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Bio film structure and properties

A mature biofilm is formed by microorganisms (representing about 10% of the biofilm) covered with an exopolymeric (EPS) matrix produced by the microorganisms contained in the biofilm, which contains proteins, nucleic acids, lipids, and polysaccharides separated by water channels (Sousa et al., 2011; Flemming and Wingender, 2000; Donlan, 2001, 2008; Mayer et al., 1999; Beech, 2004). Depending on the environment in which the biofilm grows, the matrix may also contain particles resulted from the substrate corrosion or blood components (Donlan, 2002). Water channels are vital for the survival of biofilm formed, allowing the flow of nutrients and metabolic products sharing within the biofilm community (Sousa et al., 2011; Donlan and Costerton, 2002).

EPS represents 50-90% of the total organic carbon atoms from the biofilm structure and is represented by polysaccharides that are different in chemical and physical terms, some of them being neutral or polyanionic polysaccharides (e.g., EPS from Gram-negative bacteria) Donlan, 2002; Flemming et al., 2000). The presence of uronic acid (D-glucuronic acid, D-galacturonic acid, and mannuronic acid) and pyruvate anion influence the biofilm properties (Sutherland, 2001), by binding divalent cations such as calcium and magnesium ions, which contribute to the stabilization of biofilm (Donlan, 2002; Flemming et al., 2000).

In the case of Gram-positive bacterial biofilms, the EPS composition is different and may have a cationic character (Mack et al., 1994). Hussain et al. (1993) showed that the slime factor produced by coagulase-negative staphylococci consists of a teichoic acid mixed with small amounts of protein. Subsequently, the slime was chosen to designate the glicocalix or EPS produced by strains of coagulase-negative staphylococci (S. epidermidis) with strong adherence properties isolated from infected medical implants (Lazar, 2003)

The EPS amount found in a biofilm increases with biofilm age. The EPS matrix influences microbial adherence to the substrate surface and contributes to the resistance of the biofilm to anti-microbial agents, preventing the transport of the antibiotics inside the biofilm, perhaps by binding these agents (Donlan, 2000, 2002). In some instances the antibiotic resistance of microorganisms contained in the biofilm can be 1000 times higher compared to planktonic cells (Kaali et al., 2011; Ceri et al., 1999).

In this context, it is not surprising that chronic infections associated with biofilms are resistant to antibiotics, and to the clearance mechanisms of the host organism (Donlan and Consterton, 2002; Lazar, 2003). The EPS matrix protects the biofilm from physicochemical factors of the external environment, therefore, the microbial biofilms have increased resistance to the action of toxins and disinfectants (Donlan, 2002).

While discussing the structure of the biofilm in general, each biofilm has a unique structure, influenced by several factors, such as the surface of the substrate, the properties of the interface, the availability of nutrients, the composition of the microbial community, the architecture of the three-dimensional polymer matrix (the dense area, presence of pores and ducts), and hydrodynamics (exclusive feature of the environment in which the biofilms develop) (Sousa et al., 2011; Stewart and Franklin, 2008).

The structure of a biofilm may vary from a film with a smooth surface (Wimpenny and Colasanti, 1997) to one with a heterogeneous mosaic structure (Keevil and Walker, 1992) or to a more complex one involving aggregates separated by water channels considered to be the most typical biofilm architecture (Costerton et al., 1994; Costerton and Lewandowski, 1995).

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