Saturday, March 29, 2014

A General Model for Biofilm Development



Biofilm formation is a developmental process in which bacteria undergo a regulated lifestyle switch from a nomadic unicellular state to a sedentary multicellular state where subsequent growth results in structured communities and cellular differentiation. Results of prior work by many groups allow the construction of a hypothetical developmental model for biofilm formation that can be generalized for many different bacterial species. This model can be adjusted to fit either of two general modes of unicellular lifestyle: nonmotile and motile.


In the case of non-motile species, when conditions are propitious for biofilm formation, individual bacteria appear to increase the expression of adhesins on their outer surface, i.e., they increase their “stickiness”. This increased stickiness promotes both cell-cell adherence and cell-surface adherence when these bacteria encounter a surface. For example, in the case of some strains of staphylococcal species, surface-expressed proteins, including Bap, promote cellcell interaction and contribute to the extracellular matrix. Many other species, both nonmotile and motile, harbor homologs of Bap. A unifying feature of these large extracellular proteins is the presence of repeated domains.

At the level of the bap gene, these repeats have been shown to be recombinogenic, resulting in the production of proteins of variable length within a biofilm population. Yet, the significance of this variability in the size of Bap proteins within a biofilm remains unknown. Nonmotile species also produce exopolysaccharides (EPS) that form an integral part of the extracellular matrix. One example of this is the PIA or PNAG EPS, produced by the gene products of the ica operon of staphylococcal species. Thus, in nonmotile bacteria, changes in cell surface proteins, along with the production of EPS, play a critical role in the initiation of biofilm formation.

In the case of motile species, when conditions favor biofilm formation, individual bacteria localize to a surface and initiate a dramatic lifestyle switch. Motility is lost and bacteria begin to produce an extracellular matrix that holds the cells together. For a number of motile organisms, the dominant role for flagella in initiation of biofilm formation is to provide motility as flagella-minus and paralyzed flagella mutants are comparably defective in biofilm formation. In fact, in Listeria monocytogenes supplying exogenous cell movement directed toward the surface via centrifugation restores wild type levels of initial surface adhesion to nonmotile mutants. In these cases, it appears that motility is the driving force that overcomes repulsive forces between the bacteria and the surface. Initial encounters with a surface usually lead to transient adherence. This transient adherence can result in either a stable surface association, and a subsequent switch to biofilm development, or in a return to planktonic existence.

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