Biofilms are microbial communities quite different from planktonic cells and most of common microbiological concepts had to be updated in recent years. The peculiar capacity to resist to disinfectants and antibiotics results in biofilms being a public health problem mainly when modern medical devices are used. All artificial surfaces used in medicine may be prone to biofilm attachment and could therefore represent a cause of acute or chronic infectious diseases. Uremic patients are at higher risk from biofilms as not only traditional causes, such as indwelling catheters, but also hemodialysis apparatuses contribute to bacterial exposure. Chemical or physical disinfections have been demonstrated partially active on sessile microorganisms and biofilm avoidance remains the goal to assure an adequate quality of dialytic treatment.

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Biofilm represents a community of microorganisms attached and growing on a solid surface. Bacteria, fungi, yeasts, protozoa and other microorganisms may aggregate to form biofilm. Microorganisms are enveloped in an extracellular matrix of polymeric substances while biofilm is characterized by structural heterogeneity, genetic diversity, and complex community interactions.

Biofilm develops on virtually all surfaces submerged in or exposed to some aqueous solutions irrespective of whether the surface is biological (plants and animals) or inert (glass, plastics, metal, stones). It forms particularly rapidly when the solution contains an abundant nutrient supply. The main component of biofilm is water (97%) organized in channels carrying, by convection, bulk fluid to the community, containing microbial live and dead cells (15%), exopolysaccharides (85%) and a small amount of macromolecules such as bacterial DNA, proteins and other products of bacterial lysis [1]. The initial event in biofilm formation is the adhesion of free-floating microbes to surfaces through weak, reversible van der Waals forces. If the microorganisms are not immediately separated from the surface they can anchor themselves more permanently using cell adhesion molecules such as surface proteins, pili and fim-briae. Some human proteins such as connective matrix (collagen) or plasma (fibronectin and fibrinogen) adsorbed on the biomaterial surface are recognized by specific staphyloccal membrane adhesins, defined as Microbial Surface Components that Recognize Adhesive Matrix Molecules (MSCRAMM), and seem to be determinant for initiating the colonization process [2]. The first microbes begin to synthesize an exopolysaccharide and proteic matrix (slime) that holds the biofilm together and helps in deposition of other cells by providing more varied adhesion sites. The compositions of extracellular polysaccharide matrixes are different between microbe species and play an important role in determining the final architecture of biofilms. The main component of bacterial extracellular matrix is cellulose, but in addition to cellulose other polysaccha-rides are now recognized as important components. Staphylococcus epider-midis and Staphylococcus aureus produce polysaccharide intercellular adhesion (PIA) or the related poly-N-acetyl glucosamine polymer whose synthesis is regulated by the ica locus. PIA supports cell-to-cell contact by means of multilay-ered biofilm. Now it is recognized that PIA-like polymers are produced by several gram-negative bacterial species (e.g., E. coli MG1655) [3].

Only some species are able to attach to a surface on their own, while others are often able to anchor themselves to the matrix or directly to earlier colonists. Once colonization has begun the biofilm survives by its own life, growing through a combination of cell division, recruitment and detachment.

The polymeric matrix of microbial origin protects the cells within it, facilitates the communications among microbes through chemical and physical signals, and provides a physical and chemical barrier to the diffusion of antimicrobial substances and to environmental insults.

Biofilm is a dynamic complex system that evolves according to local microenviromental conditions (hydrodynamics and biochemical conditions, thickness, shear stress and possibly others) and has a spatial heterogeneity (channels, towers) that is linked to the type of bacteria and differs in relation to oxygen limitation, pH, nutrient access and growth rates.

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