Probiotics are defined by the FAO/WHO as “live microorganisms which when administered in adequate amounts confer a health benefit on the host.” The most extensively documented genera are Lactobacillus (L. rhamnosus GG, L. casei Shirota, L. acidophilus) and Bifidobacterium (B. lactis BB-12, B. longum subsp. infantis), together with the yeast Saccharomyces boulardii. The definition requires that strains be identified by genus, species, and strain-level characterization, and that health benefits be demonstrated in human studies for specific claims.
The mechanisms of probiotic action are multifactorial. Competitive exclusion involves occupying adhesion sites on intestinal epithelial cells and competing with pathogens for nutrients. Production of short-chain fatty acids (SCFAs; acetate, propionate, butyrate) through fermentation of undigested carbohydrates lowers luminal pH, inhibits pathogens, and serves as an energy source for colonocytes. Immune modulation includes enhancement of secretory IgA, modulation of dendritic cell and T-regulatory cell responses, and reduction of pro-inflammatory cytokines. Probiotics also strengthen the intestinal barrier by upregulating tight junction proteins.
Health benefits supported by clinical evidence include prevention and reduction of antibiotic-associated diarrhea (AAD), particularly with L. rhamnosus GG and S. boulardii. Efficacy is also established for prevention of Clostridium difficile infection, reduction of necrotizing enterocolitis in preterm infants, management of irritable bowel syndrome (IBS) symptoms, and alleviation of acute infectious gastroenteritis in children. Emerging evidence supports roles in metabolic health, mood regulation (gut-brain axis), and allergy prevention, though more studies are needed.
The required viable dose is generally 10⁶ to 10⁹ CFU per serving at the end of shelf life. Viability during production and storage is affected by oxygen sensitivity (particularly Bifidobacterium), water activity, storage temperature, and acidity in fermented products. Protection technologies include microencapsulation, freeze-drying with cryoprotectants, and strain selection for acid and bile tolerance. Regulatory frameworks (GRAS in the US, QPS in the EU) require safety assessment of novel strains. Probiotic strains must survive starter culture production and storage. Their health benefits can be enhanced by prebiotics and synbiotics. Many probiotic bacteria are lactic acid bacteria.
