Quorum sensing by 2-alkyl-4-quinolones
in Pseudomonas aeruginosa and other
Jean-Fre ´de ´ric Dubern and Stephen P. Diggle*
Pseudomonas aeruginosa produces the cell-to-cell signal molecule 2-heptyl-3-hydroxy-4-
quinolone (The Pseudomonas quinolone signal; PQS), which is integrated within a
complicated quorum sensing signaling system. PQS belongs to the family of 2-alkyl-4-
quinolones (AQs), which have been previously described for their antimicrobial activities.
PQS is synthesized via the pqsABCDE operon which is responsible for generating multiple
AQs including 2-heptyl-4-quinolone (HHQ), the immediate PQS precursor. In addition,
PQS signaling plays an important role in P. aeruginosa pathogenesis because it regulates
the production of diverse virulence factors including elastase, pyocyanin and LecA lectin
in addition to affecting biofilm formation. Here, we summarize the most recent findings
on the biosynthesis and regulation of PQS and other AQs including the discovery of AQs
in other bacterial species.
Until relatively recently, bacteria were con-
sidered autonomous single-celled organ-
isms with little capacity for collective
behaviours. However, we now appreciate
that bacterial cells are highly communica-
tive and possess an extraordinary capacity
for social behaviors. Bacteria coordinate
their activities by producing and detecting
small diffusible signal molecules which en-
ables a population of bacteria to behave
collectively rather than as individuals. Such
cooperative behaviour is known as ‘‘quor-
um sensing’’ (QS) and plays a pivotal role
in the lifestyles of both beneficial and
pathogenic bacteria. Pseudomonas aerugi-
nosa is a ubiquitous Gram-negative bacter-
ium that is capable of surviving in a broad
range of natural environments, although it
is best known as an antibiotic resistant
human pathogen associated with hospital
of death in cystic fibrosis (CF) patients. In
P. aeruginosa, cell–cell communication is
known to control the production of extra-
cellular virulence factors and promote bio-
film maturation.3–5The QS system consists
of two N-acylhomoserine lactone (AHL)
regulatory circuits (las and rhl) linked to a
2-alkyl-4-quinolone (AQ) system. In the las
system, the lasI gene product directs the
serine lactone (3-oxo-C12-HSL), which
interacts with the transcriptional regulator
LasR and activates target promoters. In the
rhl system, rhlI directs the synthesis
(C4-HSL), which interacts with the cognate
regulator RhlR and activates target gene
promoters. The las and rhl systems are
hierarchically connected and have been
found to regulate the timing and produc-
tion of multiple virulence factors including
elastase, alkaline protease, exotoxin A,
rhamnolipids, pyocyanin, lectins, superox-
ide dismutases and biofilm formation.6
Pesci et al. (1999) demonstrated that
addition of a spent culture medium extract
from a P. aeruginosa wild type (PAO1)
expression in a PAO1 AHL deficient lasR
mutant.7This data suggested that a non-
capable of activating lasB expression and
which required LasR and 3O-C12-HSL for
its biosynthesis. It was also shown that the
novel signal required a functional rhl sys-
tem for its bioactivity since lasB could not
Institute of Infection, Immunity &
Inflammation, Centre of Biomolecular
Sciences, University of Nottingham,
Nottingham, United Kingdom NG7 2RD.
Fax: +44 (0) 115 846-7951;
Tel: +44 (0) 115 846-7954
Dr Jean-Fre´de´ric Dubern studied biochemistry at the University
of Montpellier II, France. He received his PhD in molecular
microbiology in 2006 from Leiden University, The Netherlands.
His research focused on the role and genetic regulation of cyclic
lipodepsipeptide biosynthesis in plant beneficial rhizobacteria.
Since 2006 he has been a Marie Curie Research Fellow at the
Institute of Infection, Immunity & Inflammation at the Univer-
sity of Nottingham, United Kingdom. His current research
activity is focused on the metabolic consequences of quorum
sensing in bacteria.
Dr Steve Diggle has studied the role of quorum sensing on the
virulence of Pseudomonas aeruginosa in the group of Prof. Paul
Williams (University of Nottingham) since 1997. After complet-
ing his PhD in 2001, his post-doctoral work focused mainly on
the role of 2-alkyl-4-quinolones (AQs) in P. aeruginosa. In 2006,
he received a fellowship from the Royal Society to study social
behaviours in pathogenic bacteria. Much of his work now focuses
on understanding how bacterial communication systems evolve
and are maintained in natural environments. His work can be
viewed at http://www.nottingham.ac.uk/quorum/diggle.htm.
