Summary A hallmark of bacterial biofilms is a self-produced extracellular matrix of exopolysaccharide, extracellular DNA (eDNA) and proteins that hold bacterial cells together in the community. However, interactions among matrix components and how the interactions contribute to the formation of matrix remain unclear. Here, we show the physical interaction between exopolysaccharide Psl and eDNA, the two key biofilm matrix components of the opportunistic pathogen Pseudomonas aeruginosa. The interaction allows the two components to combine to form a web of eDNA–Psl fibres, which resembles a biofilm skeleton in the centre of pellicles to give bacteria structural support and capability against agents targeted on one matrix component. The web of eDNA–Psl fibres was also found in flow-cell biofilms at microcolonies initiation stage. The colocalization of eDNA or Psl fibres with bacterial cell membrane stain suggests that fibre-like eDNA is likely derived from the lysis of dead bacteria in biofilms. Psl can interact with DNA from diverse sources, suggesting that P. aeruginosa has the ability to use DNA of other organisms (such as human neutrophils and other bacterial species) to form its own communities, which might increase the survival of P. aeruginosa in multispecies biofilms or within a human host.

Modern maize (Zea mays L.) hybrids are generally regarded as strongly population dependent because maximum grain yields (GYs) per area are achieved primarily in high-density populations. is study was conducted to analyze changes in density independence with plant density based on the response of GY, dry matter (DM) accumulation, and the harvest index (HI) to changes in plant density. Two modern cultivars, ZhengDan958 and ZhongDan909, were planted at 12 densities ranging from 1.5 to 18 plants m -2 . e experiment was conducted for 3 yr, with drip irrigation and plastic mulching, at the 71 Group and Qitai Farms located in Xinjiang, China. With increased plant density, DM accumulation per area increased logarithmically, the HI decreased according to a cubic curve, and GY per area increased quadratically; the optimum density was 10.57 plants m -2 . Further analysis showed that the response of GY per area, DM per area, and the HI to changes in plant density could be divided into four density ranges: Range I (£4.7 plants m -2 ), in which DM per area, the HI, and GY per area were signi cantly a ected by density; Range II (4.7-8.3 plants m -2 ), in which the HI was una ected by density but DM per area and GY per area were signi cantly a ected; Range III (8.3-10.75 plants m -2 ), in which GY per area was una ected by density but DM per area and the HI were signi cantly a ected; and Range IV (³10.7 plants m -2 ), in which DM per area was una ected by density but the HI and GY per area were signi cantly a ected. ese results indicated that Range II is a density-independent range and Range III is a GY-stable range.

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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...