Bacteria use quorum sensing to regulate virulence gene expression in response to population density. As bacteria density increases and autoinducer molecule concentration rises, bacteria transition from individual to group behaviors. This allows precise coordination of virulence factors. The document examines quorum sensing in pathogenic bacteria like S. aureus, P. aeruginosa, and E. coli. It describes the autoinducer molecules and regulatory pathways that control virulence factors important for bacterial infection and pathogenesis.
2. What is quorum sensing?
• Bacteria can behave either as individual single-celled
organisms or as multicellular populations. Bacteria
exhibit these behaviors by chemically "talking" to one
another through a process called quorum sensing.
• Quorum sensing involves the production, release, and
community-wide sensing of molecules called
autoinducers ( AI ) that modulate gene expression, and
ultimately bacterial behavior, in response to the
density of a bacterial population.
3.
4. A brief overview of the process
Bacterial genes code for the production of signaling molecules called autoinducers
that are released into the bacterium's surrounding environment.
These signaling molecules then bind to signaling receptors either on the bacterial
surface or in the cytoplasm.
When these autoinducers reach a critical threshold level, they activate bacterial
quorum sensing genes that enable the bacteria to behave as a multicellular
population rather than as individual single-celled organisms.
The autoinducer/receptor complex is able to bind to DNA promoters and activate the
transcription of quorum sensing-controlled genes in the bacterium.
In this way, individual bacteria within a group are able to benefit from the activity of the
entire group.
5. • MECHANISM IN GRAM
NEGATIVE BACTERIA.
• InGram-negative bacteria, the
autoinducers are typically
molecules called acyl-
homoserine lactones or AHL.
• AHLs diffuse readily out of
and into bacterial cells where
they bind to AHL receptors in
the cytoplasm of the bacteria.
• When a critical level of AHL is
reached, the cytoplasmic
autoinducer/receptor
complex functions as a DNA-
binding transcriptional
activator.
• MECHANISM IN GRAM POSITIVE
BACTERIA.
• InGram-positive bacteria, the
autoinducers are oligopeptides,
short peptides typically 8-10
amino acids long.
• Oligopeptides cannot diffuse in
and out of bacteria like AHLs, but
rather leave bacteria via specific
exporters. They then bind to
autoinducer receotors on the
surface of the bacterium.
• When a critical level of
oligopeptide is reached, the
binding of the oligopeptide to its
receptor starts a phosphorylation
cascade that activates DNA-
binding transcriptional regulatory
proteins called response
regulators.
6. Canonical bacterial quorum-sensing (QS) circuits.
Canonical bacterial quorum-sensing (QS) circuits. Autoinducing peptide (AIP)
QS in Gram-positive bacteria by (A) two-component signaling, or (B) an AIP-
binding transcription factor. Small molecule QS in Gram-negative bacteria by (C)
a LuxI/LuxR-type system, or (D) two-component signaling.
Gram
positive
Gram
negative
7. ADVANTAGES OF MULTICELLULAR
BEHAVIOUR
• As the entire population of bacteria simultaneously turn on
their virulence genes, the body's immune systems are much
less likely to have enough time to counter those virulence
factors before harm is done.
• Virulence factors such as exoenzymes and toxins can damage
host cells enabling the bacteria in the biofilm to obtain
nutrients.
• Quorum sensing enables some of the bacteria to escape the
biofilm and return to individual single-celled organism
behavior in order to find a new site to colonize.
• Bioluminiscence is also produced by some bacteria by quorum
sensing.
11. Important features
• Despite its widespread prevalence in healthy subjects,
S. aureus is also a very dangerous opportunistic pathogen
which has been increasingly associated with antibiotic
resistance.
• S . aureus has multiple virulence factors and can display very
rapid transmission, aiding its importance as a human
pathogen.
• S. aureus forms biofilms on many surfaces, including
indwelling devices such asurethral stents . These indwelling
devices, and subsequent biofilms formed on them, pose a
serious risk for Staphylococcus infection
12. Virulence factors of Quorum sensing
• S. aureus has a peptide based quorum sensing system ,encoded by
accessory gene regulator (agr) locus.
• The autoinducer (AI) is an oligopeptide termed as Auto inducer peptide
(AIP).
• AIP is encoded by agrD.
• Genes involved in agr system are agr A,agrB,agrC,agrD.
• agrD, encodes pre-AIP.
• AgrB protein (membrane bound) trims and releases AIP from its precursor
form to the extracellular environment.
• AgrC (a membrane bound sensor kinase) to which extracellular AIP
binds,leading to autophosphorylation of AgrC- activates AgrA.
