Hybrid Pathogenicity Island PAGI-5 Contributes to the Highly Virulent Phenotype of a Pseudomonas aeruginosa Isolate in Mammals

Most known virulence determinants ofPseudomonas aeruginosa are remarkably conserved in this bacterium's core genome, yet individual strains differ significantly in virulence. One explanation for this discrepancy is that pathogenicity islands, regions of DNA found in some strains but not in others, contribute to the overall virulence ofP. aeruginosa. Here we employed a strategy in which the virulence of a panel ofP. 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. One 99-kb island, designatedP. aeruginosa genomic island 5 (PAGI-5), was a hybrid of the knownP. 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 contributed to the highly virulent phenotype of PSE9. These results indicate that targeting highly virulent strains ofP. aeruginosa may be a useful strategy for identifying pathogenicity islands and novel virulence determinants.