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Disruption of bacterial quorum sensing by other organisms
Wolfgang D Bauer* and Jayne B Robinson†
Higher plants and algae produce compounds that mimic
quorum sensing: signals used by bacteria to regulate the
expression of many genes and behaviors. Similarly, various
bacteria can stimulate, inhibit or inactivate quorum sensing in
other bacteria. These discoveries offer new opportunities to
manipulate bacterial quorum sensing in applications relevant to
medicine, agriculture and the environment.
Addresses
*Horticulture & Crop Science, 2021 Coffey Road, Ohio State
University, Columbus, OH 43210, USA; e-mail: [email protected]
† Biology Department, University of Dayton, Dayton, OH 45469-0002,
USA; e-mail: [email protected]
Current Opinion in Biotechnology 2002, 13:234–237
0958-1669/02/$ — see front matter
© 2002 Elsevier Science Ltd. All rights reserved.
DOI 10.1016/S0958-1669(02)00310-5
Abbreviations
AHL
N-acyl homoserine lactones
QS
quorum sensing
Introduction
In bacteria, the regulation of many important changes in
gene expression relies on a system of signaling between
cells known as ‘quorum sensing’ (QS). Because genes
known to be under QS regulation are often crucial to
virulence, biofilm formation and colonization of eukaryotic
hosts [1,2], there is currently great interest in discovering
ways to disrupt or manipulate QS signaling in bacteria
[3,4]. Recently, both plants and algae have been found to
secrete substances that mimic the QS signals of bacteria.
These QS ‘signal-mimic’ compounds thus have the potential
to suppress or activate QS-regulated gene expression in
susceptible bacteria. In addition, several soil bacteria have
been found capable of disrupting QS in other species.
Studies on how these plants, algae and bacteria disrupt QS
in bacteria have begun to reveal new opportunities for
therapeutic or environmental applications.
QS in bacteria works by the production of small signal molecules that diffuse in and out of the cell (see [5,6•] for recent
reviews of QS). When several signal-producing bacteria are
in close proximity to each other, the concentration of the QS
signal in these cells increases. Binding of the QS signal to
specific receptor proteins leads to receptor activation and the
enhanced transcription of genes with appropriate promotor
sequences. Thus, bacteria can use QS signals to monitor the
density/proximity of other signal-producing cells and thereby
adjust the expression of certain genes in response to how
many sibling cells (or ‘look-alikes’) are nearby.
This kind of population density regulated behavior makes
good biological sense for pathogenic bacteria, enabling
them to maintain a low profile of virulence gene expression
to avoid triggering host defenses until a sufficient number
of cells is present to mount an effective attack. QS may also
make sense in switching from the behaviors appropriate to
isolated, free-living bacteria to the behaviors appropriate to
cells in a colony or a biofilm. This review summarizes what
has been learned recently about how QS in bacteria can be
manipulated or disrupted by other bacteria, algae and
higher plants.
Quorum sensing signal-mimics
It was the applied search for compounds capable of
preventing or disrupting bacterial biofilm formation and
the subsequent ‘fouling’ of ships and nets in marine waters
that led to the discovery of the first QS ‘signal-mimic’
compounds. The marine red alga Delisea pulchra was found
to produce substances that were highly effective in
preventing biofouling [7]. The active compounds were
shown to comprise a range of halogenated furanones.
Kjelleberg and colleagues [8] recognized that these
halogenated furanones were fairly similar in structure to
N-acyl homoserine lactones (AHLs), the principal QS
signals in Gram-negative bacteria. They subsequently
demonstrated that the halogenated furanones from Delisea
specifically inhibited AHL-regulated behaviors in a variety
of Gram-negative bacteria [8]. Thus, the furanones appear
to mimic the AHL signals of these bacteria, and most likely
do so by binding to the AHL receptor proteins [9]. The
furanone QS signal-mimics of Delisea appear to have potent
effects on the community of bacteria that colonize the algal
surface in its natural environment, shifting that community
from the Gram-negative bacterial species that normally
dominate colonization towards Gram-positive species that
are relatively poor colonizers of most marine surfaces [10•].
This suggests that the furanone signal-mimics have
considerable potential to disrupt biofilm formation and QS
regulation by AHL-producing bacteria in natural environments. Current efforts include the synthesis and testing of
chemical analogs of the furanone mimics for their effects
on pathogenic bacteria such as Pseudomonas aeruginosa and
on the prevention of fouling in marine waters.
