By Michael Wade
If you wanted to move a piano from your old apartment on the
second floor to your new house, you wouldn’t try to move it by yourself because
it’s too heavy. The logical thing to do would be to call a bunch of your
friends and have them help, distributing the weight of the piano among many
people instead of just one. Well…bacteria can do a similar thing of working
together for a common good called quorum sensing. A few decades ago, everyone
thought of bacteria as individuals doing their own thing. However, that is not
the case, for if it were, bacteria would most likely not be as successful of an
organism as they are.
Quorum sensing works by bacteria secreting a soluble small
molecule called an “autoinducer” (AI) or a “pheromone”. The AI then diffuses through the environment
and can be taken up by the same bacteria or another bacteria in the same area.
At low numbers of bacteria, there isn’t a lot of AI in the environment, but
after a few rounds of cell division, the numbers of bacteria increase
exponentially and so does the concentration of AIs. At this point, the AI’s are
binding to receptors or diffusing into the cell at a high concentration. The
bacteria “senses” that there are a lot of other bacteria around, and once the
concentration reaches a certain point, gene expression changes for all the
bacteria in that localized area, allowing the bacteria to work together for a
common good or goal.
AIs are not all the same, nor does each bacteria only
produce one AI. The structure of AIs and how many AIs an organism produces or
responds to is different for each organism. Gram-negative bacteria tend to
synthesize some variation of an acyl-homoserine lactone (AHL) molecule, while
Gram-positive bacteria tend to use small amino acid sequences. Due to the
differences in molecules, the mechanisms are slightly different. Gram-negative
AIs can diffuse in and out of a cell and can interact with intracellular
proteins or with surface sensor protein, whereas Gram-positive AIs have to be
actively transported out of the cell and only interact with surface sensor
proteins.
What kind of goal could bacteria be trying to achieve by working
together? Well, there are many. When bacteria reach the critical density and
start changing gene expression, they tend to shift toward making “public goods”
instead of “private goods”. What that means is bacteria will make extracellular
enzymes that break down the macromolecules in their environment into smaller
molecules that can then be used for energy instead of keeping their enzymes
tethered or inside the cell, or make and secrete molecules used for biofilms. These
secreted enzymes are “public goods” because all the bacteria in the area
benefit from products of the enzymes. The benefits of quorum sensing can be
seen in various situations and environments, from Vibrio fischeri in the light organ of it’s squid host, to Vibrio cholerae and many other species
in biofilms, to Pseudomonas aeruginosa
in an opportunistic infection in a cystic fibrosis patient. However, making the
“public goods” comes at a high cost in that it takes a lot of energy, which
leads to the evolution of social cheaters. Social cheaters are bacteria that
benefit from “public goods” of cooperators but don’t respond to quorum sensing
signals. Although they tend to arise only after many generations of
replication, they outcompete cooperators once they have been established as a
subpopulation, but then rapidly collapse, a tragedy of the commons.
There are many examples of bacteria that are known to
communicate, but I’ll only mention a few. Vibrio
fischeri, and Vibrio harveyi are
bioluminescent marine bacteria that are used as models for quorum sensing. V. fischeri is a symbiont that colonizes
the light organ of the Hawaiian bobtail squid and provide light during the
night as counter-illumination to prevent predation on the squid. Once V. fischeri enters the light organ, it
begins to divide, and in a short amount of time, the concentration reaches a
threshold amount, as signaled by AI concentration, that V. fischeri changes global gene expression through a
phosphorylation cascade that results in a stable master regulator (LitR/HapR)
that activates many target genes and operons, one of which is responsible for
luminescence. The quorum sensing mechanism in V. harveri is very similar to that of V. fischeri with similar gene products. However, since V. harveyi is an opportunistic fish
pathogen, the AIs also target genes for toxins and virulence factors.
Because humans are selfish, research in quorum sensing has
recently been in bacteria that cause disease in humans. During infections,
survival and colonization in hostile host environments is important for
bacteria. The ability to remain undercover from the host defenses until the
bacteria population is dense enough to overwhelm the host immune defenses is a
common theme, and is modulated through quorum sensing. Pseudomonas aeruginosa is an opportunistic pathogen that causes
both acute and chronic infections in immunocompromised individuals, and is a
major concern in cystic fibrosis patients. P.aeruginosa
is a Gram-negative bacteria, so it uses AHL molecules to asses its population
density. Once it reaches a critical density, P. aeruginosa launches a full blown attack against the host,
secreting exotoxins, proteases, cyanide, and many other exoproducts-some of
which are dependent upon the environmental factors. However, the AIs also have
a secondary role to assessing population density-they also can inhibit host
defenses such as macrophage proliferation or switch the T helper response from the
antibacterial Th1 response to the Th2 response. Another pathogen known to
quorum sense is Yersiniae pestis, the
causative agent of plague.
The ability to assess the population is not limited to
assessing individual species numbers-bacteria can communicate across species.
Cross-species communication may be important for survival in various niches
such as biofilms. Although some AIs are very specific for a particular species,
others are more generalized and can function across Genus’s. In the figure
below, the V. harveyi mutant can’t
make the autoinducer HAI. However, the E.coli
next to the V. harveryi does produce
a HAI-like autoinducer, allowing only the V.
harveyi cells next to the E. coli
cells to respond to that signal, resulting in luminescence (a quorum sensing
product)
So, now that we know bacteria communicate to work together,
what can we do to prevent this communication in pathogenic bacterial
infections? Well, chemists have teamed up with microbiologists to synthesize
quorum sensing analog molecules for both Gram positive and Gram-negative
bacteria. The goal of the analogs is not to kill the bacteria, but to
down-regulate virulence factors, which as we learned earlier, are products of
quorum sensing. However, although many bacteria posses the AI system, this type
of approach may be very limited becuase P.
aeurginosa is the only major human pathogen known to use AHLs for
controlling virulence factors. In related research, chemists have developed a
molecule that blocks the quorum sensing system in Staphylococcus aureus that is responsible for virulence factors.
Although it’s only been tested in a mouse skin model, this novel treatment
could be of importance due to the growing antimicrobial resistance in many
species, especially S. aureus (methicillin,
vancomycin).
Quorum sensing is a young subject that is attracting a lot
attention from scientists, with more and more labs dedicated to understanding
this awesome phenomenon of bacteria as complex, social organisms.
References:
Bassler, B.L., Greenberg, E.P., and A.M. Stevens. 1997. Cross-Species
Induction of Luminesence in the Quorum-Sensing Bacterium Vibrio harveyi. Journal of Bacteriology 179:4043-4045.
Dandekar, A.A., Chugani, S., and E.P. Greenberg. 2013.
Bacterial Quorum sensing and Metabolic Incentives to Cooperate. Science
338:264-266.
Nyholm, S.V., and M.J. McFall-Ngai. 2004. The
Winnowing-Establishing the Squid-Vibrio
Symbiosis. Nature 2:632-642.
Sandoz, K.M., Mitzimberg, S.M., and M. Schuster. 2007.
Social cheating in Pseudomonas aeruginosa
quorum sensing. Proceedings of the National Academy of Science 104:15876-15881.
Williams, P., Camara, M., Hardman, A., Swift, S., Milton,
D., Hope, V.J., Winzer, K., Middleton, B., Pritchard, D.I., and B.W. Bycroft.
2000. Quorum sensing and the Population-Dependent Control of Virulence. The
Royal Society 355:667-680.
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