Genomic analysis reveals that Pseudomonas aeruginosa virulence is combinatorial

The PA14 and PAO1 genomes are remarkably similar, although PA14 has a slightly larger genome (6.5 megabses [Mb]) than does PAO1 (6.3 Mb). We identified 58 PA14 gene clusters that are absent in PAO1 to determine which of these genes, if any, contribute to its enhanced virulence in a Caenorhabditis elegans pathogenicity model. First, we tested 18 additional diverse strains in the C. elegans model and observed a wide range of pathogenic potential; however, genotyping these strains using a custom microarray showed that the presence of PA14 genes that are absent in PAO1 did not correlate with the virulence of these strains. Second, we utilized a full-genome nonredundant mutant library of PA14 to identify five genes (absent in PAO1) required for C. elegans killing. Surprisingly, although these five genes are present in many other P. aeruginosa strains, they do not correlate with virulence in C. elegans.Genes required for pathogenicity in one strain of P. aeruginosa are neither required for nor predictive of virulence in other strains. We therefore propose that virulence in this organism is both multifactorial and combinatorial, the result of a pool of pathogenicity-related genes that interact in various combinations in different genetic backgrounds.The potential virulence of bacterial pathogens is significantly modulated by the presence of pathogenicity islands [1,2], which are clusters of one or more virulence-related genes that are often acquired by horizontal gene transfer. The introduction of these virulence islands can allow a previously nonvirulent isolate to infect a particular host. Commonly, this switch to a simpler and more stable environment within a host (as opposed to the more complex outside environment) is followed by gene loss and genome reduction that improve the ability of the pathogen to survive in the host but also restrict the range of hosts available to the bacterium [3,4]. In contrast, free-living bacteria that dominate in complex environments (such