Bacterial signaling can occur within bacteria (intracellular) or between bacteria (intercellular). At times, a group of bacteria behaves like a community. To achieve this, they engage in quorum sensing, the perception of higher cell density that causes changes in gene expression. Quorum sensing involves both extracellular and intracellular signaling. The signaling cascade starts with a molecule called an autoinducer (AI). Individual bacteria produce AIs that move out of the bacterial cell membrane into the extracellular space. AIs can move passively along a concentration gradient out of the cell or be actively transported across the bacterial membrane.

The Extracellular Concentration of AIs Gives Signals to Bacteria

When cell density in the bacterial populations is low, the AIs diffuse away from the bacteria, keeping the environmental concentration of AIs low. As bacteria reproduce and continue to secrete AIs, the concentration of AIs increases, eventually reaching a threshold concentration. This threshold permits AIs to bind membrane receptors on the bacteria, triggering changes in gene expression across the whole bacterial community. Examples of phenotypes induced by quorum sensing include bioluminescence, motility, formation of biofilms, and expression of virulence factors.

Signaling in Bacteria That Produces Light

Bioluminescent bacteria, such as Photorhabdus luminescens, produce light as a result of a complex signaling process. These bacteria use quorum sensing, which allows them to coordinate the production of bioluminescence. For example, P. luminescens produces autoinducer 2 (AI-2) as a quorum sensing signal and as an intracellular signal. The bacteria release AI-2 into the environment. When AI-2 reaches threshold levels outside the bacterial cells, AI-2 binds to an ATP-binding cassette (ABC) transporter on the bacterial membrane and it is re-internalized by the ABC transporter. Then, an intracellular kinase, LsrK, phosphorylates AI-2 itself. Once activated in this way, AI-2 itself can act as a transcription factor, activating genes that encode the enzyme luciferase. Luciferase produces light after catalyzing specific reactions. As a result, bioluminescence is observed only when the P. luminescens population reaches a critical density.

Quorum Signaling and Bacterial Infections of Implanted Medical Devices

The spread of bacteria across the surface of medical implants occurs through quorum signaling and can cause life-threatening infections. There is a lot of current research into stopping the formation of bacterial biofilms in medical settings. Much of this research focuses on developing new materials that inhibit bacterial growth.