| Pseudomonas aeruginosa is a ubiquitous environmental gram-negative bacterium that can be found in lakes, streams, soil, and plant matter, and is also an opportunistic pathogen of humans and a significant cause of nosocomial infection. P. aeruginosa has a large genome that encodes a broad spectrum of virulence factors, allowing it to thrive in multiple hosts and environments. Most known virulence determinants of P. aeruginosa are remarkably conserved within this bacterium's core genome, yet individual strains differ significantly in their virulence. One explanation for this discrepancy is that pathogenicity islands, regions of DNA found in some strains but not others, contribute to the overall virulence of P. aeruginosa.;Here, we employed a strategy whereby the virulence of a panel of P. aeruginosa isolates was tested in mouse and plant models of disease, and a highly virulent isolate, PSE9, was chosen for comparison by subtractive hybridization to a less virulent strain, PAO1. The resulting subtractive hybridization sequences were used as tags to identify genomic islands found in PSE9 but absent in PAO1. Nine genomic islands were identified in PSE9 that were absent in PAO1; seven of these were novel. Two of these islands were Rhs elements, or recombination hot spot elements. One island contained an ORF similar to DotA, a component of some type IV secretion systems, and another contained a gene cluster predicted to play a role in the oxidation of phosphite to phosphate. One large island was similar to a prophage genome. The largest island, a 99 kb island designated PAGI-5, was a hybrid of the known P. aeruginosa island PAPI-1 and novel sequences. Whereas the PAPI-1-like sequences were found in most tested isolates, the novel sequences were found only in the most virulent isolates. Deletional analysis confirmed that some of these novel sequences in PAGI-5 contributed to the highly virulent phenotype of PSE9 in the mouse model of acute pneumonia. These results indicate that targeting highly virulent strains of P. aeruginosa may be a useful strategy for identifying pathogenicity islands and novel virulence determinants. |
