VIRULENCE PROPERTIES OF PSEUDOMONAS-AERUGINOSA LACKING THE EXTREME-STRESS SIGMA-FACTOR ALGU (SIGMA(E))

A discerning feature of Pseudomonas aeruginosa strains causing chronic endobronchial infections in cystic fibrosis is their conversion into the mucoid, exopolysaccharide alginate-overproducing phenotype. This m orphologically prominent change is caused by mutations which upregulat e AlgU (sigma(E)), a novel extreme-stress sigma factor with functional equivalents in gram-negative organisms. In this work, we investigated the role of algU in P. aeruginosa sensitivity to reactive oxygen inte rmediates, killing by phagocytic cells, and systemic virulence of this bacterium. Inactivation of algU in P. aeruginosa PAO1 increased its s usceptibility to killing by chemically or enzymatically generated halo genated reactive oxygen intermediates and reduced its survival in bact ericidal assays with J774 murine macrophages and human neutrophils. Su rprisingly, inactivation of algU caused increased systemic virulence o f P. aeruginosa in mouse models of acute infection. The increased leth ality of the algU-deficient strain was also observed in the endotoxin- resistant C3H/HeJ mice. Only minor differences between algU(+) and alg U mutant cells in their sensitivity to human serum were observed, and no differences in their lipopolysaccharide profiles were detected. Int riguingly, while inactivation of algU downregulated five polypeptides it also upregulated the expression of seven polypeptides as determined by two dimensional gel analyses, suggesting that algU plays both a po sitive and a negative role in gene expression in P. aeruginosa. While the observation that algU inactivation increases systemic virulence in P. aeruginosa requires further explanation, this phenomenon contrasts with the apparent selection for strains with upregulated AlgU during colonization of the cystic fibrosis lung and suggests opposing roles f or this system in chronic and acute infections.