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A species of gut bacteria that breaks down a form of oestrogen may contribute to depression in premenopausal women. Future therapies targeting this microbe could help treat the condition.

Depression is about than in men. While it is unclear why, previous research has suggested the difference could be due to changes in – a form of oestrogen that has been found to be related to positive mood.

at the Renmin Hospital of Wuhan University in China and his colleagues measured oestradiol levels in blood samples from 189 premenopausal women, 91 of whom had depression. They found that, on average, oestradiol levels were nearly 43 per cent lower in those with the condition than those without it.

The researchers then extracted gut microbes from participants’ faecal samples and mixed them into a solution with oestradiol. Gut bacteria from women with depression more quickly broke down the oestradiol than bacteria from women without the condition, suggesting that differences in the gut microbiome contribute to lower oestradiol levels in depression.

The team identified the oestradiol-degrading microbe as Klebsiella aerogenes, which was 14 times as prevalent in stool samples from participants with depression as those without. Female mice fed K. aerogenes for four weeks had lower oestradiol levels and displayed more depressive symptoms, such as grooming less, than female mice not given the bacterium. Together, these findings indicate that K. aerogenes lowers oestradiol levels, potentially contributing to depression in women.

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at Emory University in Atlanta, Georgia, says this study represents the next stages of microbiome research where findings could guide the development of new drugs targeting specific microbes and their metabolic processes. However, it is unclear how significant of an effect K. aerogenes has on mood in humans, he says. It may be so small that future treatments won’t improve depressive symptoms in women.