Probiotic supplements provide live bacterial (and sometimes yeast) strains intended to colonize or transit the gut and modulate the microbiome.
Mechanism — Most probiotics don't colonize permanently — they transit, interact with the resident microbiome and immune system, and produce metabolites (short-chain fatty acids, antimicrobial peptides). Specific strain-by-condition effects are reasonably well-documented (S. boulardii for diarrhea, L. rhamnosus GG for IBS).
Use case — Post-antibiotic recovery, IBS subtypes, certain immune support contexts. Match strain to condition; ignore most multi-strain marketing.
Caveats — 'More CFU is better' is mostly marketing. Refrigeration and shelf stability matter. Some probiotic blends worsen SIBO or histamine-driven symptoms — discontinue if worse, not better.