Quorum sensing is a bacterial communication process involving the production and detection of signaling molecules called autoinducers, which regulate gene expression based on bacterial population density. It allows bacteria to coordinate behaviors such as virulence and bioluminescence and occurs in both gram-negative and gram-positive species. Quorum quenching, mechanisms that disrupt this communication, can prevent pathogenic bacteria from successfully colonizing hosts.

1. QUORUM SENSING
Presentation by:
Areeba Nameen
Menahil Khalid
Alizay

2. Introduction
‘Quorum’ is a Latin word.
•It means the number of
members of a group
required to be present
to transact business or
carry out an activity
legally

3. •Quorum sensing is a
process of bacterial cell-to-
cell communication
involving the production
and detection of
extracellular signaling
molecules (autoinducers).
•“Autoinducers”
contribute to the regulation
of the expression of
particular genes.

4. • This is the intra communication
system that is used by like
bacteria to establish presence of
their own kind.
• Each bacteria is able to send out
a signals and receive one as
well. This allows each individual
bacteria to count how many other
bacteria there are.
• Once the bacterium can assess
that there is a proper amount of
other bacteria present, it can
simultaneously, along with the
other bacteria emit a response
such as virulence or
bioluminescence

5. Discovery
• Quorum sensing was originally discovered in
the luminescent bacterium Vibrio fischeri
• These bacteria exist as free-living cells or as
symbionts in the light producing organ of an
animal host, such as the Hawaiian bobtail squid.
• It was observed that liquid cultures of V.fischeri
produced light only when large numbers of
Bacteria were present.

7. History
• Nealson et al. (1970) – luminescence in the marine Gram-
negative bacterium Vibrio fischeri controlled by self-produced
chemical signal
• Eberhard et al. (1981) identified the V. fischeri autoinducer
signal to be N-3-oxo-hexanoyl-L-homoserine lactone
• Engebrecht et al. (1983) cloned the genes for the signal
generating enzyme, the signal receptor and the lux genes

8. •Fuqua et al. (1994)
introduced the term
quorum sensing to
describe cell-cell
signaling in bacteria

9. Occurrence
• Within a single bacterial
species as well as between
diverse species.
• Quorum sensing allows
both Gram-
negative and Gram-
positive bacteria to sense
one another and to regulate
a wide variety of
physiological activities.

10. Examples
Name of the organism
Aliivibrio fischeri Gram-negative
Curvibacter sp Gram-negative
Escherichia coli Gram-negative
Salmonella enterica Gram-positive
Pseudomonas aeruginosa Gram-positive
Acinetobacter sp. Gram-positive
Aeromonas sp. Gram-positive
Yersinia Gram-positive
Archaea
Methanosaeta harundinacea 6Ac
methanogenic archaeon
Social insects Ants
Social insect Honey bees

12. A brief overview of the process
Quorum sensing can be divided into at least 4 steps:
(1) Production of small biochemical signal molecules by the
bacterial cell
(2) Release of the signal molecules, either actively or passively,
into the surrounding environment
(3) Recognition of the signal molecules by specific receptors
once they exceed a threshold concentration, leading to
(4) Changes in gene regulation

13. A brief overview of the process in luminous bacteria

14. Molecules & Mechanism of
Quorum Sensing

15. QUORUM SENSING MOLECULES
15
• Each individual bacterium is capable of producing a signaling molecule
(inducer) and each bacterium also has a receptor for the inducer.
• Signals lead to activation and suppression of certain genes leading to changes
in metabolic activity, morphology, mobility, aggregation and association with
other cells of same species or different species.
Three main types of inducer molecules :
1) Acyl-homoserine lactones (AHLs)
2) Autoinducer peptides (AIPs)
3) Autoinducer-2 (AI-2)

16. 16

17. 17
A single
bacterium has a
genetic
sequence that
codes for an
autoinducer, or a
signaling
molecule that will
be released into
the environment.
This molecule
can vary for
different types of
bacteria.

18. General
Mechanisms

19. 19
Quorum sensing in Gram Positive
Bacteria
01. Auto
Inducer
In Gram-
Positive
bacteria the
autoinducers
are
Oligopeptides ,
short peptides
typically 8-10
02. Signal 03. Diffusion 04. Critical
LevelIt uses these
short peptides
as a signal.
Oligopeptides
cannot diffuse
in and out of
bacteria like
AHLs , but
rather leave
bacteria via
specific
exporters.
When a critical level of
oligopeptide is reached , the
binding of the oligopeptide
to it’s receptor starts
phosphorylation cascade
that activates DNA binding
transcription regulatory
proteins called response
regulators.

