Evolution of bacteria seen through their essential genes: the case of Pseudomonas aeruginosa and Azotobacter vinelandii

Pseudomonas aeruginosa is a metabolically versatile bacterium and also an important opportunistic pathogen. It has a remarkable genomic structure since the genetic information encoding its pathogenicity-related traits belongs to its core-genome while both environmental and clinical isolates are part of the same population with a highly conserved genomic sequence. Unexpectedly, considering the high level of sequence identity and homologue gene number shared between different P. aeruginosa isolates, the presence of specific essential genes of the two type strains PAO1 and PA14 has been reported to be highly variable. Here we report the detailed bioinformatics analysis of the essential genes of P. aeruginosa PAO1 and PA14 that have been previously experimentally identified and show that the reported gene variability was owed to sequencing and annotation inconsistencies, but that in fact they are highly conserved. This bioinformatics analysis led us to the definition of 348 P . aeruginosa general essential genes. In addition we show that 342 of these 348 essential genes are conserved in Azotobacter vinelandii, a nitrogen-fixing, cyst-forming, soil bacterium. These results support the hypothesis of A. vinelandii having a polyphyletic origin with a Pseudomonads genomic backbone, and are a challenge to the accepted theory of bacterial evolution.