Motion to Form a Quorum
behavior in chemotaxis has been modeled usingthe Keller-Segel equations (7). Our analysis ofthese equations indicates that for a small volume
Sungsu Park,1 Peter M. Wolanin,2 Emil A. Yuzbashyan,1
connected by a small opening to a large volume,
Pascal Silberzan,3 Jeffry B. Stock,2* Robert H. Austin1
such as our enclosure within a relatively largemicrofluidic chamber (Fig. 1A) or the dead endsin a maze (Fig. 1B), random fluctuations in cell
Bacterial gene expression is frequently regu-
redox-sensing aerotaxis receptor, Aer, is present
number that cause an increase in the density of
lated by small molecules secreted into the
at low levels but still effectively modulates che-
bacteria inside the small volume can increase irre-
motaxis. Tsr binds L-serine with the highest
versibly to produce a dense accumulation of cells.
build up with increasing cell number until a
affinity, but it also binds L-alanine, L-cysteine,
Our results suggest that self attraction could
critical population density or "quorum" is
and glycine (4). Whereas strains with tsr deleted
readily produce local cell densities that exceed
achieved, at which point the cells produce a
were unable to accumulate in the enclosure, tar
the threshold necessary for quorum-dependent
response such as virulence or biofilm forma-
or aer deletion had little or no effect. Moreover,
processes. This notion was supported by the ob-
tion that requires the coordinated activity of
addition of saturating levels of L-serine (0.5
servation that, after coming together within the
large numbers of individuals. It has generally
mM), which effectively competes with glycine,
chamber, wild-type E. coli tended to exhibit a
been assumed that quorum formation derives
completely blocked accumulation of wild-type
further association to form dense granular aggre-
cells. Saturating concentrations of L-aspartate
gates. A proposed E. coli quorum sensing signal,
growth to high cell density (1).
had no effect. The particular amino acid that is
AI-2, is produced by the LuxS enzyme (8). A
Numerous studies have shown that motility
most important in mediating self attraction
strain with luxS deleted accumulated into the
seems to depend on the conditions of growth
enclosure just like wild type, but was never ob-
before nutrient depletion. It has previously been
served to form dense aggregates. Vibrio harveyi,
a highly motile marine bacterium, exhibits a sim-
ilar tendency to accumulate in confined spaces.
These cells produce light in regions of high pop-
ulation density (Fig. 1, B and C). This biolumi-
nescence, which is one of the most well-studied
of quorum-dependent responses (8), confirms
that chemotaxis-mediated associations facilitated
by closed geometries can lead to activation of
quorum sensing- dependent genes and their as-
Fig. 1. E. coli and V. harveyi accumulation and quorum sensing. (A) Epifluores- References and Notes
cence images of green fluorescent protein (GFP)-labeled E. coli in M9 minimal
1. S. Swift et al., Adv. Microb. Physiol. 45, 199 (2001).
media as they accumulate into a central 250 m by 250 m enclosure via a
2. J. W. McClaine, R. M. Ford, Biotechnol. Bioeng. 78,
40-m-wide channel through 100-m-wide walls. After 3 hours the density of
cells is more than seven times greater inside than outside. The rectangles are
3. S. Clarke, D. E. Koshland Jr., J. Biol. Chem. 254, 9695
silicone pillars that support the roof of the chamber. (B) Dark-field image of V.
4. J. Adler, Annu. Rev. Biochem. 44, 341 (1975). harveyi after 8 hours in the maze. The narrowest passages are 100 m wide.
5. E. O. Budrene, H. C. Berg, Nature 376, 49 (1995).
Lines corresponding to the walls of the maze are overlaid for clarity. (C)
6. J. E. Segall, S. M. Block, H. C. Berg, Proc. Natl. Acad.
Photon-counting image of the intrinsic luminescence, indicating active quorum
Sci. U.S.A. 83, 8987 (1986).
sensing in areas where the cells have accumulated at high density (9).
7. M. P. Brenner, L. S. Levitov, E. O. Budrene, Biophys. J.74, 1677 (1998).
8. M. G. Surette, M. B. Miller, B. L. Bassler, Proc. Natl.
lishing the high local cell densities required
shown that E. coli grown in succinate secrete
Acad. Sci. U.S.A. 96, 1639 (1999).
aspartate, which acts through Tar to cause cells
9. Materials and Methods are available as supporting
online material on Science Online.
A culture of Escherichia coli grown to mod-
to associate into dense colonies in soft agar (5).
10. We thank E. C. Cox for insightful comments on the
erate density (approximately 2 ϫ 108 cells/ml) in
Chemotaxis has generally been considered as a
manuscript; J. S. Parkinson for strains RP437, RP2361,
either rich media or in minimal media was used
mechanism for cell dispersal. In nutrient-deplet-
RP5700, and UU1117, and for helpful advice; H. C. Berg
for strain HCB317 and for comments and advice; and B.
to uniformly fill a microfluidic chamber (7 mm
ed environments, however, the cells themselves
Bassler for strains BB120 and BB170 and for the gift of
by 3 mm by 30 m) with a small central enclo-
become sources of attractant molecules. Move-
AI-2 produced in vitro. We also gratefully acknowledge
sure (250 m by 250 m) constructed from
ment toward the amino acids secreted by the
M. Taga and K. Xavier for advice and assistance; N. C.
Darnton, P. Silberzan, H. Lin, and C. Gabel for discus-
silicone elastomer. Over the course of 1 to 3
cells is enhanced by the ability of the chemo-
sions; W. Austin for technical assistance; and J. Chen for
hours (depending on the media), the cells migrate
taxis system to adjust its sensitivity so that it can
swarm plate assays. Supported by grants from DARPA
from the chamber into the central enclosure
respond to very low concentrations of attractant
(MDA972-00-1-0031), NIH (R01 HG001506 and F32
through a narrow (40 m) channel (Fig. 1A).
chemicals (6). Accumulation of a high local
GM064228 to P.M.W.), and the State of New Jersey
This behavior is not observed with a mutant
density of cells may offer advantages such as
Supporting Online Material
strain that is motile but deficient in chemotaxis.
enhanced genetic exchange or communal deg-
Cells accumulate in the enclosure because
radation of antibiotics, as well as the enabling of
they are attracted to each other due to their
secretion of amino acids, such as glycine, that
The sites at which cells accumulate depend
are chemoattractants. We detected this secretion
on the geometry of their surroundings. Results
by analysis of the free amino acid content of the
with cells in percolated lattices formed from sil-
1Department of Physics, 2Department of Molecular
growth media over time. The serine receptor,
icone elastomer indicate that E. coli tend to ac-
Biology, Princeton University, Princeton, NJ 08544,
Tsr, is the most abundant chemotaxis receptor in
cumulate in any areas of these mazes where the
USA. 3Institut Curie, 75005 Paris, France. E. coli, whereas the aspartate receptor, Tar, is
geometry provides a sufficiently enclosed space,
*To whom correspondence should be addressed. E-
also present at relatively high levels (3). The
such as dead ends and cul-de-sacs. Self-attractive
11 JULY 2003 VOL 301 SCIENCE www.sciencemag.org
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