Figure 1.

Successful invasion of a microorganism conceptualized as a 4-stage process. (A) The microbe needs to be introduced in sufficient numbers and in an active form, and possess the traits to withstand the pressures of the gut environment. (B) Habitat filters will select for microbes that possess the traits necessary to overcome them, while the host specifically selects for symbionts by a variety of mechanisms (glycans, epithelial capture, etc.). (C) The microbe needs to compete with resident members to access resources to grow and persist in an ecological niche. (D) Successful occupation of niches may result in metabolic activities and/or competitive or synergistic interactions that impacts the resident community’s composition and/or function. Adopted from Mallon et al. 2015 [13*].