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OCR for page 37
Colloquium
Chemical communication among bacteria
Michiko E. Taga and Bonnie L. Bassler*
Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014
Cell-cell communication in bacteria is accomplished through the
exchange of chemical signal molecules called autoinducers. This
process, called quorum sensing, allows bacteria to monitor their
environment for the presence of other bacteria and to respond to
fluctuations in the number and/or species present by altering
particular behaviors. Most quorum-sensing systems are species- or
group-specific, which presumably prevents confusion in mixed-
species environments. However, some quorum-sensing circuits
control behaviors that involve interactions among bacterial spe-
cies. These quorum-sensing circuits can involve both intra- and
interspecies communication mechanisms. Finally, anti-quorum-
sensing strategies are present in both bacteria and eukaryotes, and
these are apparently designed to combat bacteria that rely on
cell-cell communication for the successful adaptation to particular
niches.
quorum sensing I autoinducer I cell-cell communication
In a process called quorum sensing, bacteria monitor the
~ presence of other bacteria in their surroundings by producing
and responding to signaling molecules known as autoinducers.
The concentration of autoinducer in a given environment is
proportional to the number of bacteria present; therefore,
detecting autoinducers gives bacteria a mechanism for "count-
ing" one another. Responding to autoinducers by altering gene
expression gives bacteria a means to perform particular behav-
iors only when living in a community but not when living in
isolation. Most quorum-sensing controlled behaviors are pro-
ductive only when a group of bacteria carries them out in
synchrony; they include bioluminescence, secretion of virulence
factors, biofilm formation, sporulation, conjugation, and pig-
ment production (1-3~. Because quorum sensing allows bacteria
to coordinate the behavior of the group, it enables them to take
on some of the characteristics of multicellular organisms.
There are two general types of bacterial quorum-sensing
systems: Gram-negative LuxIR circuits and Gram-posit~ve oli-
gopeptide two-component circuits. Gram-negative quorum-
sensing bacteria typically possess proteins homologous to the
LuxI and LuxR proteins of Vibrio fischeri, the bacterium in which
they were initially discovered (Fig. lA) (4~. The LuxI-type
proteins catalyze the formation of a specific acyl-homoserine
lactone (AHL) autoinducer that *eely diffuses into and out of
the cell and increases in concentration in proportion to cell
population density. The LuxR-type proteins each bind a specific
AHL autoinducer when the concentration of autoinducer
reaches a threshold level. The LuxR-AHL complexes activate
transcription of target genes by recognizing and binding specific
DNA sequences at quorum-sensing-regulated promoters (4-6~.
Some functions controlled by LuxIR-type quorum-sensing sys-
tems include plasmid conjugation in Agrobacterium tumefaciens
(7), antibiotic production in Erwinia carotovora (8)~ biofilm
production and virulence gene expression in Pseudomonas
aeruginosa (9, 10), and expression of factors necessary for
symbiosis in Sinorhizobium meliloti (11~. Currently, there are
>70 known LuxIR quorum-sensing systems in Gram-negative
bacteria (1, 12-14~.
www.pnas.org/cgi/doi/ 10.1 073/pnas. 1934514100
Low G+C Gram-positive bacteria typically use modified
oligopeptides as autoinducers (15-17~. These signals are gener-
ically referred to as autoinducing polypeptides (AIPs) (Fig. 1B).
AIPs are produced in the cytoplasm as precursor peptides and
are subsequently cleaved, modified, and exported. AIPs specif-
ically interact with the external domains of membrane-bound
two-component sensor kinase proteins. Interaction of the auto-
inducer with its cognate sensor stimulates the kinase activity of
the sensor kinase protein, resulting in the phosphorylation of its
partner response regulator protein. The phosphorylated re-
sponse re:,ulator protein binds DNA and alters the transcription
of target genes. Some examples of behaviors controlled by AIP
quorum-sensing systems include genetic competence and sporu-
ration in Bacillus subtilis (18, 19), competence for DNA uptake
in Streptococcus pneumoniae (20), and virulence factor expres-
sion in Staphylococcus aureus (21) and Enterococcus faecalis (22~.
Despite the fact that the autoinducer signals and detection
apparatuses can be highly similar, both LuxIR and oligopeptide
two-component-type quorum-sensing systems function such that
a response is elicited only to the auto~nducer of the bacterial
species that produced it. This signaling specificity stems in large
part from subtle differences in the autoinducer molecules and
their receptors. Here we discuss what is known about the
requirements for species-specific cell-cell communication.
Quorum-sensing circuits also exist that are species nonspecific;
their properties are discussed below.
Specificity in AHL Communication Systems
AHL autoinducers all share a common homoserine lactone
moiety and differ only in their acyl side chain moieties (Fig. 2A)
(3, 12~. However, an autoinducer of one bacterial species rarely
influences expression of target genes in another species. Two
primary factors control this exquisite signaling specificity: first,
the substrate specificity of the LuxI-like proteins, and second,
specificity in the binding of the LuxR-type proteins to their
cognate AHLs.