Although all of the D. pulchra QS mimic compounds have
inhibitory effects, higher plants secrete a variety of signalmimics that stimulate QS-regulated behaviors as well as
mimic substances that inhibit AHL-regulated behaviors
[11••]. The compounds responsible for these signal-mimic
activities in higher plants have not yet been identified, but
their effects seem to be specific to QS-regulated gene
expression. The active compounds secreted by pea
seedlings have different solvent partitioning properties
than bacterial AHLs, making it likely that they are chemically different from known AHLs. A diversity of higher
plant species, including pea, soybean, rice, tomato, crown
Disruption of bacterial quorum sensing by other organisms Bauer and Robinson
235
Table 1
Organisms capable of disrupting or manipulating bacterial QS.
Organism(s)
Mechanism of disrupting QS
References
Delisea pulchra
Higher plants (pea, rice, tomato,
M. truncatula)
Soil bacteria
Many isolates
Many isolates
V. paradoxus
Secretion of halogenated furanones that inhibit QS
Secretion of unknown compounds that inhibit/stimulate AHLor AI2-mediated QS
[7–9,10•,11••]
[12]
Stimulation of QS by rhizosphere co-inhabitants
Enzymatic inactivation of AHL QS signals
Metabolic degradation of AHL QS signals
[15]
[16••,17•]
[14]
vetch and Medicago truncatula (a model legume closely
related to alfalfa), were found to secrete substances that
stimulated specific AHL reporters [11••]. More recently,
rice and M. truncatula seedlings have been found (M Gao,
M Teplitski, JB Robinson, WD Bauer, unpublished
results) to produce compounds that mimic the effects of
AI2, the QS signal used by various enteric bacteria including
Escherichia coli, Salmonella typhimurium and Vibrio harveyi
[12]. Thus, higher plants are a rich source of compounds
that positively and negatively affect at least two major QS
systems in bacteria. This implies that the QS signal-mimic
compounds are produced by plants to help them deal with
the diversity of bacteria that they encounter, just as the
furanone mimics of D. pulchra help the alga to control the
colonization and fouling of its surfaces. If this is the case,
then the engineering or breeding of plants to produce
specific QS mimics to affect specific bacterial pathogens or
symbionts could be an area of important future application.
Bacteria, as well as plants and algae, have found ways to
disrupt QS signaling. A strain of Variovorax paradoxus capable
of utilizing AHL QS signal compounds as sole carbon,
nitrogen and energy source was recently isolated from soil
by enrichment culture, suggesting that there may be
various bacterial species in natural environments that can
metabolize AHLs and disrupt QS regulation in nearby
bacteria [13]. Further evidence for the occurrence of
bacteria capable of disrupting QS comes from studies by
Pierson and colleagues [14]. These researchers first
showed that about 8% of random bacterial isolates from
wheat root surfaces were able to stimulate QS-regulated
phenazine synthesis in Pseudomonas aureofaciens cells when
co-inoculated and growing in situ on the root surface [14].
Subsequently, they found that about 7% of bacterial
isolates from the wheat rhizosphere were able to specifically
and reversibly inhibit QS-regulated gene expression in
P. aureofaciens (L Pierson, personal communication). Thus,
it seems that different bacterial species can exchange QS
signals in natural environments, giving them the potential
to stimulate important behaviors in neighbors and possibly
establish functional mixed communities. In addition, it
appears that an appreciable percentage of bacteria are
somehow able to disrupt or block QS in other species.
Recent studies by Zhang and colleagues [15] provide some
mechanistic insight as to how one bacterial species might
interfere with QS regulation in another species. They
found that about 5% of the several hundred soil bacteria
tested were able to inactivate AHLs. The AHL inactivation
activity in a Bacillus cereus isolate was due to its synthesis
and secretion of a lactonase capable of opening the
homoserine lactone ring of AHLs, thereby reducing the
effectiveness of the signal molecule by about 1000-fold
[15]. The authors tested the potential utility of this
discovery by expressing the AHL lactonase in potato and
tobacco. The transgenic plants were found to be substantially
more resistant to the soft rot pathogen Erwinia caratovora,
which depends on AHL-mediated QS for expression of
genes required for pathogenicity [16••]. This work
provides the first clear evidence that a host can control the
virulence of bacterial pathogens if the host is able to
reduce the effective concentration of QS signals in situ.
Table 1 summarizes the kinds of organisms presently
known to be capable of disrupting QS.
