The broad host range pathogen Pseudomonas aeruginosa strain PA14 carries two pathogenicity islands harboring plant and animal virulence genes

Abstract

The ubiquitous bacterium Pseudomonas aeruginosa is the quintessential opportunistic pathogen. Certain isolates infect a broad range of host organisms, from plants to humans. The pathogenic promiscuity of particular variants may reflect an increased virulence gene repertoire beyond the core P. aeruginosa genome. We have identified and characterized two P. aeruginosa pathogenicity islands (PAPI-1 and PAPI-2) in the genome of PA14, a highly virulent clinical isolate. The 108-kb PAPI-1 and 11-kb PAPI-2, which are absent from the less virulent reference strain PAO1, exhibit highly modular structures, revealing their complex derivations from a wide array of bacterial species and mobile elements. Most of the genes within these islands that are homologous to known genes occur in other human and plant bacterial pathogens. For example, PAPI-1 carries a complete gene cluster predicted to encode a type IV group B pilus, a well known adhesin absent from strain PAO1. However, >80% of the PAPI-1 DNA sequence is unique, and 75 of its 115 predicted ORF products are unrelated to any known proteins or functional domains. Significantly, many PAPI-1 ORFs also occur in several P. aeruginosa cystic fibrosis isolates. Twenty-three PAPI ORFs were mutated, and 19 were found to be necessary for full plant or animal virulence, with 11 required for both. The large set of "extra" virulence functions encoded by both PAPIs may contribute to the increased promiscuity of highly virulent P. aeruginosa strains, by directing additional pathogenic functions.

Fig. 1.

Correspondence of the PA14 PAPI-1 (A) and PAPI-2 (B) elements with the PAO1 genome. Gene designations and linear coordinates (bp) are presented above and below the lines, respectively. Light and dark gray shading represent the conserved left and right boundaries, respectively. The figure is not drawn to scale.

Fig. 2.

The organization of PAPI-1 (A) and PAPI-2 (B). The boxes with arrows represent individual ORFs and their corresponding transcriptional orientations. Empty boxes represent pseudogenes; triangles represent tRNA genes; and the marked vertical line represents the presumptive attR “attachment” site. The numbered lines represent size (kb), and the coincident rectangles and single or double-headed arrows on the line respectively correspond to direct repeats (DR1–5), inverted repeat (IR), and insertion sequences. ORF color and pattern, respectively, correspond to the predicted protein function and the bacterial species that it is most related to, according to the key. Pathogenesis-related ORFs are indicated by red shading. Gene clusters functions are marked and correspond to the ORFs above the notations. The shaded regions show the homology between PAPI-1 and PAPI-2.

Fig. 3.

Presence of PAPI-1 in P. aeruginosa clinical isolates. The thick black line represents the PAPI-1 coordinates (kb). The arrowheads indicate the position of the DR. The black rectangles correspond to the probes used for hybridization. + denotes positive hybridization; N denotes experiment not done. All strains giving positive hybridization are shown.