Lactobacillus
Lactobacillus is one of the most common genera of probiotic bacteria. The genus contains hundreds of species — and within each species, hundreds of strains. Clinical evidence is strain-specific, not genus-wide.
Definition
Lactobacillus is a genus of Gram-positive, rod-shaped bacteria that produce lactic acid as their primary metabolic product. Historically, it was one of the most studied probiotic genera. In 2020, a major reclassification reorganised many Lactobacillus species into new genera — for example, Lactobacillus rhamnosus is now formally Lacticaseibacillus rhamnosus in some taxonomies, though the older naming remains common in consumer contexts.
The Critical Taxonomy: Genus > Species > Strain
Key Lactobacillus Strains and Their Evidence
| Strain | Evidence | Key Indication |
|---|---|---|
| L. rhamnosus GG (LGG) | Very strong — 300+ RCTs | Antibiotic-associated diarrhoea; IBS-D |
| L. rhamnosus HN001 | Moderate | Eczema prevention (perinatal) |
| L. plantarum 299v | Moderate | IBS symptom reduction |
| L. reuteri DSM 17938 | Strong (infants) | Infant colic |
| L. acidophilus LA-5 | Moderate | General gut health; IBS |
| L. casei Shirota | Moderate | Transit time; immune function |
Lactobacillus in Probiotic Products
Most consumer probiotic products contain one or more Lactobacillus strains. The presence of Lactobacillus on a label does not tell you which strain — and therefore which evidence base applies. Always look for the full strain designation (species + strain code).
Lactobacillus vs Bifidobacterium
Lactobacillus strains are predominant in the small intestine; Bifidobacterium strains are predominant in the colon. For conditions involving colonic function (constipation, IBS-C), Bifidobacterium strains may be more directly relevant. For small intestinal conditions (AAD, some IBS subtypes), Lactobacillus strains have the stronger evidence base.
Bottom Line
Lactobacillus is a genus containing hundreds of scientifically distinct strains. Clinical evidence is strain-specific. When evaluating a product, check the full strain designation and verify the evidence for that specific strain against your specific health goal.