Figure 3 |. interactions between host and microbiota.

a | For luminal microbiota that do not interact with host cells, the host-specific regulatory networks are essentially unaffected by transient microbiota or by luminal microbiota that are distant from the epithelium. This could represent a large class of organisms in the microbiota. b | For microbiota that interact with epithelial cells, there is an equilibrium relationship involving signalling between the microbial populations and the host³⁵. c | For co-evolved microbiota that interact with multiple cell types, the cross-signalling in the most fully co-evolved states involves multiple host cell types, including epithelial, immunological and neuroendocrine cells. The interaction of Helicobacter pylori with the gastric mucosa is representative of this model. The introduction of drugs, such as the commonly used proton pump inhibitors that reduce gastric acidity, affects equilibrium values and selects for a differing microbiota^59,114. d | In the absence of a microbiota, the indigenous host regulatory mechanisms predominate and reach different homeostasis values than are reached when interacting microbiota are present. In the situation of a disappearing microbiome, a and d predominate.