Quorum sensing allows bacteria to communicate and coordinate behaviors in a population-density dependent manner. Bacteria produce and release signaling molecules called autoinducers that accumulate over time. When a threshold concentration is reached, it triggers a change in gene expression across the population. This process regulates behaviors like bioluminescence, biofilm formation, and virulence factor production. The mechanism was first described in Vibrio fischeri, where the lux operon controls luciferase expression in response to autoinducer concentration. Quorum sensing inhibition is now being explored as a potential antimicrobial therapy by blocking bacterial communication and virulence.

1. Quorum sensing
A social networking among bacteria
Dr. SURESH S.S. RAJA & R. GIRISH
NAIR
DEPARTMENT OF MICROBIOLOGY
BHARATHIDASAN UNIVERSITY
COLLEGE
PERAMBALUR

2. Introduction
 “Quorum” is a Latin word which means the number of
members of a group required to be present to transact
business or carry out an activity legally.
 Quorum sensing is a method of communication among
bacteria living in population.
 Bacteria communicate via secreted signaling molecules
called “autoinducers” which contribute in regulation of
expression of particular genes
 Occurrence: Within a single bacterial species as well as
between diverse species and in some local insects to
determine where to nest.

3. Introduction (Contd.)
 Quorum sensing regulates complex
cooperative behaviors, such as conjugal
plasmid transfer, biofilm maturation and
virulence.
 Each individual bacterium is capable of
producing a signaling molecule (inducer)
and each bacterium also has a receptor
for the inducer.
 When the inducer binds to the receptor, it
activates the transcription of certain
genes, including those responsible for the
synthesis of the inducer itself.

4. Why bacteria need quorum sensing
 As environmental conditions often change rapidly,
bacteria need to respond quickly in order to
survive.
 Quorum sensing enables bacteria to coordinate
their behaviour.
 It is very important for pathogenic bacteria during
infection of a host to coordinate their virulence in
order to be able to establish a successful infection.

6. History
• Journal of Bacteriology 1970, Microbiol.Rev, 1979.
• Identification of density dependent bioluminescence in the marine
symbiotic bacterium Vibrio fischeri and another free living bacteria
called Vibrio harveyi.
Nealson and
hastings
• Cell 1983, PNAS 1984
• Identified isolated and cloned genes responsible for the density
dependant gene regulation in Vibrio fischeri and the genes encoding the
luciferase enzyme complex.
Engbrecht
and
Silverman
• Used the term quorum sensing (Journal of bacteriology, 1994)
• He traced the threshold level of individual cell required to initiate a
population response in concert.
Fuqua et al.

7. History (Contd.)
Experiments by Neselson and hastings
The first such system was described in
Vibrio fischeri (Nealson & Hastings, 1979), a
symbiotic species that provides its marine
eukaryotic hosts i.e., Squid with light.

8. History (Contd.)
 Both the Vibrio species produce and respond to a signalling
molecule called Acylated Homoserine Lactone (AHL) which
is called an auto inducer that shoe extracellular
accumulation with cell growth.
 When the concentration of AHL crosses the threshold level
it initiates a signal transduction cascade with the population
density, leading to the production of luciferase.

9.  In other words the entire circuit relies on the intracellular
production and export of AHL, whose extracellular
concentration grows with population density.
 This signalling molecule is sensed and reimported into the
cells facilitating the whole population to respond to a critical
cell density.
History (Contd.)

10.  Engbrecht and Silverman (Cell 1983, PNAS 1984) identified isolated
and cloned genes responsible for the density dependant gene
regulation in Vibrio fischeri and the genes encoding the luciferase
enzyme complex.
 Their experiments demonstrated the involvement of two regulatory
proteins LuxI and LuxR where LuxI is responsible for the synthesis of
the auto inducer AHL and LuxR is a transcriptional activator protein
which binds to the auto inducer and enables transcription of the
structural operon LuxCDABE of luciferase.
Experiments by Engbrecht and Silverman
History (Contd.)

11.  Traced the threshold level of individual cell required to initiate a
population response in concert.
 The phenomenon of quorum sensing is accomplished by the
accumulation of small chemical signalling moieties which accumulate
extracellularly in the system and induce a population dependent
phenotype in the system.
Experiments by Fukua et al. 1994
History (Contd.)

12. Quorum sensing molecules

13. Acyl-homoserine lactones (AHLs)
 In bacterial signalling, N-AHLs are produced
within the bacterial cell and released into the
environment.
 N-AHLs produced by different bacteria differ in
the length of the R-group side-chain. Chain
lengths vary from 4 to 18 carbon atoms and in
the substitution of a carbonyl at the third carbon.
 They signal changes in gene expression, such as
switching between the flagella gene and the gene
for pili for the development of a biofilm.

15. Synthesis of Acyl-homoserine lactones (AHLs) by LuxI protein
S-adenosylmethionineacyl-acyl carrier protein
LuxI type of enzymes

16. Autoinducer peptides (AIPs)

17. Autoinducer-2 (AI-2)

18. Structure of some quorum sensing molecules

19. Mechanism of Quorum sensing

21. Lux operon
The lux operon encodes genes for self-
regulation and for the production of
luminescent proteins.
The most well studied operon is
originally isolated from the bacterium
Vibrio fischeri, whose operon-produced
luciferase produces a yellowish colored
bioluminescence of about 490 nm.
Component Genes
pLuxR: "right hand" lux promoter region
LuxR:
LuxI:

22. LuxI gene
LuxI is a synthase that converts SAM into a small molecule
called an acyl-homoserine lactone (AHL).
It can diffuse across cell membranes and is stable in growth
media at a range of pH.
In the natural system, the pLuxR promoter controls
transcription of the LuxI enzyme leading to a positive feedback
loop that increases transcription from the right hand lux
promoter.
In addition controlling the transcription of luxI, the promoter
also controls transcription of luciferase.

23. LuxR
LuxR is a constitutively expressed protein that can bind AHL.
When bound to AHL it can stimulate transcription from the right
hand lux promoter (pLuxR).
In the natural system, this promoter controls transcription of the
LuxI enzyme leading to a positive feedback loop that increases
transcription from the right hand lux promoter.
In addition controlling the transcription of luxI, the promoter also
controls transcription of luciferase.

29. Mediated by Autoinducer – peptide (oligopeptides).
Secreted by – ABC transporter (ATP
binding cassette transporter
Eg.- Bacillus subtulis & Streptococcus
pneumonia use quorum sensing to develop
bacterial competence.

35. Quorum sensing leads to biofilm formation

36. Quorum sensing leads to virulence expression

38. Biotechnological Applications of Quorum Quenching
 Naturally occurring quorum-quenching processes are being tested as novel
antimicrobial therapies. Over expression of aiiA in tobacco and potato plants confers
resistance to E.carotovora, which requires AHL-controlled virulence factor expression to
cause disease.
 Like wise, co culture of Bacillus thuringiensis decreased E.carotovora– mediated
plant disease in an aiiA-dependent manner.
 Mice treated with synthetic antagonists of S.aureus AIPs how resistance to infection.
Similarly, purified halogenated furanones appear to attenuate virulence of bacteria in
mouse models.
These and other examples predict that inhibition of quorum sensing which
offers an attractive alternative to traditional antibiotics because these strategies are not
bactericidal and the occurrence of bacterial resistance therefore could be reduced.
Like wise, approaches aimed at promoting bene ficia lquorum sensing associations may
enhance industrial scale production of natural or engineered bacterial products.

40. Questions?
THANK YOU