ABSTRACT
The composition and metabolism of the gastrointestinal microbiota affects the perform-
ance of farm animals in many ways, especially young ones subjected to environmental
stress. The indigenous microbiota that is established after birth interacts with the diges-
tive and immune systems of the body, and its activities can be both beneficial and
harmful to the host. The colonization of the different compartments of the gut by
specific commensal bacteria, partly by means of association with the mucus layer or
adhesion to the surface or epithelial cells, serves as a first defense barrier against invad-
ing microorganisms or toxic substances in the diet. In some species, especially in adult
ruminants, the digestion of a fibrous diet is mainly based on the fermentative action of
the bacteria in the rumen. In addition to digestive aid, the gut microbiota may produce
substances or reprocess the refluxed host metabolites that are absorbed and utilized or
In healthy animals, each part of the intestines is colonized by a typical microbiota,
which is adapted to grow in a beneficial symbiosis with the host. Due to the intensive
management methods of today, farm animals are very susceptible to enteric bacterial
imbalance, leading to inefficient digestion and absorption of nutrients and retarded
growth. To overcome these difficulties, diets have been supplemented with antibiotics,
which have indeed proven very effective in decreasing diarrhea and promoting growth.[3-5]
However, the development of resistant strains of harmful bacteria may interfere with
the use of veterinary antibiotics[6,7] and decrease the efficiency of antibiotics per se.
