Fig. 1.

Graphical representation of different microbiome concepts. A: The organ metaphor suggests that the microbiota contains our second genome, which spawned the concept that the microbiome is composed of the microbiota, its genes, and gene products. B: ecological frameworks define the microbiome as the microbiota and its environment. The host shapes the microbial environment using habitat filters that control the size, biogeography, and species composition of the microbiota. Thus, ecologically, the microbiome can be viewed as a host-microbe chimera.