Probiotics are defined as living micro-organisms, that upon ingestion in certain numbers, exert health benefits to the host. Their use is linked to proven efficacy on the gastrointestinal microbial equilibrium as well as immunomodulation. The positive effect in animals exerts not only in an improved health status, especially in young animals, but also in improved animal performance, including growth rate and feed conversion efficiency. Microorganisms that are used in animal feeds in the EU are mainly Gram-positive bacteria belonging to genera Bacillus, Enterococcus, Lactobacillus, Pediococcus and Streptococcus and yeasts, such as Saccharomyces cerevisiae or Kluyveromyces species. While most of the species are apparently safe, certain microorganisms may exert harmful properties for animals as well as humans. Enterococci, for example, might harbour transmissible antibiotic resistance determinants, which have the potential to spread in animal and human-associated pathogenic microbial populations. Recent developments in synthetic and systems biology, coupled with bioinformatics and novel tools for genetic engineering, will soon enable the construction of 'artificial' probiotic microorganisms with virtually any combination of properties. Whether and when these 'designer probiotics' will reach out of the labs depends on legislation as well as public opinion.
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