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Probiotics and prebiotics are potential to produce beneficial effects in farm animals via modifications of the microbial population within the digestive tract. In animal nutrition probiotics are viable microorganisms used as feed additives. At present 7 microorganism-preparations are authorised in the EU for the use in poultry mainly for broilers but some also for layer hens or turkey. The organism authorised for the use as feed additives in poultry belong to the bacterial genera of Enterococcus, Bacillus and in one case Pediococcus. For the majority of trials with broilers and turkey a trend towards improved performance data due to the use of probiotics were reported. The modes of action, which lead to beneficial effects are only partly known. It is very probable that the impact of probiotics on pathogenic and nonpathogenic intestinal bacteria are of prime importance. However, modifications of microstructures and barrier functions of intestinal tissues as well as reactions of the immune system seem to be involved in the overall effect, directly or as a consequence of modified bacterial populations. In order to specifically understand and improve probiotics and their application more research is needed. Several methods have to be applied, including 16 RNA group specific and species specific probes as well as microbial metabolites and enzymes. In poultry nutrition, prebiotics like inulin, fructo-oligosaccharides (FOS) and mannanoligosaccharides (MOS) are under study to stimulate growth or metabolic activity of desired intestinal bacteria in order to enhance zootechnical performance. Bifidobacteria (5 strains) and isolated bacteria from crop, stomach, jejunum, ileum and caecum of Broiler chicken (81 microorganisms) were tested whether they are able to metabolise inulin in vitro. It has been shown that some of the bifidobacteria and most of the isolated strains belonging to Enterobacteriaceae and Streptococcaceae groups had enzymes for inulin fermentation. Thus, inulin is not strictly selective for befidobacteria.