LuxI proteins link the side chain group of specific acyl-acyl
carrier proteins to the homocysteine moiety of S-adenosylmeth-
ionine (23-25~. These reactions are very precise, because LuxI
proteins recognize only the ACP containing a specific acyl chain
moiety. The reliability of this reaction is vital to maintaining
species-specific communication within each quorum-sensing sys-
tem, because it ensures that each LuxI protein generates only the
correct AHL signal molecule.
The molecular interactions underlying the specificity in LuxI-
directed signal production have been explored by structural
analyses of EsaI, the LuxI homologue of Pantoea stewartii (26~.
This paper results from the Arthur M. Sackler Colloquium of the National Academy of
Sciences, "Chemical Communication in a Post-Genomic World," held January 17-19, 2003,
at the Arnold and Mabel Beckman Center of the National Academies of Science and
Engineering in Irvine, CA.
Abbreviations: AHL, acyl-homoserine lactone autoinducer; AIP, autoinducing peptide;
Al-1, 3-hydroxybutanoyl-homoserine lactone; Al-2, 3a-methyl-5,6-dihydrofurol2,3-
dl[1,3,2]dioxaborole-2,2,6,6a-tetraol.
*To whom correspondence should be addressed. E-mail: bbassler~molbio.princeton.edu.
2003 by The National Academy of Sciences of the USA
PNAS 1 November 25, 2003 1 vol. 100 1 suppl. 2 1 14549-14554
OCR for page 38
A o °
° Oo o
o
to
o
° o o
o
o
o
o
o
o o
o o ~ o
o
target Merle expression
B o
o
o
o o
o
~ _
· Sensor
U Response
) Regulator
-
~ target gene expression ~
O O
O O
_~
1 ~
Precursor
Peptide
Fig. 1. Canonical quorum-sensing circuits. (A) In typical Gram-negative LuxlR
circuits, the Luxl-type protein catalyzes the synthesis of an AHL autoinducer
(pentagons). The LuxR-type protein binds the AHL and controls the expression
of target genes. (B) In typical Gram-positive AIP two-component quorum-
sensing circuits, precursor peptides are cleaved, modified, and exported by
dedicated transporters. The resulting AlPs (circles) are detected by two-
component sensor-histidine kineses, and sensory information is relayed by
phosphorylation (P) of cognate response regulators that, in turn, control
target gene expression. The proteins responsible for autoinducer binding are
shaded in gray.
EsaI catalyzes the formation of the AHL 3-oxohexanoyl-
homoserine lactone. The x-ray crystal structure of EsaI shows
that a hydrophobic cavity in the protein likely encapsulates the
acyl moiety of the acyl-acyl carrier proteins. The extreme
preference of EsaI for a six-carbon acyl side chain is due to the
size of the binding pocket. In addition, preference for a 3-oxo
AHL results from a favorable hydrogen bond between the C3
carbonyl of the AHL and the hydroxyl group of a threonine
residue on EsaI. Modeling studies suggest that the correspond-
ing pocket in LasI of P. aeruginosa is larger, and consistent with
this, LasI produces a longer autoinducer, 3-oxododecanoyl-
homoserine lactone This study suggests that the length and
derivatization of the acyl side chains of AHLs are determined by
differences in the structures of the binding cavities of the
LuxI-type proteins.
Specificity in LuxR-AHL interaction is critical for bacteria to
distinguish AHLs produced by their own species from AHLs of
other species. The structural basis for ligand-receptor interac-
tion has been most thoroughly examined with the Trait pro-
tein of A. tumefaciens and its cognate AHL autoinducer, 3-
oxooctanoyl-homoserine lactone (27~. The x-ray structure of
14550 1 www.pnas.org/cgi/doi/10.1073/pnas.1934514100
Trait in complex with its cognate AHL and DNA shows a precise
interaction between Trait and the acyl moiety on the AHL. In
this case, the C3 keto group of the AHL is stabilized by hydrogen
bonding to a water molecule present in an autoinducer-binding
cavity. Presumably, alterations in the size and shape of the
AHL-binding pocket in other LuxR proteins will correspond to
side chain lengths and substitutions of their cognate ligands.
The specificity of the AHL-LuxR pair does not extend to DNA
binding and transcriptional activation, because the LuxR pro-
teins all bind to similar DNA regulatory elements termed "lux
boxes" (28, 29~. Although interchangeability between AHLs and
their LuxR partners is not tolerated, together the pairs can
control gene expression at noncognate lux boxes. For example,
the AHL molecule produced by ~ fischeri (3-oxohexanoyl-
homoserine lactone) together with LuxR, expressed in Esche-
richia coli, can induce transcription of lasB, a gene normally
regulated by LasR and 3-oxododecanoyl-homoserine lactone in
P. aeruginosa. Similarly, 3-oxododecanoyl-homoserine lactone,
coupled with LasR and expressed in E. coli, will activate the V.
fischeri quorum-sensing controlled target luxCDABE (291.
