Evening picnics in a park, sunset beers by a lake and warm
nights with the windows open are just some of the delights of
midsummer. But as dusk falls, one of the most infuriating
creatures on the planet stirs: the mosquito. Outdoor activities
are abandoned in an ankle-scratching frenzy and sleep is
disturbed as we haplessly swat at the whining source of our
torment.
Of course, all these discomforts are nothing compared to the
damage mosquitoes do as transmitters of diseases such as
malaria, dengue or yellow fever. According to the World Health
Organization, mosquito-borne yellow fever alone causes
more than 30,000 deaths annually.
But now, in the on-going battle between human and mosquito, we
might just have gained the upper hand. Scientists at Texas A&M
University believe they have found a way to outsmart the
bloodsuckers by tricking them into deciding not to bite us, and
their main allies in this ruse are the
billions of bacteria that live on our skin.
Bacteria "talk" to one another using a chemical system called
quorum sensing. This cell-to-cell communication is used to
control or prevent particular behaviors within a community, such
as swarming or producing biofilm, like the formation of plaque
on our teeth. To start a conversation, bacteria produce
compounds that contain specific biochemical messages. The more
of these compounds that are produced, the more concentrated the
message becomes, until it reaches a threshold that causes a
group response. Behaviors are more likely to occur as the
message gets "louder"—and that makes it easy for other organisms
to eavesdrop on the bacterial chatter.
“Even people respond to quorum-sensing molecules," says
Jeffery K. Tomberlin, a behavioral ecologist at Texas A&M.
"For example, if something is decomposing, there are
quorum-sensing molecules that are released in that process that
tell us it is not a good environment.”
Enter the mosquito. Previous work suggests that factors such as
the volume of carbon dioxide we exhale, body temperature,
body odor and even the color of our clothes may
influence how attractive we are to the bloodthirsty insects.
According to Tomberlin, mosquitoes can also hack into bacterial
communication systems using chemoreceptors on their antennae,
rather like World War II code-breakers intercepting an encrypted
transmission: “Their radar system is extremely sensitive and can
pick up these messages that are occurring. And they have the
equipment that allows them to interrupt those messages,” he
says.
Evolutionarily speaking, quorum sensing has always occurred in
nature, and mosquitoes have evolved the ability to perceive
these communications pathways via natural selection. Mosquitoes
benefit from this hacking by gleaning information about the
quality of a blood host and being selective about who they
target. But the bacterial communication pathways continue to
evolve, resulting in a race between competing organisms—on one
side, bacteria are producing messages, and on the other,
mosquitoes are trying to interpret them.
“Your opponent is always changing the encryption of their code.
You have to break that code, and your survivorship depends on
it,” says Tomberlin. Knowing that microbial communication can
affect mosquito attraction, Tomberlin and his colleagues at
Texas A&M—including Craig Coates, Tawni Crippen and graduate
researcher Xinyang Zhang—have now shown that humans may be able
to hack the hackers and influence whether mosquitoes decide to
bite us.
Staphylococcus epidermidis is one among more than a thousand
bacterial species commonly occurring on human skin. The team
used a mutant form of S. epidermidis, in which they deleted the
genetic mechanism that encodes its quorum sensing system. With
the bacteria's biochemical pathways disrupted, the mosquitoes'
"surveillance equipment" could no longer eavesdrop.
A microscope view of the common skin bacteria Staphylococcus
epidermidis. (David Scharf/Corbis)
The team then carried out a series of experiments using blood
feeders, which were covered in sterile cloth treated with either
the silenced mutants or unmodified wild-type bacteria. The team
compared the feeders' attractiveness to the female Aedes aegypti
mosquito, the main transmitting agent for yellow fever.
The blood feeders consisted of a culture flask sealed with a
paraffin film that the mosquitoes could penetrate. A millimeter
of rabbit blood was injected between the film and the culture
flask, and warm water was pumped through the flask to keep the
blood at average body temperature. The team placed feeders
inside transparent plastic cages containing 50 mosquitoes and
left them in the cages for 15 minutes. They recorded the
insects' behavior on video, allowing them to count the number of
feeding mosquitoes at each minute.
The team tested different scenarios, such as placing blood
feeders treated with either wild-type or mutant bacteria in
separate cages, then putting both types of bacteria in the same
cage at the same time. When given a choice, “twice as many
mosquitoes were attracted to the wild type on the blood feeder
rather than the mutant on a blood feeder,” Tomberlin says.
Based on these findings, which are currently being prepared for
submission to PLOS One, the team believes that inhibiting
bacterial communications could lead to new methods for deterring
mosquitoes that would be safer than
harsh chemical repellents such as DEET. This could have
important implications for reducing the spread of mosquito-borne
diseases such as yellow fever. “Bacteria are our first line of
defence, and we want to encourage their proliferation. However,
we may be able to produce natural repellents that will allow us
to lie to mosquitoes," says Tomberlin. "We might want to modify
the messages that are being released that would tell a mosquito
that we are not a good host, instead of developing chemicals
that can be harmful to our bacteria on our skin, or to our skin
itself.”
Tomberlin notes that manipulating bacterial conversations may
have many other applications, and that these are being actively
studied in other institutions. In terms of health applications,
blocking communication between bacteria in the lungs of patients
with cystic fibrosis could lead to new treatments for the
disease. And in the energy industry, inhibiting quorum sensing
could reduce oil pipeline corrosion caused by microbes.
Researchers such as
Thomas K.
Wood of Pennsylvania State University, Rodolfo
García-Contreras of the Universidad Nacional Autónoma de Mexico
and Toshinari Maeda of the Kyushu Institute of Technology are
leaders in quorum sensing research. According to Wood, efforts
to manipulate bacterial communication need to account for the
microbes' sophisticated counter-espionage techniques: “We are
also trying to understand how bacteria evolve resistance to the
new types of compounds designed to stop bacteria from talking,”
he says.
So now, for mosquitoes and for science, the code-breaking race
is on.
Read more:
http://www.smithsonianmag.com/science-nature/stop-mosquito-bites-silence-your-skins-bacteria-180955772/#pa54Hsugtx0MmSRA.99
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