The roles of the phosphate-binding proteins as virulence factors of Pseudomonads

Abstract

Phosphate signaling and acquisition is critical for the bacterial response to phosphate limitation, and pathogenic bacteria may express a more virulent phenotype when they scavenge phosphate from a host. Among such pathogens is Pseudomonas aeruginosa, a conditional and opportunistic pathogen that can cause lethal sepsis when it colonizes the gut of critically-ill patients. Several multi-drug-resistant strains of P. aeruginosa recently isolated from such patients form newly-described outer surface appendages with abundant PstS proteins that promote bacterial adhesion to intestinal epithelial cells. In this thesis we have expanded similar investigations to DING proteins that like PstS belong to the high affinity phosphate binding proteins in P. aeruginosa but have a high level of sequence identity to eukaryotic DING proteins. Similar appendages were identified in wellcharacterized P. aeruginosa PA14, but in much smaller numbers, correlating with higher production of these proteins in clinical isolates, compared to PA14, at the threshold phosphate concentrations for DING and PstS gene expression. Appendage production could be increased in PA14 by overexpression of DING and/or PstS. Comparative analysis of commonly-used laboratory strains of P. aeruginosa and P. fluorescens indicated that the appendage assembly mechanism is strain-specific, but will incorporate heterologous recombinant DING family proteins. Production of both proteins was dependent on the lowphosphate regulatory protein PhoB, and secretion was dependent on the alternative type II secretion system Hxc, however, formation of appendages was independent of Hxc. The appendages were transformed to amorphous globules by 10 mM phosphate, suggesting that phosphate limitation is critical for both forming and maintaining appendage structure. Investigation into the mechanism of formation of these appendages revealed that they may develop as outer membrane extrusions. We further investigated the virulence potential of PstS/DING-containing appendages on the pathogenicity of P. aeruginosa against cultured intestinal epithelium. Both proteins play a significant role in bacterial adhesion only when expressed at very high levels, with a minor role in cytotoxicity towards intestinal epithelial cells. Recombinant DING proteins have the potential to disrupt the intestinal epithelial layer, inducing changes in the epithelial proteome and exerting cytotoxic effects. In summary, this thesis addresses the virulence potential of DING/PstS family proteins.