• AgrA (phosphorylated -> active) , induces transcription at the promoters P2
(higher affinity) and P3.
13. • Regulation of virulence genes in agr systeem is predominantly involved
around promoters P2 & P3 ( oppositely directed).
• P2 promotes transcription of agr operon from the RNAII transcript ,which
includes agrA,B,C,D.
• P3 transcription leads to production of RNAIII (regulatory RNA).
• RNAIII is the effector molecule of the agr system.
• RNAIII reduces the expression of surface adhesins and increases the
production of capsule toxins and proteases.
**** RNAIII also functions as the mRNA for δ-toxin ( 5’ end upregulates α-
haemolysin while 3’ is required for repression of protein A synthesis).
4 distinct group of agr polymorphism found agrI,II,III,IV .
AIP s of each group cross-inhibits other groups. (exception-group II & IV cross
activates each other).
15. Impact of agr on virulence
• RNAIII has the dual-function of activating production of a-toxin and repressing
expression of rot, fibronectin binding proteins A and B, protein A, coagulase, and
other surface proteins.
• Repression of rot, which encodes a repressor of toxins, leads to de-repression of
additional toxins, proteases, lipases, enterotoxins, superantigens, and urease .
• The net result of this QS regulatory cascade is downregulation of surface virulence
factors (such as protein A), and up-regulation of secreted virulence Factors (such
as a-toxin).
• Most of the effects of QS on regulation of virulence in S. aureus Are mediated
through direct and indirect regulation by RNAIII, however, phosphorylated AgrA
also directly activates at least two additional virulence genes encoding phenol-
soluble modulines.
• Biofilm formation :
• When agr nonfunctional(non active),bacteria S.aureus ha enhanced adhesion
abilities (i.e. first stage of Biofilm formation).
• Agr also plays a role in detachment of cells from biofilm ,important for
dissemination of during an infectious process.
• Active form triggers virulence .
• contribution of agr to disease has been studied in models like septic arthiritis,
osteomyelitis, endopthalmitis, pulmonary infections, toxic shock syndromes.
17. INTRODUCTION
• P.aeruginosa is a Gram-negetive bacterium.
• This organism is an opportunistic pathogen & commonly associated with
nosocomial infection & infection of severely burned individuals, and is
leading to cause of death in severe respiratory infections, such as chronic
lung infections in CF patients.
• Infections with P. aeruginosa are difficult to eradicate due to their high
levels of antibiotic resistance and growth in biofilms.
• Pseudomonas aeruginosa is a very versatile organism that can adapt too
many different environments and can cause diseases in plants, animals and
humans.
• This organism produces a broad range of exoproducts, which are
regulated in a population density-dependent manner via cell-to-cell
communication or “quorum sensing".
18. Quorum sensing in P. aeruginosa: a complex regulatory
network resulting in fine signal tuning
19. In P.aeruginosa at least three interwined quorum sensing (QS) system and one
orphan autoinducer receptor affect the ability of this organism to cause
disease.
Two intertwined QS systems (the las and the rhl systems),rely on the
production of acyl homoserine lactones(AHLs) as the signaling molecules(auto
inducers-AIs) have been shown to be involved in virulence, biofilm
development, and many other processes in P. aeruginosa.
In the las system lasI gene encodes an enzyme which produce an AIs molecule
called N-3-oxododecanoyl-homoserine lactone (3-O-C12-HSL).
In the other hand, the rhlI gene of rhl system encodes an enzyme that produce
another AIs molecule called N-butyryl-homoserine lactone (C4-HSL).
Then it regulates its gene expression via different receptor and via feed back
regulation.
20. Where LasR-C12-HSL activates the transcription of rhlR.
Similarly the transcriptional activator of LhlR is activates by
binding with C4-HSL (RhlR-C4-HSL).
Both the active LasR & RhlR regulates the production of
several extracellular virulence factors
Besides LasR and RhlR, P.aeruginosa encodes an orphan
receptor protein, QscR, which can sense 3O-C12-HCL to
controls its own regulon.
Besides regulating the expression of virulence factors, some
AIs have been shown to directly interact with host cells.
21. In addition to AHLs, P. aeruginosa releases a 4-quinolone signal
molecule into the extra-cellular milieu, the synthesis and bioactivity
of which has been reported to be mediated via the las and rhl
systems respectively.
It is the third P.aeruginosa AI molecules, this molecule has been
chemically identified as 2-heptyl-3 hydroxy-4(1H)-quinolone and
termed the Pseudomonas Quinolone Signal (PQS).
PqsR is a LasR transcriptional factor which is activated by HHQ and
PQS, leading to the positive activation of many virulence factors,
which includes a large no of genes also controlled by las and rhl.
