The leading cause of mortality in patients with cystic fibrosis (CF) is respiratoy failure due in large part to chronic lung infection with strains that undergo mucoid conversion, display a biofilm mode of growth and resist the infiltration of polymorphonuclear leukocytes (PMNs), which release free oxygen radicals such as HO. The mucoid phenotype among the strains infecting CF patients indicates overproduction of a linear polysaccharide called alginate. To mimic the inflammatory environment of the CF lung, PAO1, a typical non-mucoid strain, was grown in a biofilm. This was treated with low levels of HO, as if released by the PMNs, and the formation of mucoid variants was observed. These mucoid variants had mutations in which encodes an anti-σ factor; this leads to the deregulation of an alternative σ factor (σ, AlgT or AlgU) required for expression of the alginate biosynthetic operon. All of the mucoid variants tested showed the same mutation, the allele, a common allele seen in CF isolates. The mucoid variants, when compared to the smooth parent strain PA01, produced 2--6-fold higher levels of alginate|ii) exhibited no detectable differences in growth rate|iii) showed an unaltered LPS profile|iv) were ~72% reduced in the amount of inducible-β-lactamase and (v) secreted little no LasA protease and only showed 44% elastase activity. A characteristic ~54 kDa protein associated with alginate overproducing strains was identified as AlgE (Alg76) by N-terminal sequence analysis. Thus, the common phenotype of the mucoid variants, which included a genetically engineered mutant, suggested that the only mutation incurred as a result of HO treatment was in When a biofilm was repeatedly expose to activated PMNs mucoid variants were also observed, mimicking observations. Thus, PMNs and their oxygen by-products may cause to undergo the typical adaptation to the intractable mu- coid form in the CF lung. These findings indicate that gene activation in bacteria by toxic oxygen radicals, similar to that found in plants and mammalian cells, may serve as a defence mechanism for the bacteria. This suggests that mucoid conversion is a response to oxygen radical exposure and that this response is mechanism of defence by the bacteria. This is the first report to show that PMNs and their oxygen radicals can cause this phenotypic and genotypic change which is so typical of the intractable form of in the CF lung. These findings may provide a basis for the development of anti-oxidant and anti-inflammatory therapy for the early stages of infection in CF patients


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