Eukaryotic organisms have evolved in a bacterial world. As such, they enter into symbiotic relationships with their bacterial partners and the complexity of their reciprocal dialogue is only now starting to be unraveled. Together with nutrition, microbiota is one of the key forces impacting on host physiological status. The group of Integrative Physiology of Gnotobionts uses the germ-free mouse model to study the microbiota, specific bacterial strains and bacterial antigenic components impact on the physiological status of the eukaryotic host in two main research lines 1) host juvenile growth and 2) prevention/therapy of allergic sensitization and allergy development.
Host juvenile growth
We have shown for the first time that microbiota is necessary to sustain optimal weight gain and linear growth of infant mice when fed a standard diet or a nutritionally depleted isocaloric diet (Schwarzer et al., 2016) . Using monocolonized mouse model we showed that lactobacilli previously selected in Drosophila model of chronic undernutrition for their growth promoting capabilities were sufficient to increase linear growth of mice in a strain-dependent manner and recapitulated the whole microbiota's effect on growth. On the molecular level, we found that intestinal microbiota and Lactobacillusplantarum interact with the hormonal somatotropic axis (GH/IGF-1) activity to drive systemic growth. This exciting finding implies that there might be evolutionary conserved effects of Lactobacillusplantarum strains to promote juvenile growth. We are further dissecting the molecular mechanism behind the bacteria driven host growth promotion with the ultimate goal to improve the current treatment of undernutrition in human population. Further reading (Schwarzer et al., 2018; Schwarzer 2018; Poinsot et al., 2018) .
Prevention/therapy of allergic sensitization and allergy development
Allergies are widespread pathological immune reactions which are initiated by generally harmless food/airborne antigens. Their global prevalence have been increasing since the 1960s, especially in industrialized countries, suggesting environmental factors play a key role in the susceptibility and etiology of this disorder. The development of allergy is a two-step process: i) allergic sensitization and ii) the effector phase. The humoral and cellular immune responses are clearly biased toward a type 2-related phenotype, characterized by the production of specific IgE antibodies and cytokines, such as IL-4, IL-5, IL-13, or IL-10. In the effector phase, allergen-induced crosslinking of IgE bound to mast cells or basophils leads to release of allergic mediators resulting in the rapid appearance of symptoms, such as sneezing, itching, swelling or diarrhea in case of food allergies.
The hygiene hypothesis postulates that behind the increased susceptibility to allergic diseases is the lack of exposure to microbial stimuli or altered microbial stimulation, which leads to aberrant immune system maturation. Along these lines, dysbiotic microbiota leading to allergy development or associated with allergic diseases has been reported to have decreased or lack of certain groups of commensal bacteria including lactobacilli and bifidobacteria. Our group is using gnotobiotic mouse model to explore how defined bacteria consortium (Kozakova et al., 2016) , bacterial strains (Schwarzer et al., 2011, Schwarzer et al., 2013) and defined bacterial immunogenic components (Schwarzer et al., 2016) impact the development of allergic sensitization and allergic inflammation.