Gut bacteria influence the birth of new brain cells in mice

A long course of antibiotics that wipe out gut bacteria disrupts the growth of brain cells in mice – an effect that can be reversed with exercise or probiotics.

The findings provide more evidence of a link between gut bacteria and the brain, and suggest that at least some probiotics could be beneficial after all.

The trillions of bacteria that line our guts have recently been linked to brain health. Alterations in gut bacteria are thought to potentially play a part in disorders such as Parkinson’s and Alzheimer’s, and there have been a handful of cases in which a course of antibiotics has been linked to psychosis.

To explore the connection further, Susanne Wolf at the Max-Delbrück Center for Molecular Medicine in Berlin, Germany, and her colleagues set out to see if wiping out gut bacteria had any effect on neurogenesis – the creation of new neurons in the brain. This process, which is important for memory formation, is known to be impaired in a number of mental health disorders, such as depression and schizophrenia.

The team gave mice a combination of five antibiotics for seven weeks – an extreme treatment regime that would not be used in people. The researchers then looked at each mouse’s hippocampus, the area of the brain where neurogenesis takes place. They found around 40 per cent fewer newborn brain cells in mice given antibiotics than in untreated mice. The treated mice also performed worse in memory tests.

Restarting neurons

In another experiment, Wolf and her colleagues showed that it was possible to re-establish neurogenesis by giving antibiotic-treated mice a probiotic or making them exercise, something that is known to enhance neurogenesis in people. Surprisingly, transplanting faeces from an untreated mouse to repopulate the gut of a treated animal had no effect.

Gut bacteria may be altering neurogenesis via the immune system. Wolf’s team found that levels of one type of white blood cell rose and fell in line with neurogenesis. When they isolated these cells and injected them into a mouse treated with antibiotics, they were able to restore neurogenesis.

“That something happening in your gut can impact on immune cells, and that that would feed into neurogenesis, is surprising,” says Charles Mackay at Monash University in Melbourne, Australia. The finding adds to the idea that we might be able to improve mental health via our gut bacteria, whether through probiotics or diet, he says.

It is too early to tell whether the short courses of antibiotics that people are prescribed affect neurogenesis. But Wolf reckons her findings provide evidence that some probiotics at least could be beneficial for people on lengthy courses of antibiotics.

Jeffrey Blanchard at the University of Massachusetts Amherst says it is comforting to know that even when the microbiome is wiped out, there are ways of repairing the damage. “It doesn’t really matter what you do, as long as you go for a run afterwards,” he says.

Journal reference: Cell Reports, DOI: 10.1016/j.celrep.2016.04.074