Figure 1.

A schematic overview of potential consequences of probiotic supplementation. (A) An altered resource availability due to dietary restriction. (B) When presenting a probiotic (indicated by green microbes), these species will have to compete for available resources (represented by the blue color). (C) When the microbial diversity is low, resource availability will be sufficient for successful establishment and growth of the desired probiotic. However, when the microbial diversity is high, it usually correlates with high resource utilization and thus lower resource availability for the invading probiotic, leading to high competition and lack of establishment. This increased competition can either lead to (D) (genetic) adaptation of either the probiotic or resident species and (E) permanent or transient changes in the composition and functionality of the resident species, which in case of keystone species could lead to dramatic shifts in microbial community. These community dynamics could therefore lead to unpredictable changes in microbiome composition and potentially different effects on brain functioning then initially predicted from in vitro studies.