C2.1.2 Cell signalling by bacteria in quorum sensing Notes

Cell Signalling by Bacteria in Quorum Sensing

1. Imagine a crowded room where everyone whispers.
2. At first, the whispers are too faint to hear.
3. But as more people join in, the collective sound grows louder until it triggers a response.
4. This is how quorum sensing works in bacteria, a process that enables them to coordinate activities based on population density.

How Quorum Sensing Works

1. Production of Signaling Molecules: Each bacterium releases small chemical signals called autoinducers.
2. Diffusion and Accumulation: These molecules diffuse freely in the environment. As the bacterial population grows, the concentration of autoinducers increases.
3. Detection: When the concentration of autoinducers reaches a critical threshold, they bind to specific receptors in the bacteria.
4. Response Activation: This binding triggers changes in gene expression, leading to coordinated behaviors like biofilm formation or bioluminescence.

Bioluminescence in Vibrio Fischeri Is A Classic Example

1. Vibrio fischeri, a marine bacterium, uses quorum sensing to produce light through bioluminescence.
2. This process is most famously observed in its symbiotic relationship with the bobtail squid.

The Role of Autoinducers

1. Low Population Density: When V. fischeri bacteria are sparse, the concentration of autoinducers is too low to trigger a response.
2. 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.
3. Gene Activation: The LuxR-autoinducer complex binds to DNA, activating genes responsible for producing the enzyme luciferase.
4. Light Production: Luciferase catalyzes a chemical reaction that emits light, with over 80% of the energy released as visible light.

Mutualistic Relationship with the Bobtail Squid

1. Bacterial Benefit: The squid provides V. fischeri with a nutrient-rich environment, supplying sugars and amino acids.
2. Squid Benefit: The light emitted by the bacteria helps the squid avoid predators.

Why Quorum Sensing Matters

Quorum sensing enables bacteria to perform tasks that are ineffective at low densities but highly efficient when coordinated across a large population.

Key Activities Regulated by Quorum Sensing

1. Biofilm Formation: Bacteria produce glue-like substances to form protective layers, such as dental plaque.
2. Virulence Factor Production: Pathogenic bacteria coordinate the release of toxins to overwhelm a host’s immune system.
3. Bioluminescence: As seen in Vibrio fischeri, light production is triggered only at high densities.

Mechanism of Quorum Sensing in Vibrio Fischeri

1. Autoinducer Production: V. fischeri synthesizes an autoinducer molecule (Autoinducer-3 or AI-3).
2. LuxR Activation: At high concentrations, the autoinducer binds to LuxR receptor, forming a complex. The binding of AHL to LuxR activates the lux operon, a set of genes responsible for producing the bioluminescent proteins.
3. Gene Expression: The LuxR-autoinducer complex binds to DNA, activating genes for luciferase production.
4. Bioluminescence: The enzyme luciferase catalyzes the light-emitting reaction. The light emission occurs when luciferase catalyzes the oxidation of a substrate, producing light as a by-product.

Broader Implications of Quorum Sensing

1. Quorum sensing is not limited to Vibrio fischeri.
2. It is a widespread mechanism in bacteria, with significant ecological and medical implications.

Applications and Challenges

1. Medical Research: Understanding quorum sensing can lead to novel treatments that disrupt bacterial communication, preventing biofilm formation or reducing virulence.
2. Biotechnology: Harnessing quorum sensing could improve synthetic biology applications, such as engineered bacteria for environmental cleanup.
3. Antibiotic Resistance: Targeting quorum sensing offers an alternative to traditional antibiotics, potentially reducing the evolution of resistant strains.