Glossary
Metagenomics.
Direct answer
Metagenomics examines the total DNA extracted from a sample such as faeces, gut contents or soil, without first growing the organisms it contains. The sequences show which micro-organisms are present and which metabolic pathways their genes make possible in principle. Culture-based methods, by contrast, capture only a small part of the community.
Two techniques are often lumped together. Amplicon sequencing of the 16S gene answers the question of who is present. Shotgun metagenomics reads all the genes and shows what the community is equipped to do. Both measure presence rather than activity: a gene that is present may stay silent, and results are proportions.
In horses, many studies rely on faecal samples, because reaching the caecum in the living animal is difficult. Faeces mainly reflect the distal colon and not the site of caecal fermentation. The same limitation applies to human work, yet there the compartment of interest sits considerably closer to the material collected.
The evidence letter
Once a month: new syntheses, checked studies, and what has changed in the state of the evidence.