In sickness or in health, the billions of microorganisms that inhabit our guts are our constant companions throughout life. In the past few decades, scientists have shown how the nature of this ' microbiome ' can provide valuable clues to human diseases and their treatment. A new study from the Bork group at EMBL Heidelberg, recently published in the journal Cell, reports that a number of conditions, such as lifestyle and disease, affect the total number of microbes in the gut, making this often neglected metric one that bears further evaluation in gut microbiome research.
From composition to loads When studying microbiomes, researchers tend to focus more strongly on microbial composition – the relative proportion of different species of microbes (usually bacteria and archaea, but also protists, viruses, and other microorganisms). This tells us, for example, whether the level of one species of bacteria goes up or down compared to other species in the guts of certain disease patients. To illustrate this, imagine that only 1,000 bacteria live in your gut.
In healthy individuals, this might include 10 bacteria of species 'red', and 20 bacteria of species 'blue', so we could say red bacteria make up 2% of the microbiome while blue bacteria make up 5%. However, in individuals who have a particular disease, we might notice red bacteria make up 4% of the microbiome – a relative increase, while blue bacteria remain at 5%. We could then hypothesise that the red bacteria are associ.