Gut microbes can help mice overcome their fears Thankful Photography / Alamy Stock Photo

Mice with a disrupted gut microbiota may be unable to shake off fearful memories – a finding that suggests our gut bacteria may play an important role in the way we learn.

Over the past decade, there has been increasing interest in the role bacteria play in keeping us healthy, especially those living in our gut, mouth and on our skin. Emerging research has linked disruption of these bacterial communities to problems in the immune system and even changes in behaviour.

However, it has been unclear exactly how gut bacteria might bring about behavioural changes. To investigate, David Artis at Weill Cornell Medicine in New York and his colleagues studied the effect antibiotics have on the way mice learn and respond to fearful situations.


They trained mice to fear a sound by delivering a small shock to their paws every time they heard it. Afterwards, the mice would freeze in fear when they heard the tone.

Extinction learning

Normally, mice can gradually lose their fear of the tone if they hear it several times without receiving a shock. This is known as extinction learning.

But Artis and his colleagues found that if mice were given strong antibiotics to wipe out most of their gut bacteria before they learned to fear the tone, they were incapable of undergoing this extinction learning. They continued to freeze in response to the tone even though they were no longer receiving a shock when they heard it.

When the researchers looked at the rodents’ brains, they found that those with the disrupted gut bacteria had different genes switched on. What’s more, there were different patterns of neural activity in the amygdala and the prefrontal cortex – brain regions associated with fear and learning.

The team also found mice treated with antibiotics or those raised to have no microbes living on or in them had lower levels of four compounds linked to neuropsychiatric disorders, including schizophrenia and autism.

Microbes influence some gut functions, and these can affect both postnatal development and the chemical signalling needed for a healthy brain, says Andrew Holmes at the University of Sydney, Australia. When this was changed, it significantly affected how the animals adapted to stress, he says. However, it is important to note that this may only occur in very specific circumstances, and required a major disruption to the microbiota, says Holmes.

The research could shed light on new risk factors of conditions such as post-traumatic stress disorder, says Holmes.

Journal reference: Nature , DOI: 10.1038/s41586-019-1644-y

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