Role of inhibitory versus stimulatory mimics
Inhibitory signal-mimic compounds from a host might
effectively reduce the concentration of bacterial QS signals
in host tissues by competitively blocking signal binding
sites on the receptors. The selective advantage provided
by signal-mimics that stimulate QS signal receptors in
bacteria, as seen in higher plants, is not so clear. They
might, for example, be useful to the host by prematurely
inducing the expression of virulence genes in a pathogen,
leading to early triggering of host defenses and reduced
disease. Such protection of a eukaryotic host by production
of signals that stimulate QS receptors was shown in recent
studies where transgenic tobacco plants expressing the
AHL synthase gene from E. caratovora were found to be
more resistant to infection by wild-type E. caratovora [17•].
An earlier study showed that transgenic tobacco plants
expressing a bacterial AHL synthase gene produced
sufficient extracellular AHLs to stimulate QS-regulated gene
expression in synthase-deficient mutants of P. aureofaciens
and E. caratovora [18].
Limitations to manipulation of QS
The transformation of plants with bacterial genes that
confer AHL-synthesizing or AHL-inactivating activity
has provided important proofs-of-principle. It is clearly
possible to substantially affect the outcome of interactions
between bacteria and a eukaryotic host by manipulating
or disrupting QS signaling. However, the usefulness of
treating or engineering eukaryotic hosts to stimulate or
alter QS in associated bacteria will probably depend on the
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Environmental biotechnology
delivery of adequate concentrations of an effector and on
the level of specificity that can be achieved. Specificity is
important with respect to targeting just those bacterial
species and behaviors that are of interest (e.g. reducing the
virulence of a particular pathogen or enhancing the
co-metabolism of a xenobiotic by a mixed-species consortium).
Targeting specific bacteria and behaviors with agents that
disrupt QS may prove difficult to achieve. Various bacterial
species use the same or very similar QS signals, indicating
that they might be broadly rather than specifically affected
by mimics or inactivators of those signals. Different strains
within a given bacterial species may have diverged to use
different pathways of QS regulation for a particular behavior,
which could make it difficult to consistently target
virulence in a pathogen or surface adhesion in a bioreactor
isolate. In addition, substances that disrupt QS may
unintentionally affect unexpected species. Ahmer and
co-workers [19••] have recently shown that Salmonella
enterica, which probably does not synthesize AHLs and
might be thought immune to the effects of AHL signalmimics or inactivators, nonetheless has an AHL receptor,
SdiA, which activates gene expression in response to
AHLs from other bacterial species [19••]. Many bacterial
species may similarly prove to have receptors that ‘listen’
to one class of QS signals from other bacteria and make
appropriate responses, while maintaining a second class of
signals and receptors for their own QS regulation.
Control of biofilms
A prime applied target for interfering with QS is the
prevention and/or disruption of bacterial biofilms. In the
principal model system for biofilm studies, P. aeruginosa, it
is clear that AHL signals and QS-controlled genes are
central to both the establishment and maturation of
biofilms. Using green fluorescent protein to follow the
expression of lasI and rhlI in biofilms during the initial
stages of P. aeruginosa biofilm formation, it was found that
lasI appears to be necessary for both attachment and
microcolony formation [20]. In flowthrough systems, the
las QS system is important in development of fully differentiated biofilms of P. aeruginosa as well as their resistance
to the biocide sodium dodecyl sulfate [20] and to oxidizing
biocides [21].
In preliminary attempts to examine the effects of AHL
mimics from plants on biofilm formation, we found that
purified fractions containing a LasR stimulatory mimic
from the model legume M. truncatula significantly affected
biofilm initiation by P. aeruginosa (JB Robinson, M Gao,
unpublished results). When exposed to this QS mimic,
early log phase cells of P. aeruginosa formed biofilms that
were 5.9-fold higher in cell density than untreated cells. In
practical applications, the feasibility of using plant mimics
to enhance, prevent or disrupt bacterial biofilms will
depend significantly on the concentrations required.
Charlton et al. [22•] reported that the concentrations of
N-3-oxo-dodecanoyl homoserine lactone reached 600 µM
in P. aeruginosa biofilms, the highest level yet reported for
an AHL for a wild-type bacterium. This AHL was present
at only 14 nM in effluents of the biofilms, perhaps
suggesting that the AHL is binding to the biofilm matrix.