882 | Mol. BioSyst., 2008, 4, 882–888This journal is ? c The Royal Society of Chemistry 2008
HIGHLIGHT www.rsc.org/molecularbiosystems | Molecular BioSystems
be activated in a rhlI/rhlR double mutant
by PAO1 spent culture extracts.7The mo-
mediated QS signalling pathway was pur-
hydroxy-4-quinolone and termed the Pseu-
domonas Quinolone Signal (PQS). PQS
belongs to the AQ family of compounds
which were first chemically identified in the
1940s and studied for their antibacterial
properties.8In addition to PQS, other ma-
jor molecules produced by this organism
which belong to this family include 2-hep-
tyl-4-quinolone (HHQ), 2-nonyl-4-quino-
lone (NHQ) and 2-heptyl-4-quinolone N-
Biosynthesis of 2-alkyl-4-
Studies on the synthesis of AQs show
that they are derived via the condensa-
tion of anthranilate and a b-keto-fatty
acid.9It was demonstrated that anthra-
nilate is a precursor for PQS and that
addition of an anthranilate analogue,
methyl anthranilate (which inhibits an-
thranilate synthetases) to a P. aeruginosa
culture disrupts PQS production.10In
addition, Bredenbruch et al. (2005)
showed that the C4of PQS derives from
anthranilic acid and C2 derives from
acetate, thereby proving that anthrani-
late and a fatty acid combine to produce
Interestingly, the genome of P. aerugi-
nosa encodes multiple proteins that are
similar to anthranilate synthases, but
only two of these (TrpEG and PhnAB)
appear to supply anthranilate that is
available for general cellular functions.12
The anthranilate synthases are encoded
shown to be part of the main metabolic
pathway that provides anthranilate for
PQS biosynthesis since phnAB is co-
regulated with pqsABCDE operon.13,14
In addition, the three genes (kynA,
kynB, and kynU) of the anthranilate
branch of the kynurenine (kyn) pathway,
which convert tryptophan to anthrani-
late, are present in P. aeruginosa.15The
importance of this catabolic pathway for
PQS biosynthesis was suggested by
D’Argenio et al. (2002).16Recent experi-
ments from Farrow et al. (2007) showed
that supplementing a P. aeruginosa cul-
ture with radioactive tryptophan resulted
in the production of a radio-labeled PQS
and that kynurenine pathway mutants
could not produce radioactive PQS.15
The kynurenine pathway was proposed
to function via three enzymes, in which
2,3-dioxygenase (KynA), formyl-kynure-
nine converted to kynurenine by a
kynurenine formamidase (KynB), and
kynurenine to anthranilate by a kynur-
eninase (KynU).15In their study, Farrow
et al. (2007) showed that kynA and kynU
mutants produced no PQS and a kynB
mutant produced a reduced amount of
PQS providing evidence that an alterna-
tive pathway to phn may supply anthra-
nilate.15However, a kynurenine pathway
mutant continued to make PQS in mini-
mal medium. Under these conditions, a
phnA mutant did not produce PQS and
PQS synthesis is restored by addition of
either exogenous anthranilic acid or pro-
viding with phnAB genes in trans. Thus,
further studies of the regulation and
activities of kyn and phn pathways are
necessary to determine the specific envir-
onmental conditions which cause them
to become active (Fig. 1).15The presence
of two independent pathways leading to
anthranilate synthesis, which is the direct
precursor in AQ biosynthesis in P. aeru-
ginosa implies that anthranilate is an
important metabolite for the pathogen-
esis of P. aeruginosa.
The gene cluster which directs the
biosynthesis of AQs in P. aeruginosa,
was identified during a random transpo-
son mutagenesis screen for regulators of
pqsABCDE.13PqsA was identified as a
probable benzoate coenzyme A ligase
involved in anthranilate activation, while
PqsB, PqsC and PqsD show similarities
synthetase from shikimic acid via chorismic acid or tryptophan degradation depending on environmental conditions. The pqsABCDE operon is
regulated by the LysR-family regulator PqsR. Anthranilic acid condenses with b-ketodecanoic acid to produce 2-heptyl-4-quinolone (HHQ). The
FAD-dependant monooxygenase PqsH converts PQS precursor HHQ into PQS.
