Germ-Free Animals: A Key Tool in Unraveling How the Microbiota Affects the Brain and Behavior

A rapidly growing body of evidence demonstrates that the commensal microorganisms living within the gut influence brain and behavior. Germ-free (GF) mice (microbiota deficient from birth) have been a key tool in realizing much of this knowledge. The gut microbiota is required for normal stress responsivity, sociability, and cognition. Underlying the behavioral and physiological profile of GF mice are alterations in microglial activation, blood-brain barrier permeability, and neurogenesis. Evidence also suggests that the absence of microbiota impacts neuronal communication and synaptic plasticity. One of the advantages of the GF model is that it offers the opportunity for the introduction of select bacteria, or bacterial populations, at different developmental periods. However, developmental effects arising from a GF upbringing may confound some of the conclusions drawn from this model. An increasing understanding of the impact of the microbiota on brain function and structure has the potential to inform novel treatments for the often co-morbid neuropsychiatric and gastrointestinal disorders.