Genome-Wide Identification of Pseudomonas aeruginosa Virulence-Related Genes Using a Caenorhabditis elegans Infection Model

Abstract

Pseudomonas aeruginosa strain PA14 is an opportunistic human pathogen capable of infecting a wide range of organisms including the nematode Caenorhabditis elegans. We used a non-redundant transposon mutant library consisting of 5,850 clones corresponding to 75% of the total and approximately 80% of the non-essential PA14 ORFs to carry out a genome-wide screen for attenuation of PA14 virulence in C. elegans. We defined a functionally diverse 180 mutant set (representing 170 unique genes) necessary for normal levels of virulence that included both known and novel virulence factors. Seven previously uncharacterized virulence genes (ABC transporters PchH and PchI, aminopeptidase PepP, ATPase/molecular chaperone ClpA, cold shock domain protein PA0456, putative enoyl-CoA hydratase/isomerase PA0745, and putative transcriptional regulator PA14_27700) were characterized with respect to pigment production and motility and all but one of these mutants exhibited pleiotropic defects in addition to their avirulent phenotype. We examined the collection of genes required for normal levels of PA14 virulence with respect to occurrence in P. aeruginosa strain-specific genomic regions, location on putative and known genomic islands, and phylogenetic distribution across prokaryotes. Genes predominantly contributing to virulence in C. elegans showed neither a bias for strain-specific regions of the P. aeruginosa genome nor for putatively horizontally transferred genomic islands. Instead, within the collection of virulence-related PA14 genes, there was an overrepresentation of genes with a broad phylogenetic distribution that also occur with high frequency in many prokaryotic clades, suggesting that in aggregate the genes required for PA14 virulence in C. elegans are biased towards evolutionarily conserved genes.

Author Summary

Pseudomonas aeruginosa is an opportunistic human pathogen that can also infect a wide range of model organisms, including the nematode Caenorhabditis elegans. To identify P. aeruginosa genes that play key roles in the pathogenic process, we performed a screen for mutants that exhibited reduced ability to kill C. elegans using a previously constructed non-redundant library representing approximately 80% of the non-essential P. aeruginosa PA14 genes. We defined a functionally diverse set of 180 P. aeruginosa mutants (representing 170 unique genes) necessary for normal levels of virulence that included both known and novel virulence factors. The major contributors to P. aeruginosa virulence in the C. elegans infection model were not secretion systems or their corresponding effectors, but rather regulators (particularly ones that are involved in quorum sensing) and genes likely to play key roles in survival of P. aeruginosa within the host intestine. Moreover, these putative P. aeruginosa virulence genes are neither overrepresented in strain-specific regions nor in horizontally acquired genomic islands and furthermore tend to have orthologs that are widely distributed across sequenced prokaryotic species. These data underscore the diversity of pathways involved in virulence, and especially the importance of highly conserved genes for P. aeruginosa virulence in the C. elegans host model.