6. Social Lives of Bacteria

Quorum sensing

A mechanis by which bacteria coordinate gene expression in response to population density.

What is an autoinducer?

A small signalling molecule produced by bacteria that accumulates as population density increases.

What is the purpose of quorum sensing?

To coordinate behaviours that are only effective when many cells act together.

Examples of quorum sensing-regulated processes

Bioluminescence, virulence factor production, biofilm formation, motility and secretion of extracellular enzymes.

Which bacterium is the classic model for quorum sensing?

Vibrio fischeri.

Where does Vibrio fischeri live?

In a symbiotic association with certain marine squid.

Why is Vibrio fischeri bioluminescence useful to the squid?

The light helps camouflage the squid by matching downwelling moonlight.

At what cell density does Vibrio fischeri produce light?

Only at high population density.

What observation first suggested bacterial communication in Vibrio fischeri?

Culture supernatant from dense cultures induced light production in low-density cultures.

Homoserine lactone (HSL)

A quorum sensing signalling molecule used by many Gram-negative bacteria.

What type of molecule is HSL?

A small diffusible acyl-homoserine lactone (AHL).

Why can HSL function as a quorum sensing signal?

It diffuses across cell membranes and accumulates as cell density increases.

What happens to HSL concentration as bacterial population density increases?

HSL concentration increases.

lux operon

A gene cluster controlling bioluminescence in Vibrio fischeri.

What genes encode luciferase?

luxA and luxB.

What does luciferase do?

Catalyses the light-producing reaction.

Which genes encode enzymes involved in luciferin production?

luxC, luxD and luxE.

luxR

A transcriptional activator that responds to HSL.

luxI

The autoinducer synthase that produces HSL.

What is the function of LuxI?

Synthesises the quorum sensing molecule HSL.

What is the function of LuxR?

Binds HSL and activates transcription.

What kind of protein is LuxR?

A transcriptional activator.

What structural motif does LuxR contain?

A helix-turn-helix DNA-binding motif.

What happens to LuxR in the absence of HSL?

It cannot efficiently activate transcription.

What happens when HSL binds LuxR?

LuxR changes conformation and becomes an active transcription factor.

How does activated LuxR increase transcription?

It binds DNA and promotes RNA polymerase recruitment to the promoter.

What genes are activated by LuxR-HSL?

luxI and luxCDABE.

What is autoinduction?

A positive feedback loop where a quorum sensing signal stimulates production of more of the signal.

How does autoinduction occur in the lux system?

LuxR-HSL activates expression of luxI, causing production of even more HSL.

Why is light production not proportional to cell number?

Autoinduction creates positive feedback leading to a rapid increase in gene expression above a threshold density.

What is the threshold response in quorum sensing?

A sudden increase in gene expression once autoinducer reaches a critical concentration.

Why is quorum sensing considered population-density dependent?

Signal concentration reflects the number of bacteria present.

What is constitutive expression?

Gene expression that occurs continuously regardless of environmental conditions.

How is LuxI expressed initially?

At a low constitutive level.

What happens to HSL at low cell density?

It diffuses away and remains below the level required to activate LuxR.

What happens to HSL at high cell density?

It accumulates and activates LuxR.

What type of regulation controls bioluminescence in Vibrio fischeri?

Positive transcriptional regulation by LuxR-HSL.

What is bioluminescence?

The production of visible light by a biological organism.

What enzyme directly produces light in Vibrio fischeri?

Luciferase.

How is quorum sensing different from constitutive gene expression?

Quorum sensing depends on signal accumulation and population density, whereas constitutive expression does not.

Why is quorum sensing advantageous?

It prevents costly proteins being produced until sufficient numbers of cells are present.

What signalling molecules are most commonly used by Gram-negative bacteria?

Acyl-homoserine lactones (AHLs).

How do AHL signals move between cells?

By passive diffusion across membranes.

What signalling molecules are commonly used by Gram-positive bacteria?

Processed oligopeptides called autoinducing peptides (AIPs).

Why can't most Gram-positive bacteria use AHL diffusion?

Their thick cell wall limits passive diffusion of signalling molecules.

How are Gram-positive quorum sensing signals exported?

By dedicated secretion systems.

What type of receptor usually detects AIPs?

A membrane-bound two-component sensor kinase.

Two-component regulatory system

A signalling system consisting of a sensor kinase and response regulator.

How do Gram-positive quorum sensing systems work?

The peptide signal binds a sensor kinase, triggering phosphorylation of a response regulator.

Key difference between Gram-negative and Gram-positive quorum sensing

Gram-negative bacteria usually use diffusible AHLs and cytoplasmic receptors, whereas Gram-positive bacteria use peptide signals and membrane-bound receptors.