Cell Signalling by Bacteria in Quorum Sensing
- Imagine a crowded room where everyone whispers.
- At first, the whispers are too faint to hear.
- But as more people join in, the collective sound grows louder until it triggers a response.
- This is how quorum sensing works in bacteria, a process that enables them to coordinate activities based on population density.
Definition
Quorum Sensing
A communication system used by bacteria to coordinate group behaviors based on population density through the release and detection of signaling molecules.
How Quorum Sensing Works
- Production of Signaling Molecules: Each bacterium releases small chemical signals called autoinducers.
- Diffusion and Accumulation: These molecules diffuse freely in the environment. As the bacterial population grows, the concentration of autoinducers increases.
- Detection: When the concentration of autoinducers reaches a critical threshold, they bind to specific receptors in the bacteria.
- Response Activation: This binding triggers changes in gene expression, leading to coordinated behaviors like biofilm formation or bioluminescence.
- Think of autoinducers as text messages sent by each bacterium.
- When enough messages are sent, the group decides to act together.
Bioluminescence in Vibrio Fischeri Is A Classic Example
- Vibrio fischeri, a marine bacterium, uses quorum sensing to produce light through bioluminescence.
- This process is most famously observed in its symbiotic relationship with the bobtail squid.
The Role of Autoinducers
- Low Population Density: When V. fischeri bacteria are sparse, the concentration of autoinducers is too low to trigger a response.
- High Population Density: In dense populations, such as those in the squid’s light organ, autoinducers accumulate and bind to a cytoplasmic receptor protein called LuxR.
- Gene Activation: The LuxR-autoinducer complex binds to DNA, activating genes responsible for producing the enzyme luciferase.
- Light Production: Luciferase catalyzes a chemical reaction that emits light, with over 80% of the energy released as visible light.
- Free-living V. fischerido not emit light because it would waste energy without a functional purpose.
- However, in the squid’s light organ, bioluminescence helps the squid camouflage itself by mimicking moonlight on the ocean surface, reducing the risk of predation.
Mutualistic Relationship with the Bobtail Squid
- Bacterial Benefit: The squid provides V. fischeri with a nutrient-rich environment, supplying sugars and amino acids.
- Squid Benefit: The light emitted by the bacteria helps the squid avoid predators.