The PQS biosynthesis of P. aeruginosa. Two distinct metabolic routes provide anthranilate for PQS synthesis by either PhnAB anthranilate
This journal is ? c The Royal Society of Chemistry 2008 Mol. BioSyst., 2008, 4, 882–888 | 883
synthetases. Anthranilate is converted
by the pqsABCD gene products into
HHQ (2-heptyl-4-quinolone), the precur-
sor of PQS.14HHQ can be passed be-
tween cells and converted intracellularly
into PQS through the action of PqsH, a
probable FAD-dependent monoxygen-
ase (Fig. 1).13,14The transcription of
the pqsH gene was shown to be positively
controlled by LasR, providing a link
between the AHL and AQ quorum sen-
sing systems.13Along with PQS and
HHQ, P. aeruginosa produces at least
55 quinolone compounds,
which are produced by the action of the
pqsABCD genes and some of which have
been chemically identified because of
their antibiotic activities.17,18
Two other genes, pqsL and pqsE, were
identified and suggested to play a role in
PQS biosynthesis. A mutation in pqsL
resulted in an overproduction of PQS.16
Interestingly, Lepine et al. found by profil-
ing the production of AQs of P. aeruginosa
that no AQ N-oxides were detected in a
pqsL mutant, indicating that this gene is
involved in the biosynthesis of these parti-
cular AQ compounds. The role of PqsE, a
putative metallo-b-lactamase, is largely un-
known. A pqsE mutant does not produce
pyocyanin or PA-IL lectin (LecA) but
produces wild type levels of PQS and
HHQ, suggesting that PqsE could possibly
be required for the cellular response to
PQS by generating an as yet unidentified
signalling molecule from PQS.13,19
their role in cell-to-cell
The regulation of AQs and
AQs including PQS can be detected during
the logarithmic phase of growth.19,20The
regulation of the AQ biosynthetic genes
including the phnAB and pqsABCDE oper-
ons, occurs through a LysR family tran-
(MvfR),13,14which is positively controlled
by LasR/3-oxo-C12-HSL.21PqsR was pre-
dicted to be membrane associated and its
discovery was made during the search for
genes which regulate pyocyanin production
when it was demonstrated that a pqsR
mutant was unable to produce either pyo-
cyanin or PQS.13,22PqsR binds the promo-
ter of pqsABCDE but this binding increases
dramatically in the presence of PQS, sug-
gesting that PQS acts as a PqsR co-indu-
cer.21,23The pqsABCD gene products direct
the synthesis of HHQ and originally this
molecule was suggested to be an extracel-
lular messenger, released by one cell, taken
up by a neighboring cell and converted into
PQS by the action of PqsH.14However,
PQS does not work alone in activating
pqsABCDE. It is now known that HHQ
can also potentiate PqsR binding to the
pqsABCDE promoter (Fig. 2).23Not only
that, PQS is fully dispensable, as a pqsH
mutant which produces HHQ but no PQS
displayed normal PqsR-dependent gene ex-
pression and virulence.23This demonstrates
that HHQ can also function as a signal
molecule per se and shows that there is still
work to do to fully understand the mechan-
isms as to how AQs regulate gene expres-
It has also been established that PQS
first indication that PQS was intercon-
nected with the rhl system was that the
bioactivity of PQS (measured through the
ability of PQS to activate a lasB–lacZ
fusion in late stationary phase) required a
functional RhlR gene, suggesting that PQS
was acting through the rhl QS system.7
Later, it was shown by McKnight et al.