These authors also detected high levels of N-3-oxotetradecanoyl homoserine lactone in the biofilms, an AHL
previously unreported for P. aeruginosa. These results
emphasize the need for a greater understanding of the
kinds and levels of AHLs present in biofilms before we
can devise ways to disrupt or prevent their formation. Such
studies must include not only model biofilms formed by
pure monocultures, but also those formed by mixed
species such as the AHL cross-signaling biofilms produced
by P. aeruginosa and Burkhoderia cepacia [23]. The lactonase
enzymes that inactivate AHLs [15] may provide an alternative approach to the disruption of bacterial biofilms. For
example, the lactonase encoded by the aiiA gene of
Bacillus sp. 240B1 was shown to degrade both of the
ominant AHLs of P. aeruginosa.
Conclusions
The discovery of a diversity of substances in higher plants
that specifically stimulate or inhibit QS regulation in
various bacteria raises the possibility that these ‘signalmimics’ may represent new classes of antibacterial
compounds with potential uses in the control of biofilms or
disease. Chemical identification of the active mimic
compounds is clearly needed to explore these possibilities.
It will also be important to explore in depth the role that
mimics from a particular plant have in influencing the
interaction of the plant with specific bacteria or the
interactions of bacteria with each other, as in biofilms.
Similarly, it will be important to learn more about the
molecular mechanisms that enable some bacteria to
stimulate, inhibit, inactivate or simply listen to QS in
neighboring bacteria. Such interspecies communication is
likely to play crucial roles in both competition and cooperation between bacterial species in natural environments,
and is likely to provide us with additional molecular tools
for manipulating bacteria.
Update
Recent studies have tested the effects of a synthetic
halogenated furanone QS mimic compound on biofilm
formation by P. aeruginosa [24] and the effects of a natural
halogenated furanone from Delisea on swarming and
biofilm formation in E. coli [25]. The synthetic mimic
compound was found to penetrate P. aeruginosa microcolonies, to inhibit QS-regulated virulence factors, to alter
the architecture of the biofilm and to promote loss of
bacterial cells from the biofilm. The natural furanone
mimic was found to be a potent inhibitor of E. coli surface
swarming, but did not affect swimming and had modest
effects on biofilm formation.
Acknowledgements
Some of the preliminary work from the author’s laboratories was supported
by a grant from the Ohio Plant Biotechnology Consortium to WDB and
JBR. The authors wish to thank Max Teplitski, Brian Ahmer, Mary Connelley
and Mengsheng Gao for stimulating discussion.
Disruption of bacterial quorum sensing by other organisms Bauer and Robinson
bacteria in the wheat rhizosphere. Mol Plant Microbe Interact
1998, 11:1078-1084.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
Parsek MR, Greenberg EP: Acyl-homoserine lactone quorum
sensing in Gram-negative bacteria: a signaling mechanism
involved in associations with higher organisms. Proc Natl Acad
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2.
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Finch RG, Pritchard DI, Bycroft BW, Williams P, Stewart GS: Quorum
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4.
Pollack A: Drug makers listen in while bacteria talk. New York Times
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5.
Fuqua C, Parsek MR, Greenberg EP: Regulation of gene
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6.
•
Whitehead NA, Barnard AM, Slater H, Simpson NJ, Salmond GP:
Quorum-sensing in Gram-negative bacteria. FEMS Microbiol Rev
2001, 25:365-404.
An especially thoughtful review of QS in bacteria.
7.
de Nys R, Steinberg P, Willemsen P, Dworjanyn SA, Gabelish CL,
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8.
Givskov M, de Nys R, Manefield M, Gram L, Maximilien R, Eberl L,
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homoserine lactone-mediated prokaryotic signalling. J Bacteriol
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9.
Manefield M, de Nys R, Kumar N, Read R, Givskov M, Steinberg P,
Kjelleberg S: Evidence that halogenated furanones from Delisea
pulchra inhibit acylated homoserine lactone (AHL)-mediated gene
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Microbiology 1999, 145:283-291.
10. Kjelleberg S, Steinberg P: Defenses against bacterial colonisation
•
of marine plants. In Phyllosphere Microbiology. Edited by
Lindow SE, Poinar E, Elliott V: American Phytopathological Society
Press; 2002:157-173.
The authors provide a comprehensive discussion of how the colonization of
marine algae is influenced by secretion of QS mimic compounds.
11. Teplitski M, Robinson JB, Bauer WD: Plants secrete substances
•• that mimic bacterial N-acyl homoserine lactone signal activities
and affect population density-dependent behaviors in associated
bacteria. Mol Plant Microbe Interact 2000, 13:637-648.
