A Survival Strategy for Pseudomonas aeruginosa That Uses Exopolysaccharides To Sequester and Store Iron To Stimulate Psl-Dependent Biofilm Formation

Abstract

Exopolysaccharide Psl is a critical biofilm matrix component in Pseudomonas aeruginosa, which forms a fiber-like matrix to enmesh bacterial communities. Iron is important for P. aeruginosa biofilm development, yet it is not clearly understood how iron contributes to biofilm development. Here, we showed that iron promoted biofilm formation via elevating Psl production in P. aeruginosa. The high level of iron stimulated the synthesis of Psl by reducing rhamnolipid biosynthesis and inhibiting the expression of AmrZ, a repressor of psl genes. Iron-stimulated Psl biosynthesis and biofilm formation held true in mucoid P. aeruginosa strains. Subsequent experiments indicated that iron bound with Psl in vitro and in biofilms, which suggested that Psl fibers functioned as an iron storage channel in P. aeruginosa biofilms. Moreover, among three matrix exopolysaccharides of P. aeruginosa, Psl is the only exopolysaccharide that can bind with both ferrous and ferric ion, yet with higher affinity for ferrous iron. Our data suggest a survival strategy of P. aeruginosa that uses exopolysaccharide to sequester and store iron to stimulate Psl-dependent biofilm formation. IMPORTANCEPseudomonas aeruginosa is an environmental microorganism which is also an opportunistic pathogen that can cause severe infections in immunocompromised individuals. It is the predominant airway pathogen causing morbidity and mortality in individuals affected by the genetic disease cystic fibrosis (CF). Increased airway iron and biofilm formation have been proposed to be the potential factors involved in the persistence of P. aeruginosa in CF patients. Here, we showed that a high level of iron enhanced the production of the key biofilm matrix exopolysaccharide Psl to stimulate Psl-dependent biofilm formation. Our results not only make the link between biofilm formation and iron concentration in CF, but also could guide the administration or use of iron chelators to interfere with biofilm formation in P. aeruginosa in CF patients. Furthermore, our data also imply a survival strategy of P. aeruginosa under high-iron environmental conditions. P seudomonas aeruginosa is a ubiquitous environmental microorganism. It is also an opportunistic pathogen that is often found in the hospital environment. P. aeruginosa can cause severe acute nosocomial infections in immunocompromised individuals or chronic lung infections in cystic fibrosis (CF) patients (1). These infections generally persist despite the use of intensive antimicrobial therapy and have been linked to the formation of antibiotic-resistant biofilms, wherein bacterial cells were encased in an extracellular polymeric substance (EPS) matrix (2, 3). It is generally accepted that the biofilm matrix functions as both a structural scaffold and protective barrier for bacteria in biofilms (4, 5). The iron content of CF sputum is often highly increased compared to that in the sputum of healthy controls, which has also been considered a potential factor in the persistence of P. aeruginosa (6-8). Mineral elem...

Pulcherrimin formation controls growth arrest of theBacillus subtilisbiofilm

“…The low accessibility of iron within the soil stimulates competition between plants and microbes for its acquisition. Bacteria have evolved many mechanisms to capture and sequester iron from the environment including iron uptake systems (35), siderophore production (32) and sorption methods (36). Here we have uncovered the contribution that iron chelation, mediated by pulcherrimin formation, has on the physiology of the B. subtilis biofilm.…”

“…The sequestration of iron within the extracellular matrix is not unique to B. subtilis as in biofilms formed by the Gram-negative bacterium Pseudomonas aeruginosa the Psl polysaccharide fibres of the matrix bind and sequester both ferric and ferrous ions. This is not a passive process as high levels of iron in the environment trigger Psl synthesis and therefore promote accumulation of iron in the local environment of the bacterial population (36). Similarly a polysaccharide produced by Klebsiella oxytoca biofilms has also been shown to bind iron (43) and electron dense iron-enriched deposits accumulate in the fibrous matrix of Enterococcus faecalis biofilms (44).…”

Interdependence between iron acquisition and biofilm formation in Pseudomonas aeruginosa

“…3). In brief, alginate binds ferric iron, Pel binds ferrous iron, and Psl binds to both (Yu et al ., 2016). Importantly, P. aeruginosa is capable of utilizing iron bound to Psl to support its growth during in iron-limiting environments (Yu et al ., 2016).…”

Inflammation: A Double-Edged Sword in the Response to <b><i>Pseudomonas aeruginosa</i></b> Infection

“…The sputum samples of CF patients have relatively high iron levels compared to healthy controls [87], and therefore their lungs may provide a more permissive environment for biofilm formation: high iron levels have been found inhibit repressors of Psl synthesis, a polysaccharide component of biofilms. Psl in turn binds iron for storage and may sequester it for biofilm community use [88]. Changes in iron availability may also underlie increased susceptibility to P. aeruginosa after viral infection: Respiratory syncytial virus (RSV) increases iron availability in murine airway epithelial cells due to apical release of transferrin, promoting the transition of P. aeruginosa to growth in biofilms [51].…”