(2000) that PQS positively regulates rhlI
accumulation.25Recently, it was observed
that pqsABCDE expression depends on the
ratio between the two AHL molecules,
3-oxo-C12-HSL having a positive effect,
and C4-HSL having a negative effect26
and that pqsR was a target for a negative
regulation by rhl system.22Thus las and rhl
systems act antagonistically on pqsR regu-
lation (Fig. 2). However, it is important to
note that under certain growth conditions
there can be considerable LasR-indepen-
P. aeruginosa is adaptable at producing
AQs.19This could be especially important
in environments such as the cystic fibrosis
lung where lasR mutants are often isolated
and yet AQs can still be isolated.27,28
Given that PQS can act as a cell-to-cell
signalling molecule, how then is it released
from a cell? Firstly it is important to note
that the activity of a cell-to-cell signaling
molecule depends on its ability to dissolve
and diffuse freely through aqueous solu-
tions. PQS is relatively insoluble in aqueous
solutions, suggesting that P. aeruginosa
produces a PQS-solubilizing factor. It was
shown that the biosurfactant rhamnolipid
produced by P. aeruginosa greatly increases
solubility of PQS in aqueous solution, re-
sulting in an increase in PQS bioactivity.29
A mechanism for PQS release has been
proposed as it has been demonstrated that
P. aeruginosa packages the molecule into
membrane vesicles that serve to traffic PQS
showed that a large amount of PQS was
present within membrane vesicles (MVs).30
LC-MS/MS analysis also showed that the
MVs contained HHQ and HNQ.30These
findings illustrate that P. aeruginosa could
use a signal trafficking system with features
common to those used by higher organisms
for the delivery of a signal critical for
coordinating group behaviour.
Whilst the molecular mechanisms gov-
erning AQ production in P. aeruginosa are
being unravelled, it is now known that the
environmentalconditionsalso play arole in
the levels of AQs produced by P. aerugino-
sa. The role of phosphate limitation on
PQS and pyocyanin production was re-
cently established by Jensen et al. (2006).31
AQ production was enhanced under low-
phosphate medium conditions, and the in-
duced AQ production was abolished in a
phoB mutant.31Since in a phoB mutant
rhlR expression was induced under iron-
limited conditions, the study examined
in a phoB-independent manner. PQS addi-
tion to the wild type and the phoB mutant
increased rhlR expression under high-phos-
phate conditions. However, although PQS
activates rhlR transcription in a PhoB-de-
pendant manner, a phoB mutant could not
be stimulated by PQS to produce a high
level of pyocyanin, suggesting that pyocya-
nin production is under the strong influence
of PhoB in addition to the effect of PQS.
These results suggest that PhoB links en-
vironmental conditions and cell density-
dependent systems to secondary metabolite
production in P. aeruginosa.
of AQs in P. aeruginosa
The biological functions
Both PQS and HHQ have both been
shown to play a role in cell-to-cell com-
munication, however recent studies have
shown that PQS appears to have a num-
ber of other biologically important func-
tions (Fig. 3).
Two recent studies24,32have demon-
strated that PQS influences P. aeruginosa
884 | Mol. BioSyst., 2008, 4, 882–888This journal is ? c The Royal Society of Chemistry 2008
iron homeostasis and this is an impor-
tant step in the understanding of com-
plex bacterial interactions as there is
growing evidence that cell-to-cell signal-
ing mediated by QS systems can also be
strongly affected by environmental fac-
tors other than cell density. Bredenbruch
et al. (2006) performed a transcriptome
analysis of PAO1 cultures supplemented
with PQS. The transcriptome profile re-
vealed an induction of the iron acquisi-
tion systems as well as the oxidative
stress response upon PQS supplementa-
tion.32In addition, when PQS was added
to a culture, a rapid loss of free iron was
observed. This was confirmed by ESI/
MS analysis showing that PQS chelates
iron in a 3 : 1 complex.32More recently,
a biophysical analysis conducted by Dig-
gle et al. (2007) revealed that PQS forms
a complex with iron(III) at physiological
pH (7.4).24However, PQS does not ap-
pear to act as a siderophore but rather as
an iron trap. When PQS was supplied to
a P. aeruginosa mutant unable to pro-
duce pyoverdine or pyochelin, PQS asso-
inhibited bacterial growth, suggesting
that PQS could play a role in iron en-
Fig. 3 Biological roles of PQS and phenotypes known to be regulated by PQS in P. aeruginosa.
required for full expression of pqsH, while pqsR is positively regulated by LasR/3-oxo-C12-HSL. Both pqsA and pqsR are repressed by the action
of RhlR/C4-HSL system. HHQ is produced via pqsA itself regulated by pqsR. Both PQS and HHQ induce the expression of pqsA in a PqsR-
dependant manner. The production of lectin and pyocyanin also require PqsE. Furthermore, PQS released from the cell is capable of binding iron,
forming a PQS-Fe3+complex. The removal of iron from the extra-cellular environment by PQS induces expression of genes involved in
siderophores production in an independent manner from cell-to-cell signalling. - represent positive regulation; B represent negative regulation.