20. 20

21. 21
Gram Positive Bacteria
Reception
1: AIPs thread reaches
receptor on bacterial
cell.
2: AIPs then bind to
membrane bound
receptors such as AgrC
(accessory genes
regulator)
Transductio
n1: When AIPs bind to AgrC ,
the process of transduction
begins .
2: The receptors then
phosphorylated inside the cell
. This results in formation of
AgrA.
3: Transcribed DNA then
make enzyme and protein
Cellular
response1: It is the final step of the
mechanism
2: AgrA protein begin to
transcribe bacterial DNA. It
creates more AIPs in a
positive feedback loop.

22. 22
Example: Mechanism in
Staphylococcus aureus

23. 23
Quorum sensing in Gram Negative
Bacteria
01. Auto
Induce
r
It uses acyl-
homoserine
lactones or
AHL.
02. Signal 03. Diffusion 04. Critical
LevelIt uses
LUXI/LUXR as
a signaling
molecule.
AHLs diffuse
readily out of
and into
bacterial cells
where they
bind to AHL
receptors in the
cytoplasm of
When a critical level of
AHL is reached , the
cytoplasmic
autoinducer/receptor
complex functions as a
DNA-binding
transcriptional
activator.

24. 24

25. 25
Gram Negative Bacteria
Receptio
n1: When AHLs
reaches the
surface of the
receptor , then
they bind to AI
Synthase in
cytoplasm of
bacteria
Transductio
n1: AHLs are activated by
the binding of AI
Synthase.
2: As a result of binding
, critical level of AHLs
are attained.
Cellular
response1: It is the final step of the
mechanism.
2: AHLs then bind to AHL
Receptor ,as a result
cytoplasmic receptors function
as DNA binding transcriptional
activator.
3: The receptor Complex that
are formed function as auto

26. 26

27. Interspecies communication
• Both gram-negative and gram-positive bacteria are able to
cross talk by recognizing and processing autoinducing signaling
molecules of other species.
• Beside AHL autoinducer molecules, another AI termed AI-2 was
first discovered in the bioluminescent marine bacterium Vibrio
harveyi.
• It was shown that AI-2 enables interspecies communication

28. Example: in gut & in Biofilms

29. It was also shown that pathogenic bacteria can interact with
eukaryotic host cells, and vice versa, by utilizing each other’s
autoinducing signals.

30. Quorum Quenching
30

31. Quorum Quenching
• Mechanisms that have evolved to interfere
with bacterial cell-cell communication in
processes are termed quorum quenching.
• Bacterial populations compete for limited
resources, the ability to disrupt quorum
sensing may give one bacterial species an
advantage over another that relies on
quorum sensing.
• Like wise, a host’s ability to interfere with
bacterial cell-cell communication may be
crucial in preventing colonization by
pathogenic bacteria that use quorum
sensing to coordinate virulence.

34. Quorum-quenching chemicals and enzymes
• Many quorum-quenching chemicals and enzymes have been
identified.
• These include halogenated furanones from the seaweed Delisea
pulchra, which are structural mimics of quorum-sensing signals.
• Enzymes such as AHL-lactonase, AHL-acylase and paraoxonases
degrade AHLs. Synthetic AHL and AIP analogues have been developed
to compete with quorum-sensing signals.
• In mammals, enzymes that inactivate AHLs have been found in serum
and airway epithelia.
• Such natural quorum-quenching mechanisms may be used to
develop a new generation of antimicrobials.

36. What does
quorum
sensing
regulate?

37. Advantages Quorum Sensing
1. To optimize and regulate a variety of activities.
2. To communicate and to alter behavior in response to
the presence of other bacteria.
3. Allow a population of bacteria to coordinate global
behavior and thus act as a multi-cellular unit.
4. Enhance pathogenicity.
5. Evade host defense.
6. Improve overall survival.

38. Uses of Quorum Quenching
1. Combat microbial resistance, pathogenesis,
virulence etc.
2. Decrease competition
3. Prevention of bio-fouling
4. Biocontrol of plant pathogens
5. Prevention of biofilm formation on membranes in
food industry and freshwater or wastewater
treatment plants

39. Thank you for your time