Evolution of Microbial Genome

Microbial genomes change over time through the gain and loss of genes.

Within a species, multiple strains consistently inherit a shared set of genes known as the core genome.

For example, the bacterial species X has been estimated to possess around 5 core gene families shared across the analysed strains.

Also, some strains can gain extra genes, often through horizontal gene transfer.

The pangenome of species X is formed by the full set of core gene families present across the strains studied, plus the extra genes found across many strains.

In some species, the pan-genome continuously expands as new sets of genes are gained.

These acquired genes can be neutral or even harmful, leading to gene deletions that are common in microbial genomes, while natural selection retains beneficial genes.

So, all the genes for basic survival are part of the core genome of a species.