Again, these findings support the hypothesis that target speci-
ficity inherent in LuxIR circuits stems exclusively from the
selection of a particular AHL by its cognate LuxR-type protein.
Specificity in AIP Communication Systems
As in LuxIR circuits, similarities exist between oligopeptide
autoinducers and cognate two-component sensors from differ-
ent groups or species of Gram-positive bacteria, yet each signal-
response system is remarkably specific. AIP synthesis is inher-
ently accurate, because precise signal generation is guaranteed
by the specific DNA sequence encoding the precursor protein.
All AIPs are cleaved from longer precursor peptides, and many
AIPs are also subject to posttranslational modifications (Fig.
2B). Signaling accuracy is achieved through the highly sensitive
nature of the AIP receptors to alterations in AIP structure (16~.
AIP signaling specificity has been analyzed extensively in S.
aureus (30-32~. Several clinically important groups within the S.
aureus species have been characterized, and they all have slightly
different AIPs, processing enzymes, and receptors (33, 34~. The
AIPs are 8 to 9 aa long and contain thiolactone rings involving
invariant cysteine residues situated 5 aa from the C terminus
(Fig. 2B) (35~. Specificity in AIP signaling, at least in the
staphylococci, is entirely determined by the AIP-sensor kinase
interaction. This hypothesis is supported by findings that the
cognate response regulators in all of the S. aureus groups are
completely conserved, as are the target promoter regions (30,
334. Interestingly, the AIP produced by one group of S. aureus
not only activates its own virulence cascade but also inhibits
virulence in all other groups. Inhibition of virulence requires far
less specificity in AIP structure, because the thiolactone ring
portion common to all of the S. aureus AIPs acts as a universal
inhibitor of AIP signaling (30-32~. The group-specific AIP
signaling in S. aureus indicates that quorum sensing in S. aureus
occurs at the subspecies level. Inhibition by one S. aureus group
of cell-cell communication in other S. aureus groups is presumed
to benefit the group that first establishes its quorum-sensing
cascade, because it facilitates the formation of a single-group
infection.
Quorum Sensing Using Multiple Autoinducers and Sensors
Quorum-sensing systems consisting of single autoinducer-sensor
pairs are sufficient to control gene expression in response to
changes in cell density. However, many bacteria have two or
more quorum-sensing systems. Use of multiple quorum-sensing
systems allows bacteria to integrate pieces of sensory informa-
tion, which presumably confers plasticity to the genetic network.
The hierarchies can be set up in series or in parallel, the former
enabling regulation of genes in a temporally defined manner, and
Taga and Bassler
OCR for page 39
A
R groups:
(AH
AH
C~16
F~1I (P. aeruginosa)
SwrI (S. liquefaciens)
LuxLM (V. harveyi)
Coil (Chromobacterium
violaceum)
Expl (E. carotovora)
EsaI (P. stewartii)
LuxI ( V. f scheri)
AinS (V. fscheri)
CepI (B. cepacia)
Tral (A. tumefacier~s)
VanI (Vibrio anguillarum)
Lasl (P. aeruginosa) C
cQ
10~16
CinI (Rhizobium
legzuminosarum)
SinI (S. meliloti)
B ADPITRQWGD
ERGMT
*
QNSPNIFGQWM
\ /
ComX (B. subtilis)
CSF (B. subtilis)
GBAP (E. faecalis)
NO—C'
o
EMRLSKFFRDFILQRKK CSP (s. pneumonias)
YSTCDFIM
1 /
S—Cur
o
GVNACSSLF
S—C<,
INCDFLL
1 /
S—C
o
YSTCYFIM
1 /
S—C
o
-o
HO,, Q'OH
HO""~("""CH3
HO''''''
AIP-I (S. aureus group 1)
AIP-II (S. aureus Coup 2)
AlP-III (S. agrees group 3)
AIP-IV (S. aureus group 4)
AI-2 (V. harveyi)
Fig. 2. Structures of different autoinducers. (A Upper) AHL autoinducers share a common homoserine lactone moiety. (Lower) Side chains of some different
AHLs are shown (R groups). The syntheses and the organisms that produce them are listed. (B) AIP peptide sequences, their designations, and the organisms that
produce them are shown. The asterisk above the tryptophan residue in ComX represents an isoprenyl group. (C) Al-2 of V. harveyi.
the latter enabling regulation of discrete groups of genes or
converging to regulate an identical set of genes.
In P. aeruginosa, two AHL quorum-sensing systems, LasIR
and RhlIR, act in series. At high cell density, the concentration
of both AHLs is high, and LasR binds its specific AHL to activate
the expression of particular target genes. One of the genes
activated by the AHL-LasR complex is rhlR, which encodes a
second AHL receptor, RhlR. RhlR, in turn, binds its cognate
AHL autoinducer and induces expression of its own target genes.
Thus, genes controlled by the LasIR system are expressed before
those controlled by the RhlIR system. This temporal pattern of
gene regulation allows P. aeruginosa to express different viru-
lence factors at various stages in the infection process (36, 37~.