P. aeruginosa cell-to-cell signaling. AHQ and AHL-dependant quorum sensing are intimately linked since LasR/3-oxo-C12-HSL is
This journal is ? c The Royal Society of Chemistry 2008Mol. BioSyst., 2008, 4, 882–888 | 885
mediated iron delivery.24In natural en-
vironments where competition for iron is
important, PQS may aid P. aeruginosa
growth by trapping iron and storing it in
the cell membrane for future use. Not
only that, PQS-mediated iron chelation
may also starve competing species of
bacteria of free iron in the environment.
PQS has also been shown to be in-
volved in biofilm formation. Addition of
PQS to cultures of P. aeruginosa PAO1
resulted in enhanced attachment to stain-
less steel coupons.19The mechanism for
this was not described but one possibility
may be PQS-induced production of the
galactophilic lectin LecA which has been
shown to be important for the full
maturation of biofilms.19,33A second
possible mechanism is the release of
extracellular DNA which is one of the
major matrix components in P. aerugi-
nosa biofilms. Evidence has been pre-
sented that QS plays a role in the
formation of extracellular DNA in P.
DNA release from P. aeruginosa strains
involves lysis of a sub-population of
cells.34Biofilms formed by a pqsA mu-
tant contained less extracellular DNA
than biofilms formed by the wild type.
Not only that, the mutant biofilms were
less susceptible to treatment with sodium
dodecyl sulfate than a wild type bio-
film.34In support of this, P. aeruginosa
strains showing a high level of autolysis
overproduced PQS.16This increase in
PQS may be a mechanism for the release
of extracellular DNA which results ulti-
mately in an increase in biofilm develop-
Interestingly there appears to be a link
between iron levels, DNA release and
biofilm formation. It has been shown
that pqs gene expression, DNA release
and biofilm formation were favoured in
media with low iron concentrations.
When levels of iron were high, it was
found that pqsA expression was signifi-
cantly lower, DNA release was repressed
and biofilms were structurally altered
resulting in biofilms
higher susceptibility to antimicrobial
showed that the pqs operon was induced
in particular sub-populations of biofilm
cells under low iron conditions and re-
pressed in biofilm cells when higher iron
level was tested.35Therefore, PQS could
be involved in a fine balance between
survival and persistence of bacterial cells
in the environment.16
A recent and interesting paper looked
at the role of the HHQNO molecule. It
has long been known that HHQNO has
properties that make it effective against
Staphylococcus aureus. This recent study
showed that HHQNO suppresses S. aur-
eus respiration.36Presumably this gives
P. aeruginosa an ecological advantage
when both species are growing together
in an environment such as the cystic
fibrosis lung. However, it was shown that
HHQNO resulted in the development of
S. aureus small colony variants (SCVs)
which show stable amino-glycoside resis-
tance.36Therefore, the production of
HHQNO may in fact also be advanta-
geous to S. aureus due to increased re-
sistance to certain classes of antibiotics.
There is little still known about the
roles of these molecules within a host.
What is important to note is that these
molecules can be detected directly from
infected hosts. P. aeruginosa is a major
cause of morbidity and mortality in cys-
tic fibrosis (CF) patients.37PQS has been
detected in sputum, bronchoalveolar la-
vage fluid and mucopurulent fluid from
P. aeruginosa infected CF patients.28The
levels of PQS estimated to be present
correlated with the population density
of P. aeruginosa in the sample.28Adap-
tation to the CF airway occurs during
the first 3 years of life and interestingly
P. aeruginosa strains isolated from 24–36
month old CF patients produced 7–15
fold more PQS than the laboratory strain
PAO1 during logarithmic growth. How-
ever, in isolates from patients older than
36 months, PQS biosynthesis was signifi-
cantly reduced suggesting that PQS pro-
duction may vary during the different
stages occurring during the establish-
ment of P. aeruginosa in the CF lung.38
The interactions between quorum sen-
response has also been explored. In early
immunological experiments, 3-oxo-C12-
HSL was shown to suppress interleukin-
12 (IL-12) and tumor necrosis factor alpha
macrophages and to eliminate T-cell pro-
liferation.39In recent studies Hooi et al.
