Figure 1.

The gastrointestinal tract is a complex system both from a physicochemical and biological point of view. Moving from the upper to the lower GI tract, there are different parameters that vary accordingly to the site, such as oxygen partial pressure, pH and intensity of the dynamic stimuli, as well as an increased number and diversity of bacteria shaping the microbiota. Regardless from the tract considered, the distribution of bacterial strains is influenced by the oxygen gradient. Figure made with biorender (https://biorender.com) [Color figure can be viewed at wileyonlinelibrary.com]