e effective than single strains in producing health benefits. For example, a 2011 study showed that a combination of Lactobacillus helveticus and Bifidobacterium longum reduced anxiety and depressive symptoms in healthy volunteers. A 2013 neuroimaging study showed that a fermented milk product containing four different probiotic bacteria was associated with the reduced response of a brain network involved in the processing of emotion and sensation. And certain strains of bacteria can reduce the symptoms of irritable bowel syndrome, a common stress-related disorder of the brain-gut axis. This is probably achieved through a reduction in levels of the “stress hormone” cortisol and of inflammatory molecules produced by the immune system.
These findings are promising, but we are still a long way from the development of clinically proven psychobiotics and it remains to be seen whether they are capable of acting like – or perhaps even replacing – antidepressants. At a time when prescriptions for antidepressants have reached record levels, effective natural alternatives with fewer side effects would be welcome. We are currently completing a study of the gut microbiota in people with severe depression. If we find consistent alterations, this will provide a strong rationale for targeting depression with a suitable psychobiotic. We are also about to start a placebo-controlled study of Lactobacillus brevis in treating anxiety in healthy volunteers.
We must, however, sound a note of caution. Despite marketing claims to the contrary, most putative probiotics have no psychobiotic activity. Until recently, lax regulation in both the US and the European Union allowed manufacturers to make outlandish claims without supporting data. This situation is changing and will protect consumers from fraudulent marketing, but the reality is that only a small percentage of bacteria tested have positive neurobehavioural effects. Some bacteria fail to survive storage in the health food store or are eliminated by acidity in the stomach. Even if they do survive gut transit, they may be devoid of health benefits.
In the 20th century, the major focus of microbiological research was on finding ways to kill microbes via antibiotics. This century the focus has changed somewhat, with a recognition of the health benefits of bacteria, not just from an immunity perspective but from a mental health one. Today, in richer nations, the impact of stress on health is perhaps as great as the threat from harmful bacteria. Psychobiotics have enormous potential.
This article appeared in print under the headline “A light on psychobiotics” Microbiota with personality
The transplantation of faecal microbes (FMT) from a healthy individual into a recipient has emerged as an effective treatment for life-threatening Clostridium difficile infection. The success of this approach has focused attention on FMT to treat gastrointestinal, immune and metabolic disorders, but could FMT be useful in treating neuropsychiatric conditions too?
Intriguingly, a 2011 study by researchers in Canada showed that anxious mice have different microbiota compared with normal mice, and that transplantation of their microbiota into the normal mice makes the normal mice anxious – and vice versa.
If such effects can be translated to humans they have marked implications for development of microbial-based therapies for mental disorders. It also means that would-be FMT donors may need to be screened for mental health issues as well as infectious disease. Profile
John Cryan is a professor of anatomy and neuroscience and Timothy Dinan is a professor of psychiatry, both at University College Cork in Ireland. They are principal investigators at UCC’s Alimentary Pharmabiotic Centre, focusing on the interaction between stress and the brain-gut axis in relation to depression, cognition and irritable bowel syndrome Issue 2953 of New Scientist magazine