Quorum sensing allows bacteria to communicate via small diffusible molecules and regulate gene expression in response to cell population density. In Gram-negative bacteria, these signaling molecules are usually acylated homoserine lactones. Quorum sensing plays an important role in biofilm formation, allowing bacteria to synchronize behaviors like polysaccharide synthesis and adherence as the biofilm matures.

1. Quorum sensing
•Mechanism of cell to cell signaling or
density sensing in bacteria
•Small diffusible molecules secreted by
bacterial cells which when cell density
becomes sufficient can be detected by
other cells in the population
–In Gram negative bacteria these molecules
are usually derivatives of acylated
homoserine lactone (AHL(
–In Gram positives they are often small
peptides
–A “universal” autoinducer AI-2 is produced by
a number of bacteria and may mediate cross-

2. adherenceadherence microcolonymicrocolony
Synthesis of theSynthesis of the
polysacharidepolysacharide
biofilmbiofilm
““Quorum sensing”Quorum sensing”
homoserine -homoserine -
lactonelactone
NutrientNutrient
OxygenOxygen
Bacteria lacking lasI produce a loose and easily disrupted biofilmBacteria lacking lasI produce a loose and easily disrupted biofilm

3. Quorum sensing – Autoinducer systems
Gram-negative bacteria:
- LuxI (autoinducer synthase)
- LuxR (activator)
- AHL (autoinducer/ co-
activator)
Bassler (2002) Cell 109: 421.
Gram-positive
bacteria:
- propropeptide
- two component
system
- peptide autoinducer
Acylated
homoserin
e lactone
(AHL)
Oligo
-peptid
e
Vibrio
fisheri
Staphylococcus aureus

4. LuxR/LuxI quorum sensing – Gram-negative
bacteria(passive diffusion in and out of
the cell)

5. Quorum sensing in Gram positive
bacteria
•Similar to Gram
negative bacteria
•Signal is a peptide
•Peptide is detected
at the cell surface
by a two component
regulatory system

6. Quorum sensing in a biofilm

7. Quorum sensing in a biofilm