenic bacteria in check through competitive exclusion.

Disruptions to the gut microbiome — from antibiotics, illness, poor diet, stress, or age — are associated with a wide range of conditions including inflammatory bowel disease, irritable bowel syndrome, obesity, metabolic syndrome, anxiety, and depression.

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. That's the WHO definition — and the key phrase is "adequate amounts." The mechanism matters too: probiotics work by temporarily colonising the gut, producing antimicrobial compounds, competing with pathogens, and modulating local immune responses. Most strains don't permanently colonise; they have to be taken consistently to maintain their presence.

The Strain Problem: Why "Probiotic" Is Almost Meaningless

This is the single most important concept in probiotic science, and the one most supplement companies actively obscure.

When a study shows that Lactobacillus rhamnosus GG reduces the duration of rotavirus diarrhoea in children, that finding does not generalise to all Lactobacillus species, let alone all probiotics. Each strain has its own mechanism, colonisation characteristics, and evidence profile. The genus and species tell you very little. The strain identifier — that final two-letter code like "GG" — is everything.

A supplement listing "10 billion CFU of Lactobacillus acidophilus" could contain any of thousands of strains of that species, each with different properties. Without the strain identifier, you have almost no idea what you're taking.

Evidence by Condition

1. Antibiotic-Associated Diarrhoea (AAD)

This is where probiotic evidence is strongest and most consistent. Antibiotics disrupt the gut microbiome and can cause diarrhoea in 15–25% of patients. Two probiotics have robust evidence:

• Saccharomyces boulardii CNCM I-745: A yeast (not bacteria), so it's unaffected by antibiotics. Multiple meta-analyses confirm significant reduction in AAD risk (OR ~0.5 vs placebo). Also reduces risk of C. difficile-associated diarrhoea. Should be started with the first antibiotic dose and continued for 1 week after finishing.
• Lactobacillus rhamnosus GG: The most studied single probiotic strain. Meta-analyses in children show ~60% reduction in AAD duration. Adult evidence is also positive. Take 2×/day during and after antibiotic course.

Importantly: take probiotics 2 hours away from antibiotic doses to avoid the antibiotic killing the probiotic before it reaches the colon.

2. Irritable Bowel Syndrome (IBS)

IBS evidence is more mixed. No single strain has the same consistent effect across studies, partly because IBS is itself a heterogeneous condition (IBS-D, IBS-C, IBS-M have different pathophysiology).

Best-evidenced strains for IBS:

• Bifidobacterium infantis 35624: O'Mahony et al. 2005 RCT showed significant improvement in bloating, distension, and bowel movement normalisation vs placebo. Strong mechanistic data on immune regulation.
• Lactobacillus plantarum 299v: Multiple RCTs in IBS-D showing reduced abdominal pain and flatulence. 10 billion CFU/day dose used in studies.
• VSL#3 (multi-strain): Contains 8 strains at 450–900 billion CFU; best evidence in IBS with constipation predominance. Also used in ulcerative colitis maintenance.

Effect sizes in IBS are modest — expect symptom improvement, not cure. The 2021 British Dietetic Association guidelines recommend probiotics as a first-line intervention for IBS management, tried for at least 4 weeks.

3. Infectious Diarrhoea (Traveller's Diarrhoea / Rotavirus)

• Lactobacillus rhamnosus GG: Multiple RCTs show 1–1.5 day reduction in diarrhoea duration in children with rotavirus. Evidence in adults is weaker but positive for traveller's diarrhoea prevention.
• Saccharomyces boulardii: Also reduces duration and severity of acute diarrhoea. Can be started before travelling to high-risk areas.

4. Immune Function and Upper Respiratory Infections

Approximately 70% of the immune system is gut-associated (GALT — gut-associated lymphoid tissue). Probiotics can modulate immune responses, and some strains have evidence for reducing the incidence and duration of common colds and upper respiratory tract infections (URTIs).

• Lactobacillus rhamnosus GG: Cochrane review found significant reduction in URTI days in children attending daycare.
• Bifidobacterium lactis BB-12 + L. acidophilus LA-5: Combination showed reduced cold duration and symptom severity in athletes (stress-induced immune suppression model).
• L. casei Shirota (Yakult): Widely available; evidence for maintaining NK cell activity and reducing frequency of common infections.

5. Inflammatory Bowel Disease (IBD)

Evidence is mixed and condition-specific:

• Ulcerative Colitis: VSL#3 has the best evidence for remission maintenance. A 2015 meta-analysis in Gastroenterology found probiotics comparable to mesalazine for maintaining UC remission in some patient groups.
• Crohn's Disease: No probiotic has demonstrated efficacy for Crohn's induction or maintenance of remission. The evidence is consistently negative. Do not use probiotics as a Crohn's treatment.
• Pouchitis: VSL#3 has strong evidence for preventing pouchitis after ileal-pouch-anal anastomosis surgery.

