Figure 2 illustrates, for an arbitrary analytical measure, the typical accumulation of biofilm at a surface as a function of time. Initially, the substratum is conditioned and cells attach reversibly, then irreversibly. Next, attached cells grow, reproduce, and secrete insoluble extracellular polysaccharidic material. As the biofilm matures, biofilm detachment and growth processes come into balance, such that the total amount of biomass on the surface remains approximately constant in time. Multiple physical, chemical, and biological processes operate in each phase (Fig.
Processes governing biofilm formation and persistence include the following:
1. Biasing or preconditioning of the substratum either by macromolecules present in the bulk liquid or, in-
tentionally, as in the case of precoating biomedical materials with adhesion molecules (e.g., fibronectin, vitronectin, fibrinogen, von Willebrand's factor).
2. Transport of planktonic cells from the bulk liquid to the substratum.
3. Adsorption of cells at the substratum for a finite time followed by desorption (release) of reversibly adsorbed cells.
4. Irreversible adsorption of bacterial cells at a surface.
5. Transport of substrates into the biofilm, substrate metabolism by the biofilm-bound cells, and transport of products out of the biofilm. These processes are accompanied by cellular growth, replication, and extracellular polymer production.
6. Biofilm removal can be either continuous, as in the case of biofilm detachment brought on by forces associated with either fluid flow or cell replication, or discrete, as in the case of biofilm sloughing.
Each of these processes is reviewed in some detail.
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