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Three-dimensional communities of surface-attached microorganisms known as biofilms are a prevailing lifestyle in most environments (Stoodley et al., 2002). Bacterial biofilms are present in industrial and medical settings. In the later case biofilms are suspected to be responsible for many contaminations of medical devices leading to nosocomial infections. Control of biofilms is therefore a major concern with important health and economic issues. Over the past decade the need to understand biofilm biology has become one of the major challenge of microbiology (Hall-Stoodley, 2004).

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a surface covered by cells .

Biofilm formation shares common traits with cell adherence and adhesion, nevertheless, it is generally assumed that the characteristics of a biofilm essentially lies in the accumulation of a high number of cells covering a surface (a film). In this film of potentially great depths (from µm to cm), some cells are in direct contact with the surface and most of them are only in contact with each other.

a highly hydrated matrix as an exoskeleton.

Most of the biofilm biomass is composed of a highly hydrated matrix which is intimately associated with the biofilm phenotype (Sutherland, 2001). The role and impact of the matrix on the physiology of the biofilm cells have received a lot of attention, with a special emphasis on issues such as gas and nutrient diffusion constraints imposed on cells in the depths of biofilms. It is now clear that the matrix could play as much a physical as a physiological role by determining the adhesiveness, structure and cohesive strength in biofilms. The analysis of the matrix (also called EPS for Extracellular Polymeric Substance) has revealed that it is a complex milieu, containing, along with water and different types of exopolysaccharides, compounds such as DNA, proteins and signaling molecules. The functions and origins of most of these molecules circulating between the biofilm cells is still largely unknown. The implications of these studies are that the matrix, beyond its role as an exoskeleton that shapes and provides structural support for the biofilm, could also be the place where functions such as nutrient circulation, signaling or spatial determination control could take place.

a profound physico-chemical heterogeneity.

The limitations imposed by the matrix on the bacteria living in a biofilm are bound to have many biological consequences. Diffusion limits, as well as physical constraints lead to the structural and physiological heterogeneity of biofilms. Furthermore, diverse species are often encountered in a biofilm, further helping to create a highly heterogeneous environment.

the biofilm phenotype.

The biofilm heterogeneity creates physico-chemical gradients leading to the development of specific biological properties that distinguish biofilm microorganisms from their planktonic counterparts : the so-called biofilm phenotype . One of the most distinctive features displayed by bacterial biofilms, besides matrix production, structures, gene transfer and communication issues, is their increased resistance to antibacterial agents compared to their planktonic counterparts. As a consequence of this physiological particularity, it is difficult to eradicate pathogenic biofilms by conventional treatments.
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genetics of biofilm formation

The physiological commitment necessary for planktonic bacteria to adopt the biofilm phenotype is now under intense scrutiny, and evidence exit that different genes are involved in the switch from a planktonic to a biofilm lifestyle (Ghigo, 2001; Sauer et al., 2002; Jefferson, 2004)
The use of simple biofilm experimental models amenable to genetic screens led to the characterization of mutants defective for biofilm formation and the subsequent identification of specific factors which mainly participate to rather early stages of biofilm formation.

This approach demonstrated that a variety of surface organelles (flagella, type I, type III, and type IV pili and fimbriae, curli, conjugative pili, MSHA pili) but also membrane adhesins, participate to the process. It has been proposed that the function of these surface structures could include transport and initial adhesion to the surface which is necessary to overcome the physical barrier between the colonizing cells and the surface. These studies contributed to the emergence of the developmental model depicted in the figure 3 (on the right), where general functions used by bacteria during biofilm formation are indicated (O'Toole et al.2000; Davey and O'Toole G, 2000; Lejeune, 2002; Beloin et al.2005 ).

•Davey, M.E. and A. O'Toole G. (2000.) Microbial biofilms: from ecology to molecular genetics Microbiol Mol Biol Rev.64:847-67..
•O'Toole,G.; H. B. Kaplan and R. Kolter. (2000.) Biofilm formation as microbial development Annu Rev Microbiol.54:49-79..
•Sutherland, I.W. (2001) The biofilm matrix--an immobilized but dynamic microbial environment. Trends Microbiol 9: 222-227..
•Stoodley, P., Sauer, K., Davies, D.G., and Costerton, J.W. (2002) Biofilms as complex differentiated communities. Annu Rev Microbiol 56: 187-209..
•Ghigo, J.M. (2003.) Are there biofilm-specific physiological pathways beyond a reasonable doubt? Res Microbiol.154:1-8..
•Sauer, K. (2003) The genomics and proteomics of biofilm formation. Genome Biol 4: 219..
•Lejeune, P. (2003.) Contamination of abiotic surfaces: what a colonizing bacterium sees and how to blur it Trends Microbiol.11:179-84..
•Hall-Stoodley,; J. W. Costerton and P. Stoodley. (2004.) Bacterial biofilms: from the natural environment to infectious diseases Nat Rev Microbiol.2:95-108..
•Jefferson, K.K. (2004.) What drives bacteria to produce a biofilm? FEMS Microbiol Lett.236:163-73.2-227.
• C. Beloin, S. Da Re and J. M. Ghigo. (2005) Chapter 8.3.1.3. Colonization of abiotic surfaces Escherichia coli and Salmonella. Cellular and Molecular Biology. [on line]. http://www.ecosal.org. ASM Press, Washington, D.C.