22. Besides controlling bacterial virulence, PQS and HHQ have
been shown to downregulate the host immune response
through NFkB and PQS can also act as an iron chelator, and
both the synthesis of PQS and activity of PqsR-PQS are
involved in iron homeostasis.
23. QUORUM SENSING INHIBITION
P.aeruginosa elaborates two main sets of QS systems: lasI-LasR and rhlI-
rhlR.
These molecules diffuse out into the environment, and when they reach a
putative threshold concentration, they activate the receptors LasR and
rhlR.
The mechanism of Quorum Sensing Inhibition (QSI) activity appeared to
be a net effect of
(1) the ability of phytochemicals to interfere with the activity of AHL and
(2) to modulate the bacterial synthesis of AHL’s. Many natural extracts are
believed to inhibit QS by either interfering with AHL activity by competing
with them due to their structural similarity and/or to accelerate the
degradation of the LuxR/LasR receptors for the AHL molecules.
24. Some subinhibitory concentrations of macrolides have been
shown to be effective in inhibiting quorum sensing dependent
virulence both in vivo and in vitro.
26. Control of virulence gene expression through quorum sensing and adrenergic signalling in E. coli.
27. Important features
• E.coli is a gram negative ,facultative
anaerobic,rod shaped bacterium.
• Commonly found in the lower intestine warm
blooded organisms.
• Most E.coli strains are harmless and resides in
gut. Can benefit hosts by producing vit K and
preventing establishment of pathogenic bacteria
within intestine.
• E . Coli has been used as an indicator organism
for faecal contamination.
28. • Certain strains of E. coli are a major cause
of foodborne illness.
• The outbreak started when several people in
Germany were infected
with enterohemorrhagic E. coli (EHEC) bacteria,
leading to hemolytic-uremic syndrome (HUS).
• Escherichia coli E. coli O157:H7 serotype was
the cause of major outbreak of HUS in 2011 at
Germany.
• Other less common serotypes, such as O104:H4,
O121, O26, O103, O111, O145,and O104:H21 can
also cause serious infection.
29. Role of quorum sensing in virulence
• This organism produces a signal molecule termed
AI-2.
• the gene responsible for AI-2 (furanosyl borate
diester)production was identified and named
luxS.
• Genetic studies in enterohaemorrhagic E. coli
(EHEC) and enteropathogenic E. coli revealed that
LuxS controls the expression of the type-3
secretion system encoded by the locus of
enterocyte effacement (LEE) pathogenicity
island.
• This virulence determinant causes Attaching &
and effacing lesions (AE),caused by pathogens.
30. • LuxS is a global regulator in EHEC, controlling the
expression of over 400 genes
• Many of these genes have functions related to
bacterial virulence such as flagellar motility, surface
adhesion and Shiga toxin production.
• Later on the molecule responsible for the
regulation of LEE and flagellar genes in E. coli
was found to be AI-3 and not AI-2.
31. • The regulatory cascade linking AI-3 sensing and virulence gene
expression in E. coli is extremely complex.
• Multiple regulatory genes are controlled by quorum sensing.
• Gene systems are namely QseBC, QseEF,QseCD etc.
• QseBC - a bacterial two-component signalling system
• QseB being the response regulator
• QseC the sensor kinase(autophosphorylation).
• The QseBC contols motility expression.
• QseC is activated by AI-3.
• QseC acts as a receptor for extracellular AI-3.
• QseC also functions as the receptor for the host
catecholamine hormones epinephrine and
norepinephrine, indicating that small molecule signalling
pathways in eukaryotes and bacteria can intertwine.
32. Control of virulence gene expression through quorum sensing and adrenergic signalling in E. coli.
33. A second two component
regulatory system
• Histidine kinase QseE and its effector QseF.
• involved in the transcriptional control of the
effector EspFu, which is translocated into host
cells by EHEC.
• Controls formation of AE lesion.
• QseG (outer membrane),aids in typeIII
secretion sytem for translocation of effector
molecules in host cells.
34. Impact of the Qse system on virulence
• Flagellar motility.
• Type 3 secretion of potent Shiga toxin (harmful
for intestinal cell lining ,causes HUS)
Hemolytic uremic syndrome (HUS) - this condition
is characterized by hemolysis (breakup of red blood
cells) and kidney failure.
The patient can develop CNS (central nervous
system) problems, including seizures, and can also
go into a coma.
36. • In some bacteria autoinducer AI-2 acts as a metabolic by-
product along with its role as a signal molecule.
