Probiotics are live microbial feed supplements that beneficially affect the host animal by improving its intestinal microbial ecosystem. The stable microbiota in the intestine helps the animal to resist infections, particularly in the intestinal tract by bacterial interactions. In EU regulation 1831/2003 updated until March 2013 (version 156) micro-organism-preparations are listed. These organisms belong to the bacterial genera of Enterococcus, Bacillus and Pediococcus etc..

There are also products which contain large amounts of live bacteria, although not the exact composition is known. These products can be applied to support the build-up of the intestinal microflora. The products are known as Competitive Exclusion products and are sometimes used in very young animals to prevent Salmonella colonization.