The Harvard Probiotics Symposium, “Gut health, microbiota & probiotics throughout the lifespan: Metabolic & brain function”, was held on September 15th and 16th at Harvard Medical School in Boston (USA).

The second day of the conference, moderated by Emeran Mayer of University of California, Los Angeles (USA), brought to the podium a stellar lineup of researchers on the gut-brain axis. Again, these speakers addressed some key issues in the field:

How do bacteria affect the brain at different life stages?

Growing evidence (at least from mouse models) suggests gut bacterial composition in early life can have effects on the brain; at the Harvard Symposium, Tracy Bale of University of Pennsylvania (USA) detailed her work showing that exposing mice to stress in early pregnancy changes their vaginal microbiome and induces similar-looking changes in the offspring’s gut microbiota at birth. These changes may alter the brain, for example, by reprogramming the young mouse’s hypothalamus, with various effects on behaviour. Bale also noted that the effects of early prenatal stress in mice are greater and more persistent in males, raising the question of whether there could be sex-specific interventions to offset these effects.

Later came the opposite end of the lifespan: in the example of Parkinson’s disease, gut microbiota may also exert effects. Filip Scheperjans of Helsinki University Hospital (Finland) presented a talk on Parkinson’s disease and the slowly growing number of studies that link it to gut microbiota compositional differences. He described three studies showing dysbiosis in Parkinson’s disease, but noted that the nature of the dysbiosis was different from study to study. Further research is required to find out whether probiotic interventions could improve motor symptoms themselves, or only the gastrointestinal symptoms (e.g. constipation) associated with the condition.

What are the effects of neurotransmitters produced by gut bacteria?

Bacteria can produce and utilize neurotransmitters for their own purposes, said Elaine Hsiao of University of California, Los Angeles (USA) as she detailed her lab’s work on the gut serotonin system. Recently, her team found a group of bacterial metabolites that stimulated serotonin in germ-free mice. Hsiao also showed evidence that, in mice, the effects of microbes on gut serotonin influenced both gastrointestinal motility and blood platelet function.

How might probiotics positively influence the brain?

Ted Dinan of University College Cork (Ireland) shared his team’s quest to find possible “psychobiotics”–probiotics that produce a health benefit related to the brain. Dinan spoke about a study that found, in healthy humans, the probiotic B. longum improved performance on a cognitive test. Yet, he said, another probiotic that seemed to work dramatically in mice had no impact on stress response or cognition in humans. He highlighted the need for more translational studies of these potential psychobiotics and their specific effects.

Kirsten Tillisch of University of California, Los Angeles (USA) tackled the subject of irritable bowel syndrome (IBS), noting evidence of structural brain changes as well as functional changes in people with the condition. She acknowledged that IBS is still not well understood (and that no good animal model exists), but that the gut microbiota are probably implicated. Tillisch shared a meta-analysis that supported the efficacy of probiotics for symptoms of IBS, while noting the need for more focused studies of probiotics in three areas: prevention of IBS, treatment of gastrointestinal symptoms, and treatment of psychological symptoms.

Find the stream of live tweets from the Harvard Probiotics Symposium by searching #HarvardMicrobiota16 on Twitter.