Raman, A. S. et al. Science 365, eaau4735 (2019).

The organization of microbiota is often described by the abundance of its members, which does not necessarily reflect the dynamic, complex interactions between microorganisms. Raman et al. explore the use of covarying units to describe the development of human gut microbiota. The idea of covarying units emerged from the premise that conserved covariation across microbiota may provide insights about the developmental process of microbial communities. The researchers thus developed a statistical workflow to identify conserved taxon–taxon covariance in bacterial communities of the human gut. After analyzing samples from a Bangladeshi birth cohort, they were able to define an ‘ecogroup’ of 15 microorganisms that could serve as a metric to measure the state of microbiota development. They demonstrated that this ecogroup can distinguish the microbiota of children with different degrees of malnutrition. In addition to the Bangladeshi birth cohort, the ecogroup can also capture the conserved features in microbiota development of healthy Peruvian and Indian birth cohorts.