They are like regular probiotics but much more resilient. Bacillus Coagulans obliterates candida, c.diff, and other harmful bacterial species. It survives stomach acid and is stable on the shelf for 2 years without refrigeration It forms spores which make it very tough. There's also bacillus subtilis but that's pretty much like taking antibiotics, potent stuff.
Get a good Bacillus Coagulans, IMO