Mucoid conversion of Pseudomonas aeruginos by hydrogen peroxide: a mechanism for virulence activation in the cystic fibrosis lung

Abstract

The leading cause of mortality in patients with cystic fibrosis (CF) is respiratory failure due in large part to chronic lung infection with Pseudomonas aeruginosa strains that undergo mucoid conversion, display a biofilm mode of growth in vivo and resist the infiltration of polymorphonuclear leukocytes (PMNs), which release free oxygen radicals such as H202. The mucoid phenotype among the strains infecting CF patients indicates overproduction of a linear polysaccharide called alginate. T o mimic the inflammatory environment o f the CF lung, P. aeruginosa PAO1, a typical non-mucoid strain, was grown in a biofilm. This was treated with low levels of H202, as if released by the PMNs, and the formation of mucoid variants was observed. These mucoid variants had mutations in mucA, which encodes an anti-o factor; this leads t o the deregulation of an alternative c factor (022, AlgT or AlgU) required for expression of the alginate biosynthetic operon. All of the mucoid variants tested showed the same mutation, the mucA22 allele, a common allele seen in CF isolates. The mucoid mucA22 variants, when compared to the smooth parent strain PAO1, (i) 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-P-lactamase and (v) secreted little or 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 mucA22 mutant, suggested that the only mutation incurred as a result of H202 treatment was in mucA. When a P. aeruginosa biofilm was repeatedly exposed to activated PMNs in vitro, mucoid variants were also observed, mimicking in vivo observations. Thus, PMNs and their oxygen by-products may cause P. aeruginosa t o 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 a mechanism of defence by the bacteria. This is the f i r s t report to show that PMNs and their oxygen radicals can cause this phenotypic and genotypic change which is so typical of the intractable form o f P. aeruginosa 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.

Sequence diversity of the mucABD locus in Pseudomonas aeruginosa isolates from patients with cystic fibrosis

“…2) is in agreement with in vitro studies showing that this homopolymeric tract is prone to single nucleotide deletion upon exposure of P. aeruginosa biofilms to reactive oxygen species, a scenario envisaged to occur in the CF lung (Mathee et al, 1999).…”

“…1). Since it is extremely unlikely that three *The alginate contents of the PAO1 strain and of its isogenic PDO300 mutant (Mathee et al, 1999) harbouring the mucA stop mutation DG430 were 0?01 and 2?7 mg uronic acid (mg protein) 21 , respectively.…”

Pseudomonas aeruginosa alginate is refractory to Th1 immune response and impedes host immune clearance in a mouse model of acute lung infection

“…The prototypic non-mucoid P. aeruginosa strain PAO1 (Holloway & Morgan, 1986) and its isogenic derivatives were used. These derivatives were the mucoid strain Alg þ PAOmucA22 (PDO300), which carries the mucA22 allele that results in constitutive production of alginate (Mathee et al, 1999), and PAOalgD, a nonmucoid strain that carries a deletion in algD (Garrett et al, 1999). Strains were cultured in LB broth (l À1 : 10 g tryptone, 5 g yeast extract and 5 g NaCl) overnight at 37 8C and bacterial cells were harvested by centrifugation and resuspended in saline to approximately 1·5 3 10 8 c.f.u.…”

“…starvation, antibiotic treatment, slow growth rate, ethanol dehydration, high osmotic pressure or high ionic strength (Evans & Linker, 1973;Govan & Fyfe, 1978;DeVault et al, 1990;Terry et al, 1991). We have demonstrated that local accumulation of activated polymorphonuclear cells (PMNs) exerts selective pressures on the bacteria from phagocytosis and oxygen free radicals, resulting in conversion from the non-mucoid to the mucoid phenotype (Mathee et al, 1999). Approximately 84 % of mucoid CF isolates have mutations in the mucA gene, which encodes a negative regulator of an alternative sigma factor, AlgT/U (Boucher et al, 1997).…”

Calcium-Induced Virulence Factors Associated with the Extracellular Matrix of MucoidPseudomonas aeruginosaBiofilms

“…Mutations in mucA are often responsible for the switch to the mucoid phenotype in clinical isolates (5). Antibiotics as well as components of host defensive processes may contribute to mucA mutations and to the accompanying phenotypic switch from non-alginate-producing strains and acute infections to alginate-producing strains and chronic infections (42,53).…”