Probiotics are live microorganisms capable of colonizing the intestinal tract, altering the microflora, and exerting a positive effect on the host. The mechanism by which probiotics exert positive effects remains to be elucidated. However, purported mechanisms include antagonizing pathogens through the production of antimicrobial or antibacterial compounds, decreasing gut pH by stimulating lactic-acid producing bacteria, preventing colonization of pathogenic microorganisms by competing for binding sites, improving immune function, competing with pathogens for nutrients and growth factors, producing lactase to aid lactose digestion, and signaling cells to stop production of virulence factors. Probiotics have been used for the treatment of diarrhea and constipation. Preoperative probiotics were examined for prevention of infection in patients undergoing colorectal surgery.