Role of Alginate O Acetylation in Resistance of MucoidPseudomonas aeruginosato Opsonic Phagocytosis

Abstract

Establishment and maintenance of chronic lung infections with mucoid Pseudomonas aeruginosa in patients with cystic fibrosis (CF) require that the bacteria avoid host defenses. Elaboration of the extracellular, O-acetylated mucoid exopolysaccharide, or alginate, is a major microbial factor in resistance to immune effectors. Here we show that O acetylation of alginate maximizes the resistance of mucoid P. aeruginosa to antibody-independent opsonic killing and is the molecular basis for the resistance of mucoid P. aeruginosa to normally nonopsonic but alginate-specific antibodies found in normal human sera and sera of infected CF patients. O acetylation of alginate appears to be critical for P. aeruginosa resistance to host immune effectors in CF patients.The predominant bacterial pathogen in chronic pulmonary infection in cystic fibrosis (CF) patients is the mucoid variant of Pseudomonas aeruginosa, which is encapsulated by and overproduces mucoid exopolysaccharide (MEP), or alginate. That alginate is the major virulence factor of P. aeruginosa in CF lung infection is evident from the epidemiology of this disease. The pulmonary function of patients with CF declines only when mucoid P. aeruginosa is isolated and associated lung pathology develops (9, 32, 33). The growth of mucoid P. aeruginosa as a biofilm in the lungs of CF patients has been suggested to be a major factor in long-term bacterium survival. Biofilm formation by P. aeruginosa has been linked to genes involved in quorum sensing (7) and motility (31), with a recent demonstration that the acyl-homoserine lactone molecules involved in the quorum-sensing system (8) can be detected in the sputa of CF patients (42). However, the genes controlling alginate production appear to be independent of control by the known quorum-sensing genes of P. aeruginosa, including lasR and rhlR (8,44,45). Therefore, the question of whether there is a regulator or environmental cue common to both alginate production and quorum-sensing systems has not yet been answered.The conversion of P. aeruginosa to the mucoid state in CF patients is often associated with mutations at the mucA locus (23). MucA and MucB (also called AlgN) act as anti-sigma factors for the alternative sigma factor E (47), encoded by algT (25), also known as algU (22). Increased activity of this sigma factor results in hyperexpression of the alginate biosynthetic operon located at 34 min on the P. aeruginosa genome (25

Airway epithelial control of Pseudomonas aeruginosa infection in cystic fibrosis

“…Although these patients are rare (<5% of the total), it has been shown that a high proportion have naturally acquired an antibody to the alginate antigen that is characterized by the antibody's ability to mediate complement-dependent opsonic killing of mucoid P. aeruginosa growing in υitro either as planktonic cells [3,4] or in a biofilm [73]. The antibodies made to alginate by most infected CF patients [3,4], and also by most humans who have natural antibodies to alginate [74], do not mediate efficient opsonic killing because of their specificity for antigenic epitopes on alginate that do not result in deposition of the critical complement opsonin, C3bi, onto the bacterial surface [73]. However, although animal studies indicate there are potential means to induce these opsonically-active antibodies by vaccination [75], this has not readily been achieved in large-scale human trials.…”

The Neisseria meningitidis Serogroup A Capsular Polysaccharide O-3 and O-4 Acetyltransferase

“…In the extracellular alginate polysaccharide polymers, produced by isolates of P. aeruginosa from patients with cystic fibrosis, Dmannuronic acid is O-acetylated at O-2 and O-3 by three genes, algI, algJ, and algF (35). Alginate O-acetylation had been shown to contribute to biofilm architecture and microcolony formation (36) and resistance to opsonic phagocytosis (37). OAcetylation is also important for Rhizobium-legume symbiosis.…”

Pseudomonas aeruginosa – Pathogenesis and Pathogenic Mechanisms

“…aeruginosa can produce a mucoid exopolysaccharide capsule, comprised of alginate, an acetylated random co-polymer of β 1-4 linked D-mannuronic acid (poly-M) and L-guluronic acid (Gacesa & Wusteman, 1990). The overproduction of alginate is believed to play a role in cell adherence within the CF lung and is also thought to be involved in resistance to host defense by reducing susceptibility to phagocytosis (Pier, Coleman, Grout, Franklin, & Ohman, 2001), also in resistance to antibiotics. The small minorities of CF patients, who are carrying only nonmucoid P. aeruginosa, have significantly better lung function over time compared to those patients infected with mucoid P. aeruginosa (Parad, Gerard, Zurakowski, Nichols, & Pier, 1999).…”