Microbial community

A microbial community is an assemblage of microorganisms occupying a defined habitat or sampled environment. Its description includes which organisms occur, their abundances and their interactions with one another and the environment. The boundary matters: the organisms on one leaf, in a patch of soil or in a defined experimental mixture answer different questions in microbial ecology.

A controlled example is the2011 experiment by Gerd Innerebner and colleagues with Arabidopsis thaliana. They compared plants with defined bacterial colonization histories before introducing a Pseudomonas syringae pathogen, then measured both symptoms and pathogen abundance. Many plant-derived Sphingomonas isolates reduced disease, whereas other isolates did not, even at comparable colonization densities. Presence and abundance alone therefore did not specify the protective function.^[ref]

Composition and activity are also distinct measurements. In an invented two-member community, counts of900 and100 cells give relative abundances90% and10%. If both double, those percentages remain unchanged although the total cell count doubles. A DNA sequencing survey supplies another kind of observation: marker counts can reflect gene-copy number and sampling effects, so they are not automatically counts of organisms or measurements of their metabolic rates.^[ref]