(2004) determined that the signal molecule
PQS was also capable of modulating the
immune response in a manner similar to
that described for 3-oxo-C12-HSL.40In
human peripheral blood mononuclear cells
(hPBMC) activated with a lectin, both
signals showed immune suppression in
terms of cell proliferation and cytokine
(IL-2) release. However, following LPS
activation of monocytes in hPBMC, the
release of TNF-a was unaffected by treat-
ment with PQS provided at relatively low
concentration (10 mM) when compared
with 3-oxo-C12-HSL. These results offer
the possibility of developing quorum sen-
sing signals for therapeutic purposes as
novel immune modulators. There are very
few reports describing what influence sig-
nals producedbya hosthaveonthecell-to-
cell communication systems in bacteria.
Interestingly, dynorphin, a K-opioid which
is released from intestinal mucosa follow-
ing ischemia in mice, activates AQ signal-
ing in P. aeruginosa.41Dynorphin was
demonstrated to stimulate expression of
HHQ, and PQS, and in turn enhance
virulence factor production including pyo-
cyanin and LecA, suggesting that P. aeru-
ginosa can respond to opioid compounds
released during host stress by regulating
key elements of the quorum sensing system
resulting in enhanced virulence.41
signaling in other bacteria
Since AQs such as PQS are derived from
fatty acid biosynthesis and anthranilate
being supplied via a common tryptophan
biosynthesis pathway, other bacterial
species may be capable of synthesizing
similar molecules. Interestingly, Diggle
et al. (2006) showed that the genome
of at least two Burkholderia species,
Burkholderia pseudomallei and Burkhol-
deria thailandensis, possess homologous
operons to the pqsABCDE biosynthetic
gene cluster and these were termed
hhqABCDE.42Complementation of a
P. aeruginosa pqsA mutant with the
hhqA gene from B. pseudomallei comple-
tely restored production of HHQ and
PQS. Not only that, complementation
of a P. aeruginosa pqsE mutant could
be achieved by the hhqE gene. This in-
dicates that the operons in B. pseudomal-
functional. In addition, LC-MS/MS ana-
lysis confirmed that B. pseudomallei,
Burkholderia cenocepacia, as well as
Pseudomonas putida produce AQs in-
cluding HHQ.42A more recent study
886 | Mol. BioSyst., 2008, 4, 882–888This journal is ? c The Royal Society of Chemistry 2008
identified a new metabolite, 2,4-dihy-
droxyquinoline (DHQ) in both P. aeru-
production of which is governed by
PqsA/HhqA.43DHQ is neither a degra-
dation product or precursor of AQs, nor
does it increase the expression of pqsA or
pyocyanin production.43Its major func-
tion, like for many of these compounds
remains a mystery. The discovery of AQs
in Burkholderia, a genus distinct from the
fluorescent pseudomonads, indicates that
AQ-dependent signalling may be more
widespread than previously thought and
offers the exciting possibility that AQs
may be involved in both inter-genus and
Conclusions and future
Our understanding of the molecular me-
chanisms governing AQ production in
P. aeruginosa have increased dramati-
cally over the last few years. QS in
P. aeruginosa is now known to be a
complicated interconnected network in-
volving AHL-dependent and AQ-depen-
dent QS systems (Fig. 2). However, there
are still many questions that need to be
addressed. With respect to AQ biosynth-
esis, the biochemical roles of PqsABCD,
PqsH and PqsL need to be established.
The role of PqsE remains an enigma and
it is key to know by what mechanism
PQS signaling is transduced via PqsE. As
AQs have been shown to be important
for the production of virulence determi-
nants, does AQ signaling provide a tar-
get for novel antibacterial agents? The
synthesis of inhibitors of AQ-dependent
QS could provide ways of tackling P.
aeruginosa pathogenicity and biofilm for-
mation. Finally, the conservation of AQ
signalling amongst different bacterial
genera needs to be further investigated.
The identification of functional AQ sys-
tems in species other than P. aeruginosa
is exciting, but may only represent the
‘tip of the ice-berg’. Detection methods
for AQs has improved dramatically and
now researchers can look for AQs using
sensitive techniques such as LC-MS/
MS,18or by using simpler but highly
effective bioreporter assays.44,45In con-
clusion, there is much scope for future
research in AQ signaling in a diverse
number of bacterial species.
We thank Matthew Fletcher for com-
ments on the manuscript and greatly
acknowledge the Royal Society (SPD)
and Marie Curie action QUORMETAB-
no. 41376 (JFD) for funding.
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