The two P. aeruginosa AHL signals promote virulence in a
variety of different hosts, the most important human example
being the cystic fibrosis (CF) lung. Not only is the CF lung
susceptible to damage caused by AHL-controlled virulence
factors, but it is also vulnerable to direct effects of the AHL
molecules themselves. Specifically, the P. aeruginosa autoinduc-
ers induce the production of the chemokine IL-8 in the CF lung.
As a result, neutrophils are recruited to the lung, and this action
facilitates the formation of a potent P. aeruginosa infection.
Thus, in this system, bacterial AHLs direct the expression of
bacterial and host factors that enhance the infection process (38~.
Taga and Bassler
In two Vibrio species, Vibrio harveyi and Vibrio cholerae,
multiple quorum-sensing systems converge to regulate a single
group of genes. ~ harv~yi uses two parallel systems to regulate
the expression of target genes, including those required for
bioluminescence, and ~ cholerae has three systems that jointly
control the virulence regulon (1, 39~. In V. harv~yi (Fig. 3), the
two autoinducer syntheses, LuxLM and LuxS, each catalyze the
synthesis of a specific autoinducer: the AHL 3-hydroxybutanoyl-
homoserine lactone (denoted AI-1) in the case of LuxLM, and
a unique furanosyl borate diester, 3a-methyl-5,6-dihydrofuro-
[2,3-d]~1,3,2idioxaborole-2,2,6,6a-tetraol (AI-2), in the case of
LuxS (Fig. 2 A and C) (40-424. Unlike other Gram-negative
quorum-sensing systems in which AHL autoinducers are de-
tected by a cytoplasmic LuxR-type protein, detection of the two
autoinducers in ~ harv~yi occurs in the periplasm via cognate
two-component sensor kinase proteins. AI-1 initiates signal
transduction in the sensor kinase protein LuxN (43), and AI-2
binds the periplasmic binding protein LuxP, which in turn
initiates signaling from the sensor kinase protein LuxQ (44~.
Information from both LuxN and LuxPQ converges at the
phosphorelay protein LuxU (45, 46), and LuxU transmits the
phosphorylation signal to the response regulator LuxO. LuxO
controls transcription of target genes including those encoding
luciferase (IuxCDABE) (47, 48~.
PNAS I November25, 2003 1 vol. loo 1 suppl. 2 1 14551
OCR for page 40
o o o no a o To o o To o or o
Otto
LUXN~ n LUXQ ~ I 1 ~111 ~ 1
AI-IO ~ >30 LUXU OAI-2 | I I I #/ O I I ~1 I
~ ~ LUXO ~ ; .r; ~ I ~ | ~ ~ = - =
. / \~ l \~ _ Isr transport genes ~ =
A
~~ target gene expression ~
Fig. 3. The V. harveyi quorum-sensing circuit. Two autoinducers, the AHL
Al-1 (pentagons) and Al-2 (diamonds), are produced by their cognate syn-
thases LuxLM and LuxS, respectively. Al-1 and Al-2 bind cognate sensors (LuxN
and LuxPQ, respectively), initiating a phosphorylation cascade (P) that travels
through LuxU and alters the phosphorylation state of the response regulator
protein LuxO. LuxO controls the expression of target genes. The proteins
responsible for autoinducer binding are shaded in gray.
It seems paradoxical that ~ harveyi uses two quorum-sensing
systems to regulate the same set of target genes, because either
system alone should be sufficient. One possible explanation for
this molecular setup is that the autoinducers play different roles
in cell-cell communication. Support for this idea stems from the
finding that AI-1, like other AHL autoinducers, is species-
specific, whereas many diverse species of bacteria possess a
conserved LuxS homologue and produce AI-2, suggesting that
AI-2 may function in interspecies cell-cell communication.
Additionally, unlike AHL and AIP signaling, which is restricted
to Gram-negatives and -positives, respectively, LuxS and AI-2
exist in both Gram-negative and -positive bacteria, suggesting
that AI-2-mediated communication arose before AHL and AIP
signaling (2, 42, 494.
The ~ harveyi quorum-sensing circuit can distinguish among
the presence of none, one, or both autoinducers. However,
maximal expression of Lux occurs only when both autoinducers
are present, suggesting that the circuit could act as a coincidence
detector for both autoinducers (50~. Because the ~ harveyi
quorum-sensing regulon has not yet been fully defined, it
remains possible that additional classes of target genes exist that
are regulated exclusively by either AI-1 or AI-2. If so, ~ harveyi
could differentially modify its behavior depending on whether it
makes up the majority or the minority of a given mixed-species
population. If, on the other hand, the ~ harveyi quorum-sensing
circuit functions primarily as a coincidence detector that regu-
lates gene expression in response to the presence of both
autoinducers, the advantage of this scheme could be that
the simultaneous detection of two signals reduces the vulnera-
bility of the circuit to noise or to "trickery." A system that
works by a combinatorial scheme could protect the cell-cell
communication circuit from molecules made by other organisms
in the environment that are similar in structure to the auto-
inducers (50~.
