Prebiotics are sometimes included with probiotics. A prebiotic is a substance that protects the probiotic bacteria as well as feeds it. Usually these prebiotics are made with a sweet compound called Fructooligosaccharide or FOS for short. This sugar is not digestible by humans, but certain bacteria can digest it. This can cause problems for some people such as gas and bloating. Not everyone is sensitive to FOS, so keep that in mind when purchasing a probiotic.
Disclaimer: I am not a medical professional. Using this information is at your own risk.