Table 1.

Underlying assumptions of conceptualizing human microbiota as a multicelled organ versus an ecological community. Some of the assumptions of the multicelled organ conceptualization also apply to the intermediate conceptualizations depicted in Figure 1.

	Multicelled organ	Ecological community
Assumptions	(1) Identification of component microbes is not necessary for prediction of function	(1) Understanding interactions among microbiota is essential to predict function
	(2) Metabolic products and immune responses are characteristic of the microbiota as a whole	(2) Metabolic products and immune responses are a consequence of community structure and microbial interactions
	(3) Static (changes in healthy microbiota over time are not functionally important)	(3) Dynamic
	(4) Boundaries exist (movement of microbes is not important)	(4) Spatially continuous and linked by immigration and emigration
	(5) Host-to-host variation in microbiota is not important	(5) Host-to-host variation is functionally important
	(6) Microbiota functions for benefit of the host	(6) Net microbiota effects can range from negative to neutral to positive
Implications	(1) Healthy microbiota function is evaluated by its metabolic products and immune responses	(1) Healthy microbiota function is evaluated by both microbial community structure and its metabolic products and immune responses
	(2) Health is restored by providing the right signals/products that are missing or by neutralizing negative signals/products	(2) Health is restored by shifting the community and component interactions, which requires an understanding of processes that control community structure and interaction webs
	(3) Appropriate therapies include broad-spectrum antibiotics, microbiota transplants, direct manipulation of metabolic products, or immune signals	(3) Appropriate therapies include carefully tailored probiotics, modification of internal, or external environment to modify specific interactions