Influence of Gut Microbes on the Brain-Gut Axis (Gut 2011;60:307-317)

Abstract

Psychological stress and its morbidity are often encountered in functional gut disorders and this emphasizes the importance of the brain-gut axis in regulating gastrointestinal physiology.1,2 The bidirectional interaction between the gut and the brain is vital in maintaining homeostasis but the exact mechanism on how both communicate remains elusive. Evidence from animal studies suggests a possible role of intestinal microbiota to influence central control in this brain-gut axis.3,4 The current study by Gareau et al5 in the Gut 2011 issue provided a direct link for the first time between enteric infection and impaired learning and memory in the presence of psychological stress. The memory impairment persisted even after bacterial clearance and resolution of intestinal pathology. In addition, the study demonstrated the beneficial effect of probiotics in preventing stress-induced memory deficits, normalizing alteration in the hypothalamic-pituitary-adrenal (HPA) axis and colonic injury. This effect was associated, in part, with the restoration of hippocampus brain-derived neurotropic factor (BDNF) and c-Fos expression in the context of normalized microbiota. Germ-free mice also displayed an absence of memory, providing support for the requirement of commensal gut microbiota in memory. In this study, female mice at 5-6 weeks were sourced commercially from 2 sites (Charles River, Canada and Taconic Farms, New York, USA) but both populations of mice behaved similarly according to the author (data not shown). Germ-free mice were contained for 72-96 hours before study to normalize the stress from shipping. The mice were infected with Citrobacter rodentium (strain DBS100, Dr David Schauer, Massachusetts Institute of Technology, MA, USA) administered by oral gavage and were tested for behavior at 10 days and 30 days after infection, respectively.6 C. rodentium is a gram-negative bacterium that causes transient colitis in mice including attaching and effacing lesions and colonic epithelial cell hyperplasia.7 A subset of mice was treated with probiotics (Lacidofil, Montreal, Canada) or placebo daily via their drinking water (orogastric gavage induces stress) starting 1 week before infection. Psychological stress was induced using water avoidance stress,8 behavioral testing included the novel object test,9 while the T-maze test10 was used for dorsal hippocampus memory function (non-spatial and working memory) and light/dark box test11 for anxiety. After behavioral testing, the mice were sacrificed. Blood was collected for serum corticosterone to measure HPA activation, colon tissue for histology to assess for colonic pathology, intra-colonic fecal samples for assessment of bacterial DNA and brain tissue to assess for intensity of BDNF and c-Fos in the CA1 region of the hippocampus. C.rodentium infection in mice was not associated with anxiety behavior whether stressed or not. This was in contrast with another study showing an anxiety response after 8 hours of infection.12 However the current study assessed for anxiety after 10 days which was at the peak of host response to the pathogen. Infected mice demonstrated water avoidance stress-induced memory impairment which was maintained until 30 days after infection, a time period where bacteria should clear and colonic lesions resolved. Probiotics when administered to mice, prevented stress-induced memory deficits with reduction in hippocampus BDNF and c-Fos, reduced serum corticosterone, ameliorated colonic injuries and normalized changes in intestinal microbiota. To confirm the role of microbiota in mediating memory, behavior was studied in germ-free mice. No anxiety was observed in the germ-free mice but memory deficits were evident and this was associated with decrease in hippocampus BDNF and c-Fos.