Figure 1.

Four conceptualizations of human microbiota that focus to varying degrees on structure and/or function of the microbiota as a whole or of the component microbial taxa. Assumptions and implications of the extremes of simplicity and tractability on one hand (the multicelled organ conceptualization, Figure 1(a)) and complexity and relative intractability (the dynamic ecological community conceptualization, Figure 1(d)) are described in Table 1. All the interactions (linking arrows) are mediated to some extent by changes in the internal environment, which is not shown to enhance clarity. Mechanisms underlying the various interactions, including the role of internal environment, are depicted in Figure 2.