Pseudomonas Aeruginosa Biofilm: Role of Exopolysaccharides in the Function and Architecture of the Biofilm

More than 90% bacteria live as a biofilm in natural habitats. This surface-attached, sessile mode of life confers many advantages to bacteria and increases their survival. Pseudomonas aeruginosa, an opportunistic pathogen, has been extensively studied as a model organism for biofilm studies. Biofilm formation is a stepwise process in which motile bacteria attach to a surface and initiate a cascade of changes. Consequently these bacteria become phenotypically different from their planktonic counterparts. Initially these bacteria attach reversibly to the surface via hydrophobic interactions between the bacterial surface proteins and the substratum. In response to environmental cues, intracellular concentration of a secondary messenger, c-di-GMP, increases in the bacterial cells. This increase in c-di-GMP results in loss of motility, expression of specific adhesins, and irreversible attachment of the bacteria. These bacteria start proliferating while residing in close proximity. The bacterial cells sense the presence of other community members in their close proximity, and start communicating with each other through a mechanism known as quorum sensing. These bacterial cells produce and recruit extracellular polymeric substances (EPS) which encase the bacteria and protect them from unfavourable environmental conditions, predators and biocidal compounds. Components of the EPS include eDNA, proteins, lipids and exopolysaccharides. The exopolysaccharides produced and secreted by the bacteria play a vital role in biofilm formation. Pseudomonas aeruginosa produces various types of exopolysaccharides, but three of them are considered very important in biofilm formation; mannose-rich Psl, glucose-rich Pel, and uronic acid-rich alginate. These three exopolysaccharides are synthesized and secreted by the protein-complex encoded by their respective operons. The role played by these exopolysaccharides in architecture and function of biofilm is currently under extensive research. This review presents the mechanism of biofilm formation and the role of exopolysaccharides in biofilm.