Traditionally bacteria have been considered as either pathogens,commensals or symbionts.The mammal gut harbors 1014 organisms dispersed on approximately1000 different species.Today,diagnostics,in contrast to previous cultivation techniques,allow the identification of close to 100%of bacterial species.This has revealed that a range of animal models within different research areas,such as diabetes,obesity,cancer,allergy,behavior and colitis,are affected by their gut microbiota.Correlation studies may for some diseases show correlation between gut microbiota composition and disease parameters higher than 70%.Some disease phenotypes may be transferred when recolonizing germ free mice.The mechanistic aspects are not clear,but some examples on how gut bacteria stimulate receptors,metabolism,and immune responses are discussed.A more deeper understanding of the impact of microbiota has its origin in the overall composition of the microbiota and in some newly recognized species,such as Akkermansia muciniphila,Segmented filamentous bacteria and Faecalibacterium prausnitzii,which seem to have an impact on more or less severe disease in specific models.Thus,the impact of the microbiota on animal models is of a magnitude that cannot be ignored in future research.Therefore,either models with specific microbiota must be developed,or the microbiota must be characterized in individual studies and incorporated into data evaluation. Traditionally bacteria have been considered as either pathogens, commensals or symbionts. The mammal gut harbors 1014 colonies on approximately 1000 different species. Today, diagnostics, in contrast to previous cultivation techniques, allow the identification of close to 100% of bacterial species. This has revealed that a range of animal models within different research areas, such as diabetes, obesity, cancer, allergy, behavior and colitis, are affected by their gut microbiota. Correlation studies may for some diseases show correlation between gut microbiota composition and disease parameters higher than 70% .Some disease phenotypes may be transferred when recolonizing germ free mice. The mechanistic aspects are not clear, but some examples on how gut bacteria stimulate receptors, metabolism, and immune responses are discussed. A more deep understanding of the impact of microbiota has its origin in the overall composition of the microbiota and in some newly recognized species, such as Akkerman sia muciniphila, of Faecalibacterium prausnitzii, which seem to have an impact on more or less severe disease in specific models. Thus, the impact of the microbiota on animal models is a magnitude that can not be ignored in future research. Before, either models with specific microbiota must be developed, or the microbiota must be characterized in individual studies and incorporated into data evaluation.