This report provides the first evidence that higher plant species secrete AHL
signal-mimic compounds that affect QS-regulated gene expression and
behavior in various bacteria. Various plant species were shown to secrete
AHL mimics, and plants were shown to produce both stimulatory and
inhibitory mimic activities.
12. Chen X, Schauder S, Potier N, Van Dorsselaer A, Pelczer I,
Bassler BL, Hughson FM: Structural identification of a bacterial
quorum-sensing signal containing boron. Nature 2002,
415:545-549.
13. Leadbetter JR, Greenberg EP: Metabolism of acyl-homoserine
lactone quorum-sensing signals by Variovorax paradoxus.
J Bacteriol 2000, 182:6921-6926.
14. Pierson EA, Wood DW, Cannon JA, Blachere FM III: LSP:
interpopulation signaling via N-acyl-homoserine lactones among
237
15. Dong YH, Xu JL, Li XZ, Zhang LH: AiiA, an enzyme that inactivates
the acylhomoserine lactone quorum-sensing signal and
attenuates the virulence of Erwinia carotovora. Proc Natl Acad Sci
USA 2000, 97:3526-3531.
16. Dong YH, Wang LH, Xu JL, Zhang HB, Zhang XF, Zhang LH:
•• Quenching quorum-sensing-dependent bacterial infection by an
N-acyl homoserine lactonase. Nature 2001, 411:813-817.
The authors show that various AHLs are inactivated by an N-acyl homoserine
lactonase, Aii, from Bacillus sp. They also demonstrate that transgenic
potato and tobacco plants expressing the aiiA gene are substantially more
resistant to infection by the soft rot pathogen Erwinia carotovora, a bacterium
in which pathogenicity is regulated by AHL QS.
17.
•
Mae A, Montesano M, Koiv V, Palva ET: Transgenic plants producing
the bacterial pheromone N-acyl-homoserine lactone exhibit
enhanced resistance to the bacterial phytopathogen Erwinia
carotovora. Mol Plant Microbe Interact 2001, 14:1035-1042.
This study demonstrates that eukaryotes can effectively disrupt bacterial
pathogenicity by overproducing QS signals.
18. Fray RG, Throup JP, Daykin M, Wallace A, Williams P, Stewart GS,
Grierson D: Plants genetically modified to produce
N-acylhomoserine lactones communicate with bacteria.
Nat Biotechnol 1999, 17:1017-1020.
19. Michael B, Smith JN, Swift S, Heffron F, Ahmer BM: SdiA of
•• Salmonella enterica is a LuxR homolog that detects mixed
microbial communities. J Bacteriol 2001, 183:5733-5742.
This report provides the first demonstration that bacteria which do not produce
any AHLs can nonetheless carry functional AHL receptors that enable them
to ‘listen’ and respond to AHL QS signals produced by other bacterial
species. QS signal-mimics produced by eukaryotes may thus affect behaviors
in those bacteria that have a receptor specifically for listening to the QS
signals of other species.
20. Davies DG, Parsek MR, Pearson JP, Iglewski BH, Costerton JW,
Greenberg EP: The involvement of cell-to-cell signals in the
development of a bacterial biofilm. Science 1998, 280:295-298.
21. Hassett DJ, Ma JF, Elkins JG, McDermott TR, Ochsner UA, West SE,
Huang CT, Fredericks J, Burnett S, Stewart PS et al.: Quorum
sensing in Pseudomonas aeruginosa controls expression of
catalase and superoxide dismutase genes and mediates biofilm
susceptibility to hydrogen peroxide. Mol Microbiol 1999,
34:1082-1093.
22. Charlton TS, de Nys R, Netting A, Kumar N, Hentzer M, Givskov M,
•
Kjelleberg S: A novel and sensitive method for the quantification
of N-3-oxoacyl homoserine lactones using gas chromatographymass spectrometry: application to a model bacterial biofilm.
Environ Microbiol 2000, 2:530-541.
These authors measure the concentrations of 3-oxo AHLs released by and
in P. aeruginosa biofilms.
23. Reidel K, Hentzer M, Geisenberger O, Huber B, Steidle A, Wu H,
Hoiby N, Givskov M, Molin S, Eberl L: N-Acylhomoserine-lactonemediated communication between Pseudomonas aeruginosa and
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25. Ren D, Sims JJ, Wood TK: Inhibition of biofilm formation and
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2001, 3:731-736.