In his paper, “Sur les colorations bleue et verte des linges à pansements”, introduced by Louis Pasteur, Carle Gessard describes the isolation of an organism causing a blue-green coloration of wound dressings . He describes this ‘accidental’ organism as colourless, globular, to . thousandths of a millimetre in length, aerobic and very motile. The bacterium was named Bacillus (rod) pyocyaneus. Today we refer to this organism as Pseudomonas aeruginosa. This species is ubiquitous in the biosphere, has wide metabolic versatility and high intrinsic and acquired resistance to antimicrobials. It can be found in a wide variety of ecological environments ranging from fresh and salt water to the rhizosphere in which they colonize the endemic fauna (e.g. nematodes), flora and fungi (e.g. Pythium spp.) . The opportunistic bacterium P. aeruginosa occasionally migrates from its natural environment and causes disease in animals (wild, domestic and livestock) and humans. In the latter it has emerged, partly due to its intrinsic antibiotic resistance, as a major pathogen in the airways of cystic fibrosis (CF) patients, causing often-fatal chronic respiratory infections, and as one of the most clinically significant opportunist nosocomial agents. Immunosuppressed patients such as those with severe burns, cancer or AIDS are particularly at risk.
Numerous research groups have demonstrated that P. aeruginosa clinical isolates are genotypically, chemotaxonomically, and functionally indistinguishable from environmental isolates. Römling et al. observed that the most frequently identified clone in CF patients was also detected at a relatively high frequency in aquatic environments and Rahme et al. demonstrated the infectivity of a P. aeruginosa strain in both plant and animal models . Similarly, P. aeruginosa strains isolated from a gasoline-contaminated aquifer were indistinguishable from clinical isolates and both oil-contaminated soil isolates and clinical isolates showed pathogenic and biodegradative properties.
Population structure
Using multilocus enzyme electrophoresis, Maynard Smith and colleagues demonstrated that bacterial population structures could range from panmictic or fully sexual, with random association between alleles, to clonal, with nonrandom association of alleles, the latter resulting in the frequent recovery of relatively few of the many possible multilocus genotypes . An intermediate type of population structure that is predominantly sexual, but harbours some epidemic clones, which show significant association between loci, was called ‘epidemic’.
The population structure of P. aeruginosa has been the subject of numerous investigations, we present an overview. Both Denamur et al. in , and Picard et al. in , suggested a panmictic population structure for the species but highlighted the need for caution in inferring the population structure from any single class of genetic marker , . In , comparative sequencing of environmental and clinical isolates revealed a net-like population with a high frequency of recombination between isolates . Using randomly amplified polymorphic DNA typing, Ruimy et al. demonstrated that bacteremia and pneumonia were not caused by specific P. aeruginosa clones . In Lomholt and colleagues suggested an epidemic population structure for a P. aeruginosa population isolated mainly from patients with keratitis and their environment . They found evidence for an epidemic clone that is pathogenic to the eye and is characterized by a distinct combination of virulence factors. In , we combined the data obtained by different typing methods, performed on a batch of unrelated clinical and environmental P. aeruginosa isolates, collected across the world and observed a clear mosaicism in the results and a non-congruence between experiments, features of a panmictic population structure . But, in this network we also observed some clonal complexes characterized by an almost identical data set. There was no obvious correlation between these dominant clones and habitat or, with the exception of some recent clones, their geographical origin. Therefore, we suggested an epidemic population structure for P. aeruginosa. Using multi locus sequence typing (MLST), Curran et al. confirmed in that P. aeruginosa exhibits a nonclonal epidemic population structure . The P. aeruginosa population in the River Woluwe in Brussels was found to be almost as diverse as the global population, harbouring members of nearly all successful clonal complexes.
Several groups found that P. aeruginosa possessed a highly conserved genome, which encoded genes important for survival in numerous environments including humans and evolved through the acquisition, loss, and reorganisation of genome islands and genome islets –. Horizontal gene transfer (HGT) might play a more important role than point mutation in the adaptation of P. aeruginosa to different habitats. Despite not believed to be naturally competent, P. aeruginosa displays a high level of interstrain genomic plasticity and contains a high number of unfixed genes. Shen et al. put forward the idea of a population-based supra-genome that is substantially larger than the genome size of any of the component strains . No two strains would be identical in terms of their genetic content and HGT continuously creates new strains with unique genetic characteristics.
Environmentally endemic bacteriophages are probably responsible for a fair amount of HGT, as they were shown to be formidable transducers of naturally occurring microbial communities of P. aeruginosa.
Pirnay J-P, Bilocq F, Pot B, Cornelis P, Zizi M, et al. (2009) Pseudomonas aeruginosa Population Structure Revisited. PLoS ONE 4(11): e7740. doi:10.1371/journal.pone.0007740