Why effects are strain-specific, not species-wide
Probiotic benefits depend on the precise strain, identified by genus, species, and a strain code, not on the broad category. Two strains of the same species can carry different genes and produce different effects, so evidence from one cannot be assumed for another. This is why reputable trials name strains exactly.
Examples studied for particular uses include certain Lactobacillus and Bifidobacterium strains explored for IBS symptom relief, and Saccharomyces boulardii, a yeast, studied for antibiotic-associated diarrhea. SIBO in particular is contested territory where probiotic effects are not well established.
What the trials support and what they do not
The strongest probiotic evidence is for specific outcomes such as reducing the risk of antibiotic-associated diarrhea and for some acute infectious diarrhea, with results tied to particular strains and doses. Broad claims that probiotics improve general health in healthy adults are weakly supported.
This is general educational information, not medical advice. Strain, dose, and indication all matter, and people who are immunocompromised or seriously ill should consult a clinician before taking live-microbe products.