1.1  Bacterial quorum sensing

It is generally believed that cellular communication systems evolved very early in the history of life on Earth. The ability to sense their environment and to communicate with each other would have provided primordial cells with distinct adaptive and replicative advantages. An example of an ancient cellular communication system is quorum sensing, which is observed in both bacteria and archaea and is therefore believed to have originated ~3 billion years ago, well before prokaryotic and eukaryotic life diverged.

Quorum sensing coordinates the behaviour of bacteria within a colony. In particular, quorum sensing is important for controlling bacterial gene expression and maintaining the viability of a bacterial colony. Under certain conditions, bacteria secrete peptides known as autoinducers into their surroundings; if the bacterial population density increases so will the concentration of autoinducer. If the concentration of autoinducer reaches a sufficient level, it can trigger the transcription of specific genes within all the cells in a bacterial colony so that they act in concert (Figure 3). Quorum sensing therefore allows planktonic (single-celled, free-swimming) bacteria to adopt group behaviours when they reach a certain density. The production of autoinducer by bacteria does not have a simple proportional relationship to bacterial population density. Rather, the release of autoinducer substantially increases at a critical population level (dashed vertical line in Figure 3). This surge in autoinducer release underlies the coordinated change from planktonic to group behaviour. Cellular signalling systems often show rapid changes in activity beyond a threshold point. This allows for cellular processes to be either switched on or off with only a relatively small change in stimulation.

Figure 3  Quorum sensing in bacteria depends on the release of signal molecules called autoinducers. Bacteria release autoinducers into their extracellular environment. As the bacterial population density increases, the production of autoinducer can dramatically rise and lead to the induction of group behaviours.

Through changes in the expression of specific genes, autoinducers help bacterial colonies to survive. Typically, autoinducers increase the transcription of genes that encode for antibiotic resistance, in addition to genes that lead to the development of biofilms (Figure 4). A biofilm is a bacterial niche that arises from the aggregation of bacteria within a polysaccharide matrix that they secrete. The bacteria within a biofilm are ~1000-fold less sensitive to antibiotics than their planktonic counterparts and are difficult to kill with disinfectants. Biofilms are found in many places including water distribution pipes and on teeth, lung and intestines. The formation of the protective biofilm niches through quorum sensing is believed to be a major contributor to antibiotic resistance.

Figure 4  Stages in development of a bacterial biofilm. (a) Initial bacterial attachment and colonisation of a surface. (b) The formation of a microcolony where quorum sensing begins to affect the bacteria. (c) Bacteria secrete polysaccharides to form a protective barrier. (d) The colony grows and releases new planktonic bacteria.

Activity 1  Signalling mechanisms

Figure 5 shows one of the signalling mechanisms that bacteria use for quorum sensing. Describe this quorum sensing mechanism in terms of the generalised cell signalling pathway discussed earlier and represented in Figure 1.

Figure 5  Simple representation of the signalling mechanisms involved in quorum sensing in bacteria.

Answer

The bacterial cells release a signalling molecule into their environment (the autoinducer). The signalling molecule binds to specific receptors that are located on the surface of the bacterial cells. The binding triggers a transduction process (involving relay molecules), which activates a response (gene transcription).

It is believed that the evolution of quorum sensing provided bacteria with a mechanism to coordinate their behaviour. In addition, the effectiveness of autoinducers can tell bacteria something about the environment they live in, such as its viscosity or chemical composition. Moreover, it has been found that different bacterial species can live synergistically within a single biofilm and can signal to each other using quorum sensing molecules, such as autoinducers (Figure 6). These bacterial niches allow interspecies cooperation that is similar to intercellular communication in multicellular organisms.

Figure 6  Chemical signalling between bacteria within a community. Vellonella atypica produces a molecule that induces the expression of amylase by Streptococcus gordonii. This helps to produce lactic acid, the preferred carbon source of V. atypica. Autoinducer secretion is also important for S. gordonii and Porphyromonas gingivalis in the formation of mixed biofilm communities that provide mutual protection.