The complexity of the microbial population of the animal gastro-intestinal trac has been recognised long ago. However, thus far, investigations have been limited to a few major groups, considered to be dominating, and pathogens that are detrimental and may case diseases and concomitant financial losses in the production animal. Thanks to the latest developments, including improved micriological detection and sampling techniques, and the application of molecular tools to monitor the presence of specific strains in the intestine, our knowlede has increased rapidly in recent years. In addition,...