The gut microbiota has a role in modifying host steroids, but the exact nature of this interaction has been unclear. New research shows that certain organisms in the gut microbiota produce the neurosteroid allopregnanolone (a derivative of progesterone), and also identified the mechanisms of this production.

“Steroids are exquisitely potent signalling molecules, but that also means they’re found in low levels in blood and tissues, making them hard to detect and quantify,” explains author Sloan Devlin. Using mass spectrometry techniques and faecal samples from healthy women, Devlin and colleagues identified that 21-dehydroxylation was dependent on the presence of certain bacteria (Eggerthella lenta and Gordonibacter pamelaeae). They also used comparative genomics to identify the bacterial gene cluster that was involved in 21-dehyroxylation. Hydrogen gas (in this instance produced by Escherichia coli) was found to be required, as the hydrogen promoted 21-dehydroxylation. “In other words, allopregnanolone production is an example of cooperative metabolism by gut bacteria: some bacteria make hydrogen gas, which promotes allopregnanolone production in other gut bacteria,” says Devlin.