6. The Gut-Brain Axis: Mental Health

This is one of the most exciting — and most overhyped — areas of microbiome research. The gut produces ~90% of the body's serotonin (though it doesn't cross the blood-brain barrier) and communicates with the brain via the vagus nerve, immune mediators, and bacterial metabolites.

The term "psychobiotic" refers to probiotics or prebiotics that confer mental health benefits via the gut-brain axis.

• L. rhamnosus JB-1: Bravo et al. (2011) showed that mice given this strain showed reduced anxiety-like behaviour and lower corticosterone levels — and effects were abolished by vagotomy, confirming the vagus nerve pathway. Human trials are ongoing but small.
• Bifidobacterium longum NCC3001: Pinto-Sanchez et al. (2017) RCT showed significant reduction in depression scores in IBS patients over 6 weeks. Mechanism: altered brain fMRI response to negative emotional stimuli.
• Multi-strain products (Lactobacillus helveticus R0052 + B. longum R0175): Dailey et al. meta-analysis showed reduced depression and anxiety scores across multiple RCTs.

The mental health evidence is real but early. Effect sizes are modest, and this should not replace established treatments for depression or anxiety. It's an interesting adjunct, not a primary intervention.

Strains Worth Knowing

CFU Count: Does More Mean Better?

CFU stands for colony-forming units — the measure of live bacteria in a dose. Products range from 1 billion to 900 billion CFU. Bigger is not automatically better.

The dose that matters is strain-specific. B. infantis 35624 has compelling IBS evidence at just 1 billion CFU/day. VSL#3 requires hundreds of billions for its therapeutic effects. A 50 billion CFU product of an unstudied strain is worth less than 1 billion CFU of a well-researched one.

Also: CFU counts on labels reflect the count at manufacturing. Probiotics are live organisms that die over time, especially when exposed to heat, moisture, and oxygen. Products should be tested for potency at end of shelf life, not at manufacture — look for "CFU guaranteed through end of shelf life" on the label.

Prebiotics: The Fuel That Makes Probiotics Work

Prebiotics are non-digestible fibres that selectively feed beneficial gut bacteria. Key types include inulin, fructooligosaccharides (FOS), galactooligosaccharides (GOS), and resistant starch. The gut microbiome ferments these into short-chain fatty acids (butyrate, propionate, acetate) which are among the most beneficial molecules for gut and metabolic health.

Many trials showing modest probiotic effects may have been hampered by poor prebiotic availability in the test diet. Probiotics work better when fed. Eating diverse plant fibres (vegetables, legumes, oats, green bananas) significantly improves probiotic colonisation and overall microbiome diversity.

Some products combine both ("synbiotics") — pairing specific probiotics with their preferred prebiotic substrates. The evidence for synbiotics is emerging and promising.

For more on prebiotic-rich foods, see our article on fermented foods and the gut microbiome.

When Probiotics Are Probably Useless

• For weight loss: No convincing evidence in humans for meaningful weight reduction.
• For general "detox": Meaningless claim; not how the gut or liver works.
• For Crohn's disease: Consistently negative evidence. Don't use as treatment.
• After every meal: Probiotics aren't digestive enzymes. They don't help break down food.
• Generic "broad spectrum" products: If you can't find the specific strains listed, the manufacturer likely can't either.

Safety

Probiotics are safe for healthy people. Serious adverse events are extremely rare and essentially limited to immunocompromised individuals (HIV/AIDS, cancer patients on chemotherapy, post-organ transplant, severe pancreatitis). If you're in a high-risk group, discuss with your doctor before taking probiotics — a 2008 Dutch RCT found increased mortality in severe acute pancreatitis patients given L. acidophilus + B. bifidum, likely due to intestinal ischaemia from the probiotic formula.

For healthy individuals: the main side effects are temporary bloating and gas when starting, particularly with high-dose products. Starting low and building up over 1–2 weeks typically resolves this.

Practical Protocol

The Bottom Line

Probiotics work — but only the right strains, for the right conditions, at the right doses. The most robust evidence is for S. boulardii and L. rhamnosus GG in antibiotic-associated diarrhoea, B. infantis 35624 and L. plantarum 299v in IBS, and VSL#3 in ulcerative colitis maintenance.

The emerging gut-brain axis research is genuinely exciting but not yet ready to replace conventional mental health treatment. The microbiome is real, its importance is real, and targeted probiotic use has real clinical applications.

What doesn't work: vague multi-strain products with no strain identifiers, probiotic claims for weight loss or detox, and using probiotics as a substitute for dietary fibre and food diversity.

The best probiotic strategy is still diet-first: diverse, fibre-rich plants that feed your existing bacteria. Supplements are adjuncts, not replacements.