Water activity does not shape the microbiota in the human colon

We recently reported an association between stool consistency as measured by the Bristol Stool Scale (BSS) and major markers of the gut ecosystem, including microbial richness, enterotypes and bacterial growth rates in 53 healthy women.1 Meanwhile, the link between stool consistency and colon microbiota composition has been confirmed in two large-scale (n>1000) cohorts including individuals of both sexes, different age and varying health statuses.2 3 In our original manuscript, we hypothesised on mechanisms that would explain the associations observed. Two potential mechanisms were put forward, namely colon ecosystem differentiation through passage rate variation and reduction of water availability. While the impact of passage rate on ecosystem composition has recently been shown to partially account for BSS-associated microbiota variation,4 the potential correlation with water availability remains unexplored.

Water activity (a_w) is a dimensionless variable that reflects the availability of water in a...