ANTIBIOTICS GIVE RISE TO NEW COMMUNITIES OF HARMFUL BACTERIA
Most people have taken
an antibiotic to treat a bacterial infection. Now researchers from the
University of North Carolina at Chapel Hill and the University of San Diego, La
Jolla, reveal that the way we often think about antibiotics -- as
straightforward killing machines -- needs to be revised.
"For a long time
we've thought that bacteria make antibiotics for the same reasons that we love
them -- because they kill other bacteria," said Shank, whose work appears
in the February 23 Early Edition of the Proceedings of the National
Academy of Sciences. "However, we've also known that antibiotics can
sometimes have pesky side-effects, like stimulating biofilm formation."
Shank and her team now
show that this side-effect -- the production of biofilms -- is not a
side-effect after all, suggesting that bacteria may have evolved to produce
antibiotics in order to produce biofilms and not only for their killing
abilities.
Biofilms are
communities of bacteria that form on surfaces, a phenomenon dentists usually
refer to as plaque. Biofilms are everywhere. In many cases, biofilms can be
beneficial, such as when they protect plant roots from pathogens. But they can
also harm, for instance when they form on medical catheters or feeding tubes in
patients, causing disease.
"It was never
that surprising that many bacteria form biofilms in response to antibiotics: it
helps them survive an attack. But it's always been thought that this was a
general stress response, a kind of non-specific side-effect of antibiotics. Our
findings indicate that this isn't true. We've discovered an antibiotic that
very specifically activates biofilm formation, and does so in a way that has
nothing to do with its ability to kill."
Shank and her team
previously reported that the soil bacterium Bacillus cereus could
stimulate the bacterium Bacillus subtilis to form a biofilm in
response to an unknown secreted signal. B. subtilis is
found in soil and the gastrointestinal tract of humans.
Using imaging mass
spectrometry, they subsequently identified the signaling compound that induced
biofilm production as thiocillin, a member of a class of antibiotics called
thiazolyl peptide antibiotics, which are produced by a range of bacteria.
At that point, Shank
and her colleagues knew thiocillin had two very specific and different
functions, but they didn't know why -- and wanted to know how it worked. That's
when they modified thiocillin's structure in a way that eliminated thiocillin's
antibiotic activity, but did not halt biofilm production.
"That suggests
that antibiotics can independently and simultaneously induce potentially
dangerous biofilm formation in other bacteria and that these activities may be
acting through specific signaling pathways," said Shank. "It has
generated further discussion about the evolution of antibiotic activity, and
the fact that some antibiotics being used therapeutically may induce biofilm
formation in a strong and specific way, which has broad implications for human
health."
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