M. Sc., Microbiology Study Material (Semester-I, General Microbiology) Department of Microbiology, School of Bioscience, Periyar University, Salem.

1. QUORUM SENSING
How Bacteria Talk to Each Other ?
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2.  Bacteria exhibit complex cooperative behaviours, such as conjugal
plasmid transfer, biofilm maturation and virulence.
 Many of these behaviours are regulated by a process known as
quorum sensing.
 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.
 Imagine that only a few bacteria of the same kind are nearby…..
 Diffusion reduces the concentration of the inducer in the
surrounding medium to a negligible amount, so each bacterium
produces a very small amount of the inducer.
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4. CELL DENSITY AND QUORUM SENSING
R gene I gene
R protein I protein
AHL diffuse out
R gene I gene
R protein I protein
AHL diffuse
out
+
AHL diffuse in
Cell
density
Time
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5.  When concentration of these signaling molecules exceed a
particular threshold value, these molecules are internalized in the
cell and activate particular set of genes in all bacterial population,
such as genes responsible for virulence, competence, stationary
phase etc .
 Quorum sensing thus enables bacteria to co-ordinate and respond
quickly to environmental changes, such as the availability of
nutrients, other microbes or toxins in their environment.
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7. QUORUM SENSING MOLECULES
 Three types of molecules :
 1: Acyl-homoserine lactones (AHLs)
 2: Autoinducer peptides (AIPs)
 3: Autoinducer-2 (AI-2)
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8. SIGNAL MOLECULES INVOLVED IN QUORUM SENSING
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9. ACYL-HOMOSERINE LACTONES (AHLS)
AHL MEDIATED QUORUM SENSING CYCLE
AI
LuxI
+
promoter target genes
LuxR
RNA
polymerase
Transcription
AI
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10. AUTOINDUCER PEPTIDES
 These are small peptides, regulate gene expression in Gram-
positive bacteria such as Bacillus subtilis, Staphylococcus aureus etc.
 Recognized by membrane bound histidine kinase as receptor.
 Regulates competence and sporulating gene expressions.
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11. AUTOINDUCER-2 (AI-2)
 Involve in interspecies communication among bacteria.
 Present in both Gram (+) and Gram (-) bacteria.
 Chemically these are furanosylborate diester.
S-ribosyl-homocysteine (SRH)
4,5-dihydroxyl-2,3 pentanedione (DPD)
Autoinducer-2 (AI-2)
LuxS
Cyclization
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12. Gram negative bacteria
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.
The host provides a nutrient-rich environment for the
bacterium and the bacterium provides light for the host.
It was observed that liquid cultures of V. fischeri produced
light only when large numbers of Bacteria were present.
The initial explanation for this was that the culture medium
contained an inhibitor of luminescence, which was removed
when large numbers of bacteria were present.
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13.  When a V. fischeri cell is alone, the autoinducer (3-oxo-
C6-HSL, an AHL) is at a low concentration.
At high cell concentrations, the level of the autoinducer
becomes sufficient to induce transcription of the genes that
produce the enzyme luciferase, leading to bioluminescence.
On reflection, this system is clearly a sensible one. Asingle cell
is not capable of producing enough luciferase to cause visible
luminescence.
Using quorum sensing, the cell can save its effort for the time
when sufficient similar cells are around, so that their combined
action produces a visible glow.
The bacteria thus behave differently in the free-living and
symbiotic states.
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14. The pathogen Pseudomonas aeruginosa uses quorum sensing
to coordinate behaviours such as biofilm formation, swarming
motility, and aggregation.
These bacteria grow inside a host organism without harming
it, until they reach a threshold concentration.
Then, having detected that their number is sufficient to
overcome the host’s immune system, they become aggressive
and form a biofilm, causing disease.
This pathogen uses AHL-mediated quorum sensing to regulate
the production of many factors needed for virulence.
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17. Gram-positive bacteria
They communicate using modified oligopeptides as signals and
“two component”- type membrane-bound sensor histidine kinases
as receptors.
Signaling is mediated by a phosphorylation cascade that
influences the activity of a DNA-binding transcriptional regulatory
protein termed a response regulator.
Each Gram-positive bacterium uses a signal different
from that used by other bacteria and the cognate receptors
are exquisitely sensitive to the signals’ structures.
Peptide signals are not diffusible across the membrane,
hence signal release is mediated by dedicated
oligopeptide exporters.
it is known that most peptide quorum-sensing signals are
cleaved from larger precursor peptides, which then are
modified to contain lactone and thiolactone rings,
lanthionines, and isoprenyl groups 17

18. S. aureus uses a biphasic strategy to cause disease:
At low cell density, the bacteria express protein factors that
promote attachment and colonization,
whereas at high cell density, the bacteria repress these traits
and initiate secretion of toxins and proteases that are presumably
required for dissemination
The system consists of an autoinducing peptide of
Staphylococcus aureus (AIP) encoded by agrD and a
two-component sensor kinase-response regulator pair,
AgrC and AgrA, respectively.
Activated AgrA induces expression of the agrBDCA.
results in increased AIP levels, which ensures that the
entire population switches from the low-cell-density to the
high-cell-density
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22. INHIBITION OF QUORUM SENSING
 Inhibition of quorum sensing has been proved to be very potent method
for bacterial virulence inhibition.
 Several QS inhibitors molecules has been discovered.
 QS inhibitors have been synthesized and have been isolated from several
natural extracts such as garlic extract.
 QS inhibitors have shown to be potent virulence inhibitor both in in-vitro
and in-vivo, using infection animal models.
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23. QUORUM QUENCHING
The ability to disrupt quorum sensing may give one
bacterial species an advantage over another that
relies on quorum sensing.
Likewise, 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.
Thus, mechanisms that have evolved to interfere
with bacterial cell-cell communication in processes
termed quorum quenching.
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24. 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.
Likewise, co culture of Bacillus thuringiensis decreased
E. carotovora–mediated plant disease in an aii A-dependent manner.
Mice treated with synthetic antagonists of S. aureus AIP show
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.
 Likewise, approaches aimed at promoting beneficial quorum sensing
associations may enhance industrial scale production of natural or
engineered bacterial products.
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25. ANTIBIOTIC RESISTANCE
Antibiotic
Antibiotic
Antibiotic sensitive bacteria
Antibiotic resistant bacteria
 Now a days most of bacteria are antibiotic resistant
 Penicillin resistant bacteria developed in 1942, just after 2 years
of its introduction
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26. STRATEGIES FOR QUORUM SENSING INHIBITION
3 strategies can be applied
Targeting AHL signal
dissemination
Targeting the signal
receptor
Targeting signal
generation
Signal precursor
Signal
Signal receptor
Signal precursor Signal precursor
Signal Signal
Signal receptor Signal receptor
X
X
X
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27. QUESTIONS ?
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