Figure 1.

Models of a core microbiome. The circles represent the microbial communities in different individuals and can be thought of as either representing different taxa (species, genera, etc.) or representing different genes. (A) “Substantial core” model. Most individuals share most components of the microbiota. (B) “Minimal core” model. All individuals share a few components, and any individual shares many components with a few other individuals, but very little is shared across all individuals. (C) “No core” model. Nothing is shared by all individuals, and most diversity is unique to a given individual. (D) “Gradient” model. Individuals next to each other on a gradient, for example, age or obesity, share many components, but individuals at opposite ends share little or nothing. (E) “Subpopulation” model. Different subpopulations, for example, those defined by geography or disease, have different cores, but nothing is shared across subpopulations. Scenarios C–E would represent situations in which the strategy of identifying core species for sequencing, then using these as a scaffold for “omics” studies, would be problematic.