Extracellular signal regulation of cell differentiation in biofilms
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Introduction From the early days of the discovery of the first bacterial cell by Antonie van Leeuwenhoek through the days of Robert Koch and for many years after that, bacteria were regarded as freeliving cells co-existing in the medium where they were growing. However, the more we learn about bacteria, the more it becomes apparent that rather than surviving independently, bacteria often form communities of cells called biofilms.1,2 Biofilms are surface-associated groups of cells that are encased in a self-produced extracellular matrix.3,4 Extracellular matrix material varies between species; however, common constituents include exopolysaccharides, proteins, or nucleic acids.4 The colony shown in Figure 1 is an example of a biofilm that is formed by the bacterial species Bacillus subtilis, a Gram-positive, spore-forming bacterium that is commonly found in the soil. B. subtilis biofilms can be grown in the lab either on semi-solid surfaces such as an agar plate in the form of colonies (Figure 1a) or at the air-liquid interface in the form of pellicles (Figure 1b).5 While this review will primarily focus on B. subtilis as a model system, biofilm formation is not limited to these two forms. For example, other organisms are able to form biofilms that adhere to submerged surfaces, and they can reside on the inner surfaces of water and oil pipes.1,6,7 Additionally, biofilms can form in various natural environments such as on the surface of teeth, in wounds, or in the lungs of cystic fibrosis patients. While not all biofilms are detrimental, they
can be extremely dangerous when they develop on artificial implants and catheters in hospitals.3,7,8 At first observation, one might think that all cells within a biofilm are similar; however, this is not the case. There is a remarkably sophisticated mechanism through which cells in these microbial communities interact. As a result of cell-cell communication, cell growth, nutrient consumption, and waste production, local microenvironments or molecular gradients are formed across a growing biofilm. These gradients lead to phenotypic differentiation, such that constituent cells express different genes in response to the local environment that they are sensing.9–11 In this article, we will discuss how extracellular signals regulate the differentiation of genetically identical cells within bacterial biofilm communities.
Cell-cell communication In the early 1960s, since Jacob and Monod’s studies on lactose metabolism, it has been known that bacteria are able to change their physiology as a consequence of extracellular inducers. They discovered that adding lactose to growing Escherichia coli cells induces the expression of the genes that are responsible for the synthesis of the lactose-processing enzyme, betagalactosidase.12 However, it was not until about a decade later that it was found that molecules secreted by bacteria can serve as signals for gene expression. This was observed when media taken from cultures of bacteria, which have already produced
Liraz Chai, Harvard Medical School, Boston, MA
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