|The Pseudomonas aeruginosa pangenome|
Different phenotypes in the P. aeruginosa population go along with a remarkable genomic diversity. Comparisons of independent isolates displayed rather different gene contents. Sequencing projects in our own and other laboratories revealed a core genome common to all P. aeruginosa comprising approximately 4,000 genes. But each genome sequenced so far displayed more than 1,200 additional genes which are either strain-specific or appear in subsets of strains. These genes make up the so-called accessory genome. Many accessory genes display similarities to DNA from phages, plasmids and other mobile elements indicating that the respective DNA blocks have been acquired by horizontal gene transfer. Since closest homologues are often found in other pseudomonads, other gamma- or even beta-proteobacteria, interspecies horizontal gene transfer is apparently a major mechanism for the uptake of additional DNA - thus P. aeruginosa has access to a large gene pool for the acquisition of accessory genome parts. Function and source predictions demonstrated that the accessory genomes are highly variable and individually composed. Whenever a strain of a yet uncharacterized P. aeruginosa clonal complex is subjected to genome sequencing, dozens or hundreds of genes previously unknown in this species are found. Therefore, a very large pan-genome can be assumed for P. aeruginosa. Sequencing a collection of 20 strains representing the most common clonal complexes has revealed a pan-genome of more than 13,000 genes. But since more than 1,000 clonal complexes were already identified, a pool of 100,000 further genes could be assumed to appear rarely in the population and contribute specific features to a few or even to individual P. aeruginosa isolates.