Figure 2.

A universe of gene functions. In a particular environment, the sum of all microbial genes corresponds to the metagenome, which is in turned formed by pan-genomes. A pan-genome is the sum of all genomes of similar strains; each having similar (core genome) or distinct (cenomes) sets of nonpersistent genes. About ∼500 persistent genes form the paleome. As an example, the addition of 1500 nonpersistent genes to the 500 persistent genes of the paleome in E. coli makes a core genome of 2000 genes, whereas the sum of all cenomes of each individual E. coli strain comprises about 18 000 genes [71]. For the time being, the pan-genome of E. coli is composed of roughly 20 000 genes (2000 of the core-genome and 18 000 of the cenomes), the majority of which (80%) is often colocalized on genomic islands [72]. For a particular E. coli strain with a genome of 4500 genes the cenome alone would be about 4000 genes.