Pseudomonas aeruginosa Psl polysaccharide reduces neutrophil phagocytosis and the oxidative response by limiting complement-mediated opsonization

Abstract

Summary Pseudomonas aeruginosa causes chronic lung infections in the airways of cystic fibrosis (CF) patients. Psl is an extracellular polysaccharide expressed by non-mucoid P. aeruginosa strains, which are believed to be initial colonizers. We hypothesized that Psl protects P. aeruginosa from host defences within the CF lung prior to their conversion to the mucoid phenotype. We discovered that serum opsonization significantly increased the production of reactive oxygen species (ROS) by neutrophils exposed to a psl-deficient mutant, compared with wild-type (WT) and Psl overexpressing strains (Psl++). Psl-deficient P. aeruginosa were internalized and killed by neutrophils and macrophages more efficiently than WT and Psl++ variants. Deposition of complement components C3, C5 and C7 was significantly higher on psl-deficient strains compared with WT and Psl++ bacteria. In an in vivo pulmonary competition assay, there was a 4.5-fold fitness advantage for WT over psl-deficient P. aeruginosa. Together, these data show that Psl inhibits efficient opsonization, resulting in reduced neutrophil ROS production, and decreased killing by phagocytes. This provides a survival advantage in vivo. Since phagocytes are critical in early recognition and control of infection, therapies aimed at Psl could improve the quality of life for patients colonized with P. aeruginosa.

Importance of the Exopolysaccharide Matrix in Antimicrobial Tolerance of Pseudomonas aeruginosa Aggregates

“…Alginate has been shown to protect P. aeruginosa against human leukocyte killing (11), in addition to inhibiting pulmonary clearing and increasing bacterial load in a mucine lung infection model (19). Psl expression is also important for reduced phagocytosis (20). In line with this, increased immune evasion of ΔyfiR mutant strains has been reported, and it has been shown that disruption of pel and psl in a ΔyfiR mutant strain renders it as sensitive to phagocytosis by macrophages as the wild type (14).…”

Insect pathogenicity in plant-beneficial pseudomonads: phylogenetic distribution and comparative genomics

“…Psl was shown contributing to biofilm production, tolerance to oxidizing agents and host defensive processes (Friedman and Kolter, 2004;Jackson et al, 2004;Mishra et al, 2012), that is, traits likely useful in insect interactions.…”

PslG, a self-produced glycosyl hydrolase, triggers biofilm disassembly by disrupting exopolysaccharide matrix

“…Psl, a repeating pentasaccharide containing D-mannose, D-glucose and L-rhamnose [23], acts as 'molecular glue' to promote bacterial cell-cell and cell-surface interactions and it can form a fiber-like web to enmesh bacterial communities [6,7,17]. This exopolysaccharide also confers resistance of P. aeruginosa biofilms to antibiotics and phagocytic cells [9,[24][25][26]. In P. aeruginosa PAO1 (a common lab strain), Psl is more important than Pel for biofilm microcolony formation and antibiotic resistance [24].…”

“…One of the most important features of biofilms is extracellular polymeric substances consisting of mainly polysaccharides, proteins, and extracellular DNA (eDNA), which function as a matrix, or glue, holding biofilm cells together and protecting cells from antibiotics and shear forces in fluid environments [2,[6][7][8]. By forming a matrix-encased multicellular aggregate, cells can also escape engulfment by phagocytic cells within a mammalian host [9]. Hence, factors that disrupt biofilm matrix may disassemble or disperse biofilm, rendering biofilm bacteria sensitive to antibiotics and phagocytic cells.…”