The absence of microbes in the human gut during early brain development may lead to reduced sociability, according to new research.
This absence causes increased activity in part of the brain which processes emotions and contributes to fear and anxiety responses, and could explain reduced sociability in people with autism spectrum disorders, they suggest.
The findings are based on research on adult mice conducted by scientists at APC Microbiome Ireland, attached to UCC, which has conducted wide-ranging research on the significance of the human "microbiome".
There are at least as many bacterial cells as human cells in the body; a great many of which are in the gut.
Known collectively as the microbiome, they are critical for our health as they influence brain activity and behaviour while also protecting against germs, breaking down food to release energy and producing vitamins.
In the new study, APC researchers led by Prof John Cryan set out to determine whether gene activity in the amygdala region of the brain during social interaction differs between germ-free mice, which have no bacteria, and mice with a normal microbiome.
Sterile environment
"Germ-free mice, which grow up in a sterile environment and thus have no microbes in or on their bodies, were less sociable and spent less time than control animals interacting with an unfamiliar mouse," said Dr Roman Stilling, first author of the paper published in the journal eLIFE – the research was funded by Science Foundation Ireland and the Irish Research Council.
“During social interaction, they displayed a strikingly different pattern of gene activity in the amygdala region of the brain compared to normal mice,” he explained.
The germ-free mice had increased levels of a process called “alternative splicing”. This enables cells to produce many different proteins from a single gene.
“Increases in gene activity in the amygdala, which processes emotions and contributes to fear and anxiety responses, may provide clues to the processes underlying reduced sociability in people with autism spectrum disorders,” said Prof Cryan.
“This new study deepens our understanding of the links between the microbiome and brain health,” he added.
In these mice, the brains don’t develop properly; their nerve cells don’t talk to each other appropriately, thus implicating the microbiome in a variety of disorders.
The UCC team has also shown changes in anxiety behaviour, fear, learning, stress response, and the blood-brain barrier in such circumstances. They have found “a deficit in social behaviour, so for social interactions we have an appropriate repertoire of bacteria in the gut as well”.
The human microbiome is the subject of an international conference attended by 700 scientists in Killarney, Co Kerry, this week. It is focusing on latest science; development of therapeutics, and the microbiome's links to brain diseases, cancer, HIV and Irritable Bowel Syndrome. It is organised by APC Microbiome Ireland and the International Human Microbiome Consortium.
Meanwhile, scientists and academic institutions throughout the world are promoting the first ever World Microbiome Day on Wednesday; "a celebration of microbes" to encourage public dialogue on the critical importance of microbes for human, animal and environmental health.
They are also providing an education resource to counter perceptions that all microbes are bad, which is available through worldmicrobiomeday.com