14552 1 www.pnas.org/cgi/doi/, 0., 073/pnas. ~ 934534100
Fig. 4. Lsr-mediated transport of Al-2 in S. typhimurium. Al-2 activates
transcription of the Isr genes, four of which (shaded in gray) encode the Lsr
transporter apparatus that functions to internalize Al-2.
Interspecies CelKell Communication: LuxS and Al-2
AI-2 is the only species-nonspecific autoinducer known. Because
of its widespread occurrence, AI-2 is proposed to act as a
universal quorum-sensing signal for interaction between species
of bacteria (42, 49~. Besides controlling light production in ~
harveyi, the various roles that AI-2 plays in other species are
beginning to be defined. Among other things, AI-2 controls the
expression of genes required for virulence in E. colt, V. cholerae,
Clostridium perfringens, and Streptococcus pyogenes; iron acqui-
sition in Porphyromonas gingivalis and Actinobacillus actinomy-
cetemcomitans; antibiotic production in Photorhabdus lumine-
scens; motility in Campylobacter jejuni; and mixed-species biofilm
formation between P. gingivalis and Streptococcus gordonii (51~.
In Salmonella typhimurium, AI-2 was recently shown to control
a seven-gene operon, called the Isr operon (for Lusts Regulated)
(Fig. 4) (52~. Four of the Isr operon genes encode an ABC
transporter whose function is to promote internalization of AI-2.
No additional AI-2-regulated genes have been identified in S.
typhimurium, suggesting that AI-2 may have a role that is
different from a classic quorum-sensing autoinducer in some
bacteria, including S. typhimurium.
In S. typhimurium, and presumably in other bacteria that
possess the Isr operon, AI-2 is produced in the cytoplasm and is
released into the extracellular environment, where it accumu-
lates. AI-2 is subsequently transported back inside the cell
through the Lsr transporter (Fig. 4) (52~. The benefit that
bacteria gain from this cyclic process is unknown, although many
possibilities exist. For example, AI-2 could be a quorum-sensing
signal that impacts as-yet-unidentified genes in S. typhimurium.
Other quorum-sensing regulated genes may exist that were not
identified in the analysis revealing the Isr genes due to redun-
dancy in the quorum-sensing circuitry or a requirement for a
specific growth condition that was not met in the laboratory. If
so, import of AI-2 could be used to terminate the quorum-
sensing cascade or to convey AI-2 to an internal detection
apparatus. An alternative hypothesis is that internalization of
AI-2 could be a mechanism that bacteria such as S. typhimurium
have evolved to interfere with the AI-2 quorum-sensing systems
of competing species. This action could serve to confound other
bacteria that regulate specific functions using AI-2, thereby
giving S. typhimurium an advantage in particular environments.
Taga and Bassler
OCR for page 41
POOH
RJ(N_?
H \
o
~ R N:OH
IAiiDI
RICH + H2N~)
o
Fig. 5. Enzymatic inactivation of AHLs. (Upper) AiiA of Bacillus species
hydrolyzes the lactone ring of AHEs. (Lower) AiiD of a Ralstonia strain hydro-
lyzes AHLs to release the acyl side chain moiety and homoserine lactone.
Interference with Cell Cell Communication: Biological Battles
and Conspiracies
Quorum sensing often promotes behaviors that are detrimental
to other organisms in the vicinity. For example, pathogens such
as P. aeruginosa and E. carotovora use AHL-mediated quorum
sensing to activate their virulence genes in specific animal and
plant hosts, respectively (12~. Likewise, in Serratia liquefaciens,
antibiotic production and swarming motility, two behaviors
required for virulence, are controlled by AHL-mediated quorum
sensing (53~. To protect themselves, some susceptible organisms
have developed defenses that interfere with quorum sensing.
Several anti-AHL-mediated quorum-sensing strategies have re-
cently been discovered, and we present a few examples here.
Additionally, in mixed-species environments, different species of
bacteria can "conspire" with one another by responding to
heterologous AHLs.
The macroalga Delisea pulchra produces a collection of haloge-
nated furanones, many of which inhibit growth of pathogenic
microorganisms on its surface. The furanones have structural
similarity to AHLs and can prevent the AHL signal of S. Izquefacier~s
from interacting with its cognate LuxR-type protein. In this model
system, inhibition of AHL signaling prevents AHL-mediated mo-
tility in S. Iiqu~aciens, which precludes bacterial colonization of the
alga (54-57~. Similarly, several organisms produce enzymes that
modify AHlLs, rendering them inactive. Marine algae such as
Laminaria digitata prevent the formation of bacterial biofilms on
their surfaces by producing haloperoxidases that generate oxidized
halogens with microbicidal activity, such as hypochlorous acid
(HOCl). Oxidized halogens react specifically with C3-oxo-AHLs
and destroy their signaling capability. This finding is particularly
significant because of the ability of the oxidized halogens to
penetrate into biofilms (58~.
