There is growing evidence that the intestinal microbiota interacts with the host central nervous system (CNS) to modify stress responses and anxiety behaviour; this is the so-called gut–brain axis. Bravo et al. now show that chronic treatment of mice with the probiotic bacterium Lactobacillus rhamnosus JB-1 can alter the transcript level for receptors of the neurotransmitter GABA (γ-aminobutyric acid) in the CNS in a region-dependent manner. GABA is the main neurotransmitter in the CNS, and its receptors are important pharmological targets. L. rhamnosus JB-1 treatment decreased corticosterone levels as well as anxiety- and stress-related behaviour when compared with mock-treated mice. Importantly, the effects of probiotic treatment were blocked in vagotomized mice, indicating the vagus nerve as the likely communication pathway in the gut–brain axis.