In a recent publication we examined whether high abundance of a probiotic species, Lactobacillus casei subsp. rhamnosus GG (LGG), impacted the overall composition of the gastrointestinal (GI) microbiota of six-month-old infants at high risk for asthma development. Profound GI microbiota restructuring and the establishment of significantly more even and putatively, functionally redundant consortia were characteristic of high LGG abundance. Here we discuss, in the context of more recently published data, support for the hypothesis that the beneficial effect of probiotic supplementation on human health lies in the formation of a stable and resilient gut ecosystem enriched for species that exert a concerted beneficial effect on the host immune system via direct and indirect mechanisms.