In addition to eukaryotes, some bacteria produce enzymes that
interfere with AHL signaling, presumably to gain an advantage over
AHL-producing bacteria in specific biological niches. For example,
an AHL lactonase (AiiA) isolated from Bacillus species has been
shown to hydrolyze the lactone ring of AHLs to form acyl-
homoserine, which does not function as a quorum-sensing signal
(Fig. 5 Upper). Expression of AiiA in either the quorum-sensing
pathogen E. carotovora or its susceptible plant host eliminates AHL
production and infection, suggesting that AiiA can be used to
prevent infection by pathogens that use quorum sensing to control
virulence (59-62~. What in vivo role AiiA plays in Bacillus species
is not known, but it could be used to interfere with AH3L-mediated
quorum-sensing behaviors of soil bacteria that compete with Ba-
cillus for the same niche. A different AHL inactivating enzyme,
AiiD, was recently discovered in a Ralstonia strain isolated from a
bactenal biofilm. Like AiiA, AiiD hydrolyzes AHLs, but in this case
Taga and Bassler
the side chain is released from the intact homoserine lactone ring
(Fig. 5 Lower). Again, the final result is elimination of quorum
sensing. Introduction of aiiD into P. aerugirnosa caused reduced
accumulation of extracellular AHLs and inhibited AHL regulated
behaviors such as swarming motility, virulence factor production,
and paralysis of Caenorhabdids elegans (63~. In another study, the
bacterium Varzovormc paradoxus was shown to be capable of using
AHLs as a sole carbon and nitrogen source, suggesting that it too
has AHL degrading activity (64~. It appears that ~ paradoxus has
evolved the ability to degrade AHLs either as a defense against
other bacteria that use AHLs as signaling molecules or as a
metabolic scavenging method for using molecules released by
neighboring bacteria. These recent discoveries suggest that the
inhibition of AHlL-mediated quorum sensing might be a widespread
mechanism that bacteria use to compete with one another.
Instead of interspecies inhibition of AHL signaling, some
bacteria use AHLs for interspecies communication during in-
fection. For example, in cystic fibrosis lungs colonized by both P.
aeruginosa and Burkholderia cepacia, in addition to producing
and responding to its own AHL, B. cepacia responds to AHLs
produced by P. aeruginosa, which promotes formation of a
mixed-species biofilm (65~. Because most bacteria reside in
mixed-species environments, other examples of bacteria "con-
spiring" to carry out different behaviors could be discovered in
examinations of quorum sensing in mixed populations.
Few cases of interference with AIP-mediated quorum-sensing
systems are currently known, with the best-studied example
being the cross-group inhibition of AIP signaling in S. aureus,
discussed above. However, other systems for interfering with
AIP-mediated quorum-sensing systems may exist. Although the
autoinducer AI-2 is used for interspecies communication, no
cases of interference with AI-2 quorum sensing are yet docu-
mented. However, as noted above, internalization of AI-2 by
enterics such as S. typhimurium is considered one such possible
anti-AI-2 communication mechanism.
Conclusion
Recent research shows that chemical communication among
bacteria is widespread and involves complex interconnected
regulatory networks that serve to fine-tune the expression of
diverse group behaviors. Specificity in autoinducer production
and recognition is a key component of quorum sensing, because
bacteria must preserve the fidelity of their communication
circuits while existing in communities containing other organ-
isms that produce molecules similar to their own autoinducers,
either as quorum-sensing autoinducers or as autoinducer antag-
onists. The use of quorum sensing to control behaviors such as
biofilm formation, symbiosis, and virulence factor expression
indicates that quorum sensing is frequently used to regulate traits
involving interactions between different organisms, both mutu-
alistic and antagonistic. Thus, the ability of bacteria to distin-
guish self from other could also be a fundamental property of
quorum-sensing systems. In support of this idea, it appears that
some bacteria have evolved species-specific as well as "generic"
signaling molecules. The recent discovery of autoinducer de-
grading enzymes demonstrates that interference with quorum
serlsing is an effective antibacterial strategy used in the wild.
Synthetic anti-quorum-sensing strategies could be developed in
the future as possible alternatives to antibiotics, because block-
ing cell-cell communication within or among bacterial species
could prevent pathogenicity. Likewise, biotechnological ap-
proaches that promote beneficial quorum-sensing behaviors
could be exploited for the production of industrial-scale natural
products in bacteria.
This work was supported by Office of Naval Research Grant N00014-
99-1-0767, National Science Foundation Grant MCB-0094447, and
National Institute of General Medical Sciences Grant GM65859.
PNAS I November25, 2003 1 vol. 100 1 suppl. 2 1 14s53
OCR for page 42
1. Miller, M. B. & Bassler, B. L. (2001) Annul Rev. Microbiol. 55, 165-199.
2. Bassler, B. L. (1999) Curr. Opin. Microbiol. 2, 582-587.
3. Fuqua, C., Winans, S. C. & Greenberg, E. P. (1996)Annu. Rev. Microbiol. 50,
727-751.
4. Engebrecht, J., Nealson, K. & Silverman, M. (1983) Cell 32, 773-781.
5. Engebrecht, J. & Silverman, M. (1984) Proc. Nall. Acad. Sci. USA 81,
4154-4158.