• in most cases the production of AI-2 is mainly involved in SAM
(S-Adenosylmethionine) utilization and SAH (S-
Adenosylhomocysteine)detoxification along with its role as a
bona fide signal in some bacteria.
Thus it can be said that the synthesis of AI-2 has two connections
with bacterial metabolism---
• The precursor of AI-2 is a major methyl donor in bacterial
cells, acting in multiple metabolic process.
• Enzyme LuxS is involved in detoxification of a metabolite
intermediate.
• In Pseudomonas aeruginosa Pseudomonas Quinolone Signal
(PQS) has iron chelating properties.
38. • Another non-signaling function observed in
bacterial signaling molecules is that of
Antimicrobials.
• Gram-positive bacteria produce a small peptide namely Lantibiotics which has
antibiotic properties. It has also been found that in some cases lantibiotics act
as a signaling molecule that regulates its own synthesis, like the activity of AIs
as signaling molecule in several bacteri
• Mersacidin and subtilin by Bacillus sp.
• Nisin production by Lactobacillus lactis.
• Streptin production by Streptococcus pyogenes.
• Bacterial AIs can also help in the competition between
species and strains.
• Here these molecules don’t act as signaling molecule rather as signaling
inhibitors.
• . It has been observed that different strains of S.aureus produce different
peptides. Thus a particular kind of peptide produced from one strain can
inhibit the gene expression of other strains.
• Therefore it can be suggested that in S.aureus quorum sensing may function as
a way to control the gene expression and also as a mechanism to survive in
the competition and allow more efficient host colonization.
39. Summary
• Quorum sensing” (QS) is the phenomenon which allows single bacterial
cells to measure the concentration of bacterial signal molecules. Two
principle different QS systems are known, the Autoinducer 1 system (AI-1)
for the intraspecies communication using different Acyl
homoserinelactones (AHL) and AI-2 for the interspecies communication.
• AHL were originally identified in marine bacteria, where they play a pivotal
role in the regulation of bioluminescence in Vibrio fischeri.
• InGram-negative bacteria, the autoinducers are typically molecules called
acyl-homoserine lactones or AHL.
• InGram-positive bacteria, the autoinducers are oligopeptides, short
peptides typically 8-10 amino acids long.
40. • Virulence factors such as exoenzymes and toxins
can damage host cells enabling the bacteria in
the biofilm to obtain nutrients.
• S. aureus has a peptide based quorum sensing
system ,encoded by accessory gene regulator
(agr) locus.
• In P.aeruginosa at least three interwined quorum
sensing (QS) system (las ,rhl,AHL)and one orphan
autoinducer receptor affect the ability of this
organism to cause disease.
• In E.coli Lux S have functions related to bacterial
virulence such as flagellar motility, surface
adhesion and Shiga toxin production.
41. Conclusion
From the above foregoing it may be
concluded that quorum sensing is a
mechanism by which bacteria can not only
communicate to each others but also
regulate their virulence gene expression in
numerous microorganism.
42. What are the advantages we get by studying quorum sensing
mechanism ?
• We can understand that how bacteria can communicate to each
others.
• We can understand the relationship between bacterial virulence and
quorum sensing.
• As quorum sensing controls bacterial virulence, It has been
considered an attractive target for development of new therapeutic
strategies.
Attractiveness of using quorum sensing inhibitor over
antibiotic
• Antibiotics are active on diving/replicating cells but quorum sensing
inhibitors are active on both diving/replicating cells .
• Antibiotics have adverse effect on host system but quorum sensing
inhibitors have no adverse effect on host.
• eg: QS inhibitor RNAIII-inhibiting peptide (RIP) has been found to
inhibit Staphylococcus aureus virulence without affecting host
system.
43. • Most of the antibiotics failed to inhibit bacteria (ie, bacteria resistent
towards antibiotics) but quorum sensing inhibitors are able to inhibit
bacterial pathogenesis.
Some of the strategies to combat bacterial virulence based on
the inhibition of bacterial quorum sensing (QS) system
• To inhibit the production of autoinducer (Ais) molecules
• The virulence of Staphylococcus aureus Is destroyed by inhibiting QS
system
• Natural QS inhibitors like cyclic sulfur compounds,halogenated
furanones ,penicillin acid,garlic extract, 4-nitro-pyridine-N-oxide were
found to inhibit the activation of virulence genes in Pseudomonas
aeruginosa
• QS inhibitors from plant species Combretum albiflorum revealed that
catechin has a negative impact on the production of QS dependant
virulence factor
Notas do Editor
In P.aeruginosa at least three interwined quorum sensing (QS) system and one orphan autoinducer receptor affect the ability of this organism to cause disease.