Biofilm formation extracellular polysaccharides

Attachment of bacteria. At low ionic strength of the medium — as in many freshwaters — bacteria-surface interactions are controlled by the effects of van der Waals attraction and electrostatic repulsion. At high ionic strength — as in seawater — steric interactions between the outer cell surface macromolecules and the substratum gain in importance (van Loosdrecht et al., 1989 Rijnaarts etal., 1999). Additionally, flagellar and twitching motility of bacteria was found to be essential in the process of attachment by bacteria onto surfaces (Pratt and Kolter, 1998 O Toole and Kolter, 1998). It seems that extracellular polysaccharides of bacteria are not involved in the adhesion process itself. However, bacterial extracellular polysaccharides are necessary for the development of a biofilm and for the formation of microcolonies (Allison and Sutherland, 1987 Hoyle et al., 1993). 

Many microbial infections and toxins are spread by biofilms. Biofilm formation occurs on virtually every surface, starting with the adhesion of planctonic cells or small dispersed biofilm fragments. Proliferation of the cells is accompanied by the expression of an extracellular polysaccharide-based matrix [6], The cells embedded in this matrix are well protected and up to 1000 times less susceptible to antibiotics [7], Once a biofilm is formed, it is extremely difficult to remove this contamination. Thus, all antimicrobial surfaces should prevent the primary attack [8], One class of antimicrobial surfaces prevents the primary attack by creating surfaces that are not sticky to microbial cells, i.e., they do not allow adhesion of these cells. The other major class of antimicrobial surfaces is based on the killing of approaching microbes (see Fig. 2). Interestingly, both approaches can be achieved either by permanent surface modifications or by releasing bioactive compounds. 

In addition, a synthetic route was recently reported for /3-(l,6)-linked glucosamine oligosaccharides containing up to 11 glucosamine residues, corresponding to fragments of the extracellular polysaccharide poly-Af-acetylglucosamine, important in biofilm formation in many bacteria (O Fig. 27) [341]. 

Capsular and extracellular polysaccharides are involved in several aspects of cellular behavior that are tied to bacterial survival and virulence [321]. The capsule layer provides a physical barrier that prevents the bacteria from drying out, aiding in survival outside a host. CPS are also involved in colonization and biofilm formation. In some bacteria CPS promote adherence to surfaces, aiding colonization and biofilm formation, while CPS in other bacteria inhibit adhesion and biofilm formation [344]. 

Membrane autopsy of desalination RO membranes that had been in service for 2.5 yr in Saudi Arabia revealed bacterial deposits that are slimy and very adherent (64). This is primarily due to accumulation of extracellular polysaccharides excreted by microorganisms, thus resulting in biofilm formation (54). Bacteria embedded in a biofilm are found to be more resistant to biocides than freely suspended ones (65). 

Rollefson JB, Stephen CS, Tien M, Bond DR. Identification of an extracellular polysaccharide network essential for cytochrome anchoring and biofilm formation in Geobacter sulfurreducens. 1 Bacteriol 2011 193 1023-1033. 

Vu B, Chen M, Crawford RJ, Ivanova EP. Bacterial extracellular polysaccharides involved in biofilm formation. Molecules 2009 14 2535-54. 

Calcium ions are fixed into the biofilm by the attraction of carboxy-late functional groups on the polysaccharides. In fact, divalent cations, such as calcium and magnesium, are integral in the formation of gels in some extracellular polysaccharides. A familiar biofilm-induced mineral deposit is the calcium phosphate scale that the dental hygienist removes from teeth. When biofilms grow on tooth surfaces, they are referred to as plaques. If these plaques are not continually removed, they will accumulate calcium salts, mainly calcium phosphate, and form tartar (scale). 

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