6. Engebrecht, J. & Silverman, M. (1987) Nucleic Acids Res. 15, 10455-10467.
7. Piper, K. R., Beck von Bodman, S. & Farrand, S. K. (1993) Nature 362,
448-450.
8. Bainton, N. J., Stead, P., Chhabra, S. R., Bycroft, B. W., Salmond, G. P.,
Stewart, G. S. & Williams, P. (1992) Biochem. J. 288, 997-1004.
9. Davies, D. G., Parsek, M. R., Pearson, J. P., Iglewski, B. H., Costerton, J. W.
& Greenberg, E. P. (1998) Science 280, 295-298.
10. Passador, L., Cool`, J. M., Gambello, M. J., Rust, L. & Iglewski, B. H. (1993)
Science 260, 1127-1130.
11. Marketon, M. M., Gronquist, M. R., Eberhard, A. & Gonzalez, J. E. (2002) J.
Bacteriol. 184, 5686-5695.
12. de Kievit, T. R. & Iglewski, B. H. (2000) Infect. Immun. 68, 4839-4849.
13. Fuqua, C., Parsek, M. R. & Greenberg, E. P. (2001) Annul Rev. Genet. 35,
439-468.
14. Parsek, M. R. & Greenberg, E. P. (2000) Proc. Natl. Acad. Sci. USA 97,
8789-8793.
15. Lazazzera, B. A. & Grossman, A. D. (1998) Trends Microbiol. 6, 288-294.
16. Kleerebezem, M., Quadri, L. E., Kuipers, O. P. & de Vos, W. M. (1997) Mol.
Microbiol. 24, 895-904.
17. Sturme, M. H., Kleerebezem, M., Nakayama, J., Akkermans, A. D., Vaugha,
E. E. & de Vos, W. M. (2002) Antonie Leeuwenhoek 81, 233-243.
18. Magnuson, R., Solomon, J. & Grossman, A. D. (1994) Cell 77, 207-216.
19. Solomon, J. M., Magnuson, R., Srivastava, A. & Grossman, A. D. (1995) Genes
Dev. 9, 547-558.
20. Haverstein, L. S. & Morrison, D. A. (1999) in Cell-Cell Signaling in Bacteria, eds.
Dunney, G. M. & Winans, S. C. (Arn. Soc. Microbiol., Washington, DC), pp.
9-26.
21. Novick, R. P. (1999) in Cell-Cell Sigrzaling in Bacteria, eds. Dunney, G. M. &
Winans, S. C. (Am. Soc. Microbiol., Washington, DC), pp. 129-146.
22. Qin, X., Singh, K. V., Weinstock, G. M. & Murray, B. E. (2000) Infect. Immun.
68, 2579-2586.
23. Hanzelka, B. L. & Greenberg, E. P. (1996) J. Bacteriol. 178, 5291-5294.
24. More, M. I., Finger, L. D., Stryker, J. L., Fuqua, C., Eberhard, A. & Winans,
S. C. (1996) Science 272, 1655-1658.
25. Val, D. L. & Cronan, J. E., Jr. (1998) J. Bacteriol. 180, 2644-2651.
26. Watson, W. T., Minogue, T. D., Val, D. L., von Bodman, S. B. & Churchill,
M. E. (2002) Mol. Cell 9, 685-694.
27. Zhang, R. G., Pappas, T., Brace, J. L., Miller, P. C., Oulmassov, T., Molyneaux,
J. M., Anderson, J. C., Bashkin, J. K, Winans, S. C. & Joachimiak, A. (2002)
Nature 417, 971-974.
28. Devine, J. H., Shadel, G. S. & Baldwin, T. O. (1989) Proc. Natl. Acad. Sci. USA
86, 5688-5692.
29. Gray, K. M., Passador, L., Iglewski, B. H. & Greenberg, E. P. (1994)J. Bacteriol.
176, 3076-3080.
30. Lyon, G. J., Mayville, P., Muir, T. W. & Novick, R. P. (2000) Proc. Natl. Acad.
Sci. USA 97,13330-13335.
31. Lyon, G. J., Wright, J. S., Christopoulos, A., Novick, R. P. & Muir, T. W. (2002)
J. Biol. Chem. 277, 6247-6253.
32. Lyon, G. J., Wright, J. S., Muir, T. W. & Novick, R. P. (2002) Biochemistry 41,
10095-10104.
33. Ji, G., Beavis, R. & Novick, R. P. (1997) Science 276, 2027-2030.
14554 1 www.pnas.org/cgi/doi/10. 1 073/pnas. 1934514100
34. Jarraud, S., Lyon, G. J., Figueiredo, A. M., Gerard, L., Vandenesch, F.,
Etienne, J., Muir, T. W. & Novick, R. P. (2000) J. Bacteriol. 182, 6517-6522.
35. Mayville, P., Ji, G., Beavis, R., Yang, H., Goger, M., Novick, R. P. & Muir,
T. W. (1999) Proc. Natl. Acad. Sci. USA 96, 1218-1223.
36. Brint, J. M. & Ohman, D. E. (1995) J. Bacteriol. 177, 7155-7163.
37. Pesci, E. C. & Iglewski, B. H. (1997) Trends Microbiol. 5, 132-134; discussion,
134-135.
38. Smith, R. S., Fedyk, E. R., Springer, T. A., Mukaida, N., Iglewski, B. H. &
Phipps, R. P. (2001) J. Immunol. 167, 366-374.
39. Miller, M. B., Skorupski, K., Lenz, D. H., Taylor, R. K. & Bassler, B. L. (2002)
Cell 110, 303-314.
40. Cao, J. G. & Meighen, E. A. (1989) J. BioL Chem. 264, 21670-21676.
41. Chen, X., Schauder, S., Potier, N., Van Dorsselaer, A., Pelczer, I., Bassler, B. L.
& Hughson, F. M. (2002) Nature 415, 545-549.
42. Surette, M. G., Miller, M. B. & Bassler, B. L. (1999) Proc. Natl. Acad. Sci. USA
96, 1639-1644.
43. Bassler, B. L., Wright, M., Showalter, R. E. & Silverman, M. R. (1993) Mol.
Microbiol. 9, 773-786.
44. Bassler, B. L., Wright, M. & Silverman, M. R. (1994) Mol. Microbiol. 13,
273-286.
45. Freeman, J. A. & Bassler, B. L. (1999) J. Bacteriol. 181, 899-906.
46. Preeman, J. A., Lilley, B. N. & Bassler, B. L. (2000) Mol. Microbiol. 35, 139-149.
47. Freeman, J. A. & Bassler, B. L. (1999) Mol. Microbiol. 31, 665-677.
48. Lilley, B. N. & Bassler, B. L. (2000) Mol. Microbiol. 36, 940-954.
49. Bassler, B. L., Greenberg, E. P. & Stevens, A. M. (1997) J. Bacter~ol. 179,
4043-4045.
50. Mok, K. C., Wingreen, N. S. & Bassler, B. L. (2003) EMBO J. 22, 870-881.
51. Xavier, K. B. & Bassler, B. L. (2003) Curr. Opin. Microbiol. 6,191-197.
52. Taga, M. E., Semmelhack, J. L. & Bassler, B. L. (2001) Mol. Microbiol. 42,
777-793.
53. Eberl, L., Winson, M. K., Sternberg, C., Stewart, G. S., Christiansen, G.,
Chhabra, S. R., Bycroft, B., Williams, P., Molin, S. & Givskov, M. (1996) Mol.
Microbiol. 20, 127-136.
54. Givskov, M., de Nys, R., Manefield, M., Gram, L., Maximilien, R., Eberl, L.,
Molin, S., Steinberg, P. D. & Kjelleberg, S. (1996) J. Bacteriol. 178, 6618-6622.
55. Givskov, M., Eberl, L. & Molin, S. (1997) FEMS Microbiol. Lett. 148, 115-122.
56. Manefield, M., de Nys, R., Kumar, N., Read, R., Givskov, M., Steinberg, P. &
Kjelleberg, S. (1999) Microbiology 145, 283-291.
57. Rasmussen, T. B., Manefield, M., Andersen, J. B., Eberl, L., Anthoni, U.,
Christophersen, C., Steinberg, P., Kjelleberg, S. & Givskov, M. (2000) Micro-
biology 146, 3237-3244.
58. Borchardt, S. A., Allain, E. J., Michels, J. J., Stearns, G. W., Kelly, R. F. &
McCoy, W. F. (2001) Appl. Environ. Microbiol. 67, 3174-3179.
59. Dong, Y. H., Gusti, A. R., Zhang, Q., Ku, J. L. & Zhang, L. H. (2002) Appl.
Environ. Microbiol. 68,1754-1759.
60. Dong, Y. H., Wang, L. H., Xu, J. L., Zhang, H. B., Zhang, X. F. & Zhang, L. H.
(2001) Nature 411, 813-817.
61. Dong, Y. H., Xu, J. L., Li, X. Z. & Zhang, L. H. (2000) Proc. Natl. Acad. Sci.
USA 97, 3526-3531.
62. Lee, S. J., Park, S. Y., Lee, J. J., Yum, D. Y., Koo, B. T. & Lee, J. K. (2002)
Appl. Environ. Microbiol. 68, 3919-3924.
63. Lin, Y. H., Xu, J. L., Hu, J., Wang, L. H., Ong, S. L., Leadbetter, J. R. & Zhang,
L. H. (2003) Mol. Microbiol. 47, 849-860.
64. Leadbetter, J. R. & Greenberg, E. P. (2000) J. Bacteriol. 182, 6921-6926.
65. Riedel, K, Hentzer, M., Geisenberger, O., Huber, B., Steidle, A., Wu, H.,
Hoiby, N., Givskov, M., Molin, S. & Eberl, L. (2001) Microbiology 147,
3249-3262.
Taga and Bassler
Representative terms from entire chapter:
target genes
