Our intestinal microbiota serve many roles vital to the normal daily function of 
the human gastrointestinal tract. Many probiotics are derived from our 
intestinal bacteria, and have been shown to provide clinical benefit in a 
variety of gastrointestinal conditions. Current evidence indicates that 
probiotic effects are strain-specific, they do not act through the same 
mechanisms, and nor are all probiotics indicated for the same health conditions. 
However, they do share several common features in that they exert 
anti-inflammatory effects, they employ different strategies to antagonize 
competing microorganisms, and they induce cytoprotective changes in the host 
either through enhancement of barrier function, or through the upregulation of 
cytoprotective host proteins. In this review we focus on a few selected 
probiotics - a bacterial mixture (VSL#3), a Gram-negative probiotic (E. coli 
Nissle 1917), two Gram-positive probiotic bacteria (LGG, L. reuteri), and a 
yeast probiotic (S. boulardii) - for which sound clinical and mechanistic data 
is available. Safety of probiotic formulations is also discussed.