| Biofilm Formation and Bacterial Communication |
How do biofilms form? The formation of a biofilm requires coordinated chemical signaling between cells. Unless an adequate number of neighboring cells are present, the costs of biofilm production to an individual bacterium outweigh the benefits. Thus, a signaling process benefits the bacteria by allowing them to sense the presence of neighboring bacteria and respond to varying conditions. The process by which a bacterium does this is called quorum sensing.
Quorum sensing uses signaling molecules, known as autoinducers. These are continuously produced by bacteria and can readily diffuse through the cell membrane. When elevated numbers of bacteria are present in an area, the concentration of autoinducers in the region will be higher. Autoinducer molecules (which include certain peptides and compounds known as homoserine lactones) can interact with specific repressor or activator sequences in DNA. The presence or absence of the autoinducer thus controls the production of mRNA, and therefore protein. These proteins are encoded by dozens of genes, including the genes for biofilm production. Laboratory strains of P. aeruginosa lacking the gene for a specific homoserine lactone will not develop into normal biofilms but pile up into a disorganized heap.
From the bacteria's perspective, intracellular signaling has many advantages. Microbes often produce antibiotics that inhibit the growth of competitive species. Intracellular signaling not only brings bacteria together in biofilms, it also regulates the coordinated delivery of high doses of these antibiotics from the denser bacterial population. It also helps bacteria coordinate the release of virulence factors (such as disease-causing toxins) to overcome animal or plant defenses. Signals between bacteria in close proximity, as in a biofilm, also seem to enhance bacterial mating and the acquisition of novel DNA by transformation, both